DISCLAIMER: I DO NOT OWN ANY OF THIS CONTENT, ALL THESE CONTENTS BELONG TO THEIR RIGHTFUL OWNERS
The joy of owning a DSLR or mirror-less system camera is the ability to change lenses. You could go for a wide angle to squeeze more in, a telephoto to magnify a distant subject, or a macro lens for taking great quality close-ups. There’s almost no limit to what you can do, with lenses to suit all occasions – and budgets – but so where do you start? In this guide I’ll explain everything you need to know about buying lenses and understanding their descriptions to help you make the right choice for your style of photography. This guide to understanding lenses accompanies my buyer's guides for specific camera systems, linked above.
But first a question for you. If you'd like a new lens, first ask yourself what kind of pictures you'd like to take, or as revealingly, how your current lens or lenses aren't delivering the effect you're after. Maybe you’re struggling to fit everything in, whether it's a large building, cramped interior or big group shot. Perhaps you'd like to make distant sports players or wildlife look bigger. Alternatively it could be small things that interest you, but you just can't focus close enough to render them into a decent size on the photo. Or you might be perfectly happy with the view from your current lens, but you fancy something which delivers better quality, focuses quicker, has anti-shake facilities or maybe works better in low light.
There’s almost always more than one lens which does what you’re after, so the next step is working your way through the options which are available. Even if you're a seasoned photographer though, the names and descriptions given to lenses can often feature a bewildering array of letters and numbers. Thankfully it’s actually easier to decipher than it first appears. Here are the key specifications to look out for.
Lens mount
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Before even discussing the numbers, it's important to remember most camera companies employ their own unique lens mountings, which means one company's lenses generally won't work on another company's cameras. For example, Canon's lenses are designed for Canon cameras and Nikon's lenses are designed for Nikon bodies.
There are however some exceptions. Third party companies like Sigma and Tamron produce multiple versions of their lenses for each of the most popular camera manufacturers, so you'll typically be able to choose versions designed to work with Canon, Nikon, Sony and Pentax cameras.
Beyond this, some adapters are available which let you use lenses designed for one type of camera on a different type of camera, such as fitting a Nikon lens to a Canon body, but in doing so you'll often lose the ability to autofocus or even use automatic exposures; but for some specialist photographers or movie makers it can be a compromise worth making simply to have access to specific lenses which aren't 'natively' available for their camera.
If you're really into using lenses designed for different systems, mirror-less cameras can give you much more flexibility than conventional DSLRs. The reason being all lenses are designed to focus on a sensor at a certain distance, so inserting adapters can move them further apart and prevent them from focusing successfully. But mirror-less cameras are normally thinner than traditional DSLRs, which means they have room to squeeze in adapters for lenses designed for thicker bodies - although again remember you'll normally lose autofocus and auto-exposure in the process.
And finally on the subject of mirror-less cameras, Panasonic and Olympus decided to develop the same lens mounting for their mirror-less models, called Micro Four Thirds, which means Olympus Micro Four Thirds lenses will work on Panasonic Micro Four Thirds cameras and vice versa.
Focal length and crop factors
Key points:
1: The focal length (coupled with the size of your camera's sensor) defines how much you'll fit in a photo.
2: The focal length of a lens is measured in millimeters, such as 50mm.
3: Shorter focal lengths are known as wide angle and squeeze more into a photo.
4: Longer focal lengths, often known as telephotos, magnifiy a smaller area.
5: Cameras with smaller sensors crop and effectively magnify the middle of the view.
The amount you'll fit into a photo is known as the coverage or field of view and it's actually down to a combination of the lens focal length and the size of your camera's sensor; not the number of Megapixels, but the actual dimensions. The focal length which most closely matches the magnification of the human eye when using a camera with a full-frame (35mm sized) sensor is 50mm. Our eyes have a much bigger field of view, but if you were to look at something in person, and then through a 50mm lens, the actual magnification, again with a full-frame camera, would be similar. This is why 50mm lenses are known as standard lenses – they’re suited to a wide variety of subjects from landscapes to portraits.
You'll note I kept referring to full-frame bodies there. This is because the coverage delivered by a lens is dependant on both its focal length and the size of the sensor behind it. If you keep the focal length the same but put a smaller sensor behind it, it'll crop the image for a more magnified view. Conversely if you put a bigger sensor behind the same lens, it'll deliver a broader field of view. This can understandably become very confusing, so the camera world has, for better or worse, standardised on describing coverage in relation to full-frame bodies. This may seem odd since cameras with smaller sensors are much more common, but we need a benchmark as there's so many different sensor sizes in the market. We ended up settling on full-frame because back in the old days of 35mm film cameras, many photographers became familiar with the coverage delivered by certain focal lengths. They knew that 50mm was roughly normal magnification and anything shorter would be wide angle and anything longer would deliver a tighter view.
So nowadays we talk about equivalent or effective focal lengths in relation to full-frame / 35mm systems. Two cameras may have completely different sensor sizes and lens focal lengths, but if they share, say, a 28-80mm equivalent focal length lens, then you'll know they both deliver the same wide angle to mild telephoto coverage and will be able to take similar photos.
So when choosing a lens, it's important to know what impact your camera's sensor will have on it. This is known as the crop-factor and once you know what you're dealing with, it's easy to calculate the coverage you'll get in practice. Starting with full-frame bodies, also known as FX in Nikon's world, there is no crop factor. Their sensors match the size of 35mm film, so all lenses will act the same as if they were mounted on a 35mm camera. Easy.
Anything smaller than full-frame is known as a cropped-sensor, but these come in different sizes. The next sensor down the scale from full-frame is known as APS-H and is only used in a handful of cameras such as Canon's older pro sports models. This applies a crop factor of 1.3 times, so if you mounted a 50mm lens on one, it would deliver coverage equivalent to 65mm (50x1.3).
Full-frame camera, left, and APS-C 'cropped-frame' camera, right.
Note the circular lens mount is the same size, but the rectangular sensor inside is different. |
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Much more common is the next size down from that, known as APS-C and employed by consumer DSLRs including Nikon's DX models, along with Sony's NEX, Samsung's NX and Fujifilm's X mirror-less systems. These all apply a crop factor of 1.5 times, so to work out the effective coverage, just multiply the lens focal length by 1.5 times. So if you mount a 50mm lens on an APS-C body, it'll deliver coverage equivalent to 75mm (50x1.5) on a full-frame body. Note Canon's APS-C sensors are fractionally smaller than those in rival cameras, resulting in a 1.6x crop-factor, so that 50mm would now act like an 80mm (50x1.6) on a full-frame body.
In the photo above you can see two cameras with their lenses removed - the metal circle is the lens mount and the rectangle inside is the sensor. The camera on the left has a full-frame sensor (looking green in this photo) and the one on the right has an APS-C sensor (looking white in this photo); clearly the APS-C sensor is much smaller. To illustrate the difference this makes in practice, I took a photo with a 36mm focal length lens on both types of sensors. You can see their relative coverage below left and below right. The image in the middle below shows the size of an APS-C sensor in relation to full-frame, and it's clear how the smaller sensor is simply acting as a crop, delivering the tighter view below right.
36mm focal length
Full-frame sensor
36mm effective focal length
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Size of APS-C sensor in relation to full-frame sensor. APS-C sensor indicated by coloured rectangle
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36mm focal length
APS-C sensor
54mm effective focal length
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36mm focal length on full-frame sensor
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APS-C effectively crops FF image
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36mm focal length on APS-C sensor
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The next sensor down the scale is Four Thirds as used in Four Thirds DSLRs and Micro Four Thirds mirror-less cameras. This has a crop-factor of two times, so a 50mm lens would deliver coverage equivalent to 100mm (50x2) on a full-frame body. After this comes Nikon's 1 mirror-less format which has a 2.7x crop-factor, so the same 50mm lens would now deliver even tighter coverage equivalent to 135mm (50x2.7) on a full-frame body.
What should be apparent by now is the crop factor is good news for anyone who's into shooting small, distant subjects, as smaller sensors effectively reduce the field of view of every lens, thereby increasing their effective magnification. But conversely if you desire wide angle coverage, you'll need lenses with shorter focal lengths than full-frame owners. Let's say you like the coverage of a 36mm lens on a full-frame body (as seen in the image above left), where it's considered mild wide angle. Mount this same lens on an APS-C or Micro Four Thirds camera and it'll suddenly perform like a 54mm or 72mm lens respectively, thereby losing its wide angle coverage and acting like a standard or short telephoto lens instead (as seen in the image above right). If you want the same equivalent coverage, you'll need to divide the desired focal length on a full-frame system by the crop-factor of your system. So if you have a camera with an APS-C sensor, you'd divide 36mm by 1.5 (or 1.6 if it's a Canon) and end up needing a 24mm (or 22.5mm) lens to deliver the same field of view. If you had a Micro Four Thirds camera, you'd need to divide it by two times, thereby requiring a 18mm lens to deliver the same field of view.
It sounds complex the first time you come across all of this, but it's actually very simple in practice. Just familiarise yourself with the coverage of lenses on full-frame bodies (my table below will help), then divide them by your sensor's crop factor to work out what actual lens you need to get. Or if you already have a lens and are wondering what effective coverage it's delivering on your camera, just multiply its focal length by your sensor's crop factor.
Note: since cropped-frame cameras aren’t using the full area of normal lenses, many manufacturers additionally offer models which are only corrected for this smaller frame. Canon, Nikon, Sony and Pentax refer to these types of lenses as EF-S, DX, DT and DA respectively. These aren’t suitable for full-frame cameras though, so if you’re thinking of upgrading to full-frame in the future, try to avoid these models. The important thing to remember is that a camera with a cropped sensor will apply a crop factor to any lens you mount, regardless of whether it's corrected for a larger format or not.
Interestingly Nikon's full-frame cameras can actually use DX-format lenses which are only corrected for its APS-C range of cameras. You can either have the camera automatically make an APS-C sized crop of the image, or capture the full-frame in case the lens performs better than you think just outside the APS-C area - many of them do. Unfortunately this option is not available to owners of Canon full-frame cameras - these cannot use EF-S lenses and trying to fit one could damage the mount or the mirror inside the body. Meanwhile lenses designed for mirror-less cameras are generally corrected for their particular sensor size, so you don't need to worry.
Lens coverage
To illustrate the views you can expect at different focal lengths I took the following images from the same spot with different lenses. Remember the focal lengths quoted here are effective for a full-frame body, so to match the coverage with a camera employing a smaller sensor, you’ll need to divide them by the crop factor of your particular model – see above for an explanation.
So if you have a Nikon, Sony or Pentax APS-C body, divide the following focal lengths by 1.5 times. If you have a Canon APS-C body, divide them by 1.6 times, and if you have a (Micro) Four Thirds body, divide them by two times. So if you like the coverage of the 28mm example below and want it with an APS-C camera, you’d divide it by 1.5 times to give you just over 18mm. Conversely, if you want to see what a Nikkor DX 18-200mm lens would give you on an APS-C body that it's designed for, just multiply it by 1.5 times to deliver an effective range of 27-300mm.
Lens coverage by focal length from same position (equivalent to full / 35mm frame)
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17mm equivalent
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20mm equivalent
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24mm equivalent
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28mm equivalent
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35mm equivalent
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50mm equivalent
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70mm equivalent
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100mm equivalent
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135mm equivalent
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200mm equivalent
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300mm equivalent
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400mm equivalent
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Wide angle and telephoto lenses
Okay, now let's talk about wide angle and telephoto lenses, and in this section all the focal lengths will be in relation to a full-frame sensor. As I mentioned above, 50mm (on a full-frame sensor) is considered standard as the magnification roughly matches that of the human eye. Lenses with focal lengths shorter than 50mm are known as wide angle because they fit more into your photo. If you’re stood in the same position, a 25mm lens will have twice the diagonal field of view of a 50mm, and could therefore be used to squeeze in larger buildings, interiors, landscapes or even big group shots – ideal when you can’t step back any further. 28mm is the most common wide angle focal length and is ideal for landscape and architecture shots, but you can go much wider still if desired, and anything below 20mm is typically known as an ultra-wide angle lens on a full-frame body.
Squeezing in such a big view means wide angle lenses inevitably suffer from some distortion, especially towards the edges, but this can be used to exaggerate subjects for a special effect; indeed a special type of ultra-wide angle lens called a fish-eye deliberately uses distortion to deliver a highly curved result typically with a 180 degree field of view between diagonal corners of the frame. Lenses with shorter focal lengths also inherently have a larger depth-of-field (see aperture section below), which means it’s easier to get more in focus from near to far. The examples right were taken with a 17mm focal length.
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Lenses with focal lengths longer than 50mm are commonly known as telephoto models when mounted on a full-frame body. These fit less in, and are therefore ideal for getting closer to distant subjects or picking out detail; they also give a more flattering effect when taking photos of people. In contrast to wide angle, lenses with longer focal lengths have an inherently smaller depth of field, which means it’s easier to get a blurred background effect – again ideal for isolating the subject in portrait, wildlife and sports photography.
Ideal focal lengths for portraits are typically between 85mm and 135mm – these are often known as short telephotos. Appropriate focal lengths for sports or wildlife are generally much longer – at least 200mm, and ideally 300mm or more. Professional sports and wildlife photographers often use 600mm lenses, or even longer still. The two examples left were taken at 400mm.
You can buy lenses with either a fixed focal length which doesn’t vary, or a zoom lens which goes from one focal length to another. Zooms are very convenient, but generally aren’t as good quality as a fixed lens. Fixed focal length or 'prime' lenses are also normally smaller, lighter and give a brighter view that’s better for low light – see the aperture section below. It’s all about weighing up convenience against quality, although some more expensive zooms can certainly be very good.
General purpose zooms usually go from wide angle to short telephoto, such as 28-80mm, although some ‘super-zooms’ could give a range from 28-300mm, covering almost every photo opportunity. There are also wide angle zooms which normally offer an ultra-wide to normal range, such as 16-35mm. Similarly there are telephoto zooms which go from short to long telephoto focal lengths, such as 70-300mm. A longer range may sound tempting, and they're certainly more convenient, but you normally pay for this convenience with reduced quality. Don't expect a lens with a big zoom range to perform as well as one with a short range, and again don't expect a zoom even with a short range to perform as well as a prime lens.
Aperture and depth of field
Key points:
1: The aperture of a lens defines its light gathering power. The bigger the aperture, the more light it'll capture.
2: Aperture on camera lenses is most commonly described by the focal-ratio, or f-number for short.
3: The smaller the f-number, the bigger the aperture and the more light it'll gather - making it better in dim conditions.
4: The smaller the f-number, the shallower the depth of field - ideal for blurring backgrounds on portraits.
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The second most important lens specification is the aperture – this refers to how much light it can gather. The bigger the aperture, the more light it can capture, and the better it can work in dimmer conditions. This is important because if the lens can't get enough light to the sensor to make a normal photo, the camera either has to use a longer exposure (increasing the risk of camera shake or motion blur), or increase the ISO sensitivity (reducing the image quality). But if a lens has a bigger aperture, it can maintain faster exposures in lower light (thereby better freezing fast action or avoiding camera shake), or employ lower ISO sensitivities (to deliver better image quality).
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Lenses with bigger apertures also allow you to achieve a 'shallower' depth of field if desired - this refers to the distance over which things look like they're in sharp focus. A shallow depth of field will have very little beyond the main subject in focus, whereas a large depth of field will have lots in front and behind the subject in focus too. Portrait, sports, wildlife or close-up photographers often like to have a shallow depth of field so that their subjects stand out against a blurred background. This means they value lenses with large apertures. You can see an example of this on the left.
So if a larger aperture lets you deliver better quality in low light and achieve shallower depth of field effects, why don't all lenses have it? The reason is a larger aperture makes a lens bigger, heavier and more expensive than one with a smaller aperture, and of course not everyone needs a shallow depth of field or takes photos in low light. If you do like the sound of a lens with a large aperture though, be prepared to spend quite a lot of money, especially if the lens has a long focal length. That said, there are a handful of exceptions which are quite affordable. Lenses with a focal length around 50mm are very easy to make with a large aperture, so are often available at low prices. As you'll know from the section above, a 50mm lens on a cropped sensor also becomes a short telephoto which, coupled with the large aperture, is ideal for portraits. I've popped two of the most affordable and best-selling 50mm f1.8 options for Canon and Nikon bodies opposite.
So how do you measure aperture? Well the aperture itself is actually just the diameter of the opening in the lens which lets the light through; if a lens had just one piece of glass, the aperture would be its diameter. But in photography it's more useful to use the term focal ratio, or f-number instead. This is because the f-number describes the amount of light which finally reaches the sensor, so if two lenses have the same f-number, even if one is really wide and the other is really long, then they'll both deliver the same amount of light to the sensor and both use the same exposure.
To work out the f-number you simply divide the focal length of the lens by the effective aperture of the lens. So if the focal length were 50mm and the effective aperture were 25mm, then the f-number would be 50 divided by 25, or f2. If the focal length were doubled to 100mm, but the aperture kept the same at 25mm, then the f-number would become 100 divided by 25, or f4. It's easy to work backwards too. If you wanted a 100mm f2 lens, then it would need a larger aperture, measuring 100 divided by two, or 50mm. If you wanted a 200mm f2 lens, then it would need a 100mm aperture. If you wanted a 400mm f2 lens, it would need a 200mm aperture - imagine that, an aperture 8in in diameter compared to the 50mm f2 lens we started with employing a mere one inch aperture, just to achieve the same focal ratio and the same exposures.
Clearly as focal lengths get longer, they need correspondingly bigger apertures to deliver the same amount of light as lenses with shorter focal lengths. This makes them very large, heavy and expensive, so long focal length lenses generally split into two categories: expensive, large and heavy models for professionals and well-funded enthusiasts which maintain small f-numbers, and consumer models which employ more modest apertures - with bigger corresponding f-numbers - to become much smaller and more affordable. For an extreme example, look at the lenses used by pros at big sporting events - many of these share the same 300mm to 400mm focal lengths as a typical consumer telephoto zoom, but boast much larger apertures and smaller f -numbers. This lets them shoot in dimmer conditions, use faster shutter speeds to freeze action, employ lower ISO sensitivities for better quality and deliver images with a shallower depth of field. But they're paying handsomely for the privelege of doing so, not to mention lugging around something which costs the same as a small car and resembles a rocket launcher.
Even with their high prices and hefty construction though, large aperture lenses remain desirable to most enthusiasts, but if they're way out of reach financially don't forget you may be able to rent one for a short time to shoot an event. I couldn't possibly afford to buy a 500mm f4 lens to photograph the final Space Shuttle launches, but rented one for a few days at a low price which worked out really well. If you're in the US, check out
Borrow Lenses and if you're in the UK, check out
Hire A Camera - they're also great for trying out a new lens before buying.
So after all this talk, what sort of f-numbers do typical lenses have anyway? On fixed focal length 'prime' lenses, there’ll be one number – for example, 50mm f1.8. On zoom lenses, there’ll typically be two numbers, one for each end of the range – for example 18-55mm f3.5-5.6 – which means f3.5 at 18mm and f5.6 at 55mm. Note some premium zoom lenses have a fixed aperture throughout their range – for example 24-70mm f2.8 which is f2.8 regardless of the zoom setting.
F-numbers of 1.4, 2.8 and 4 may sound similar, but they actually represent a significant difference in light gathering power. For example, an f1.4 lens can gather twice as much light as an f2.0 model, or four times more than an f2.8 model. Similarly, an f2.8 lens can gather twice as much light as an f4 model, or four times more light than an f5.6 model.
A lens which gathers twice as much light lets you use a shutter speed that’s twice as quick, or the same shutter speed when it’s twice as dark. A lens which gathers four times more light lets you use a shutter speed that’s four times quicker, or the same shutter speed when it’s four times darker. Clearly lenses with big apertures, and therefore smaller f-numbers, are desirable when you’re taking photos in low light or of quick action. If you're willing to keep the shutter speed the same, a lens with a smaller f-number will alternatively allow you to use a lower ISO sensitivity value for better quality images. And as mentioned earlier, smaller f-numbers also allow you to achieve a shallower depth of field, which is ideal if you want to blur the background behind your subject.
To illustrate the difference in potential depth of field between a typical kit lens and a prime with a much smaller f-number, I took a portrait shot from the same distance with both at their smallest f-numbers; the zoom was set to the same focal length as the prime. You can see the result below, where it's clear the photo on the left at f1.4 has a much more blurred background than the one on the right at f4.6. In the next section you'll see more examples of a scene taken at different aperture settings.
Panasonic Leica DG Summilux 25mm at f1.4
Using Panasonic Lumix G3 on tripod
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Panasonic G VARIO 14-42mm at 25mm f4.6
Using Panasonic Lumix G3 on tripod (same distance)
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1/20, f1.4, 160 ISO
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1/1.6, f4.6, 160 ISO
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Again though, the price you pay for a larger aperture and smaller f-number is a bigger, heavier and more expensive lens, especially if it’s a zoom or a long telephoto. The exception to the rule are standard 50mm lenses (which thanks to the crop factor on most cameras act like a short telephoto of 75mm to 100mm).
These can be surprisingly affordable, and with most models offering f1.8 apertures, they’ll actually gather over eight times more light than a typical 18-55mm kit lens when it’s zoomed-in to the same focal length. Their small f-numbers also mean you can easily blur the background. That’s why standard 50mm lenses make a perfect introduction to low light and portrait photography.
Depth of field in more detail
Key points
1: Depth of field is the amount in front and behind the main subject that looks sharp.
2: Smaller f-numbers deliver a shallower depth of field with less that's sharp in front and behind the subject.
3: Bigger f-numbers deliver a larger depth of field with more that's sharp in front and behind the subject.
4: Wider angle lenses start off with a larger depth of field, so are ideal if you want lots to look sharp.
5: Longer lenses start off with a shallower depth of field, so are ideal if you only want the main subject looking sharp.
So far I've only spoken about using the smallest f-number on a lens for the maximum light gathering power, but most lenses also feature an iris which actually lets you reduce the size of the opening. Reducing the size of the iris obviously lets in less light, which means you can use it to adjust the exposure, but it also has the effect of increasing the depth of field, or the amount that looks sharp in front and behind the main subject. To control the f-number, simply set your camera's exposure mode to Aperture Priority or full Manual. In Aperture Priority you choose the f-number and the camera will try and choose a shutter speed to deliver a 'correct' exposure, so long as there's enough, or not too much light.
As you close the iris, the effective aperture is reduced which in turn means the f-number gets bigger. As you already know, the smallest f-number is defined by the size of the aperture, and depending on the lens could be somewhere between f1.8 and f4. Beyond this though, all lenses follow the same scale, typically offering a range of f5.6 to f16 or f22. So the simple rule is to choose small f-numbers if you want a shallow depth of field where only the main subject looks sharp, and to choose big f-numbers if you want a broader depth of field where more in front and behind the subject will look sharp.
The focal length of the lens also has an impact on the depth of field. Wide angle lenses have a large depth of field to start with which means it's easier to make lots look sharp, but harder to blur the background. Conversely longer focal lengths have a shallower depth of field to start with which means it's easier to achieve blurred backgrounds, but harder to make lots look sharp in front and behind the subject. The distance to the subject also plays its part on the depth of field: the closer you are to the subject, the less that will look sharp.
So if you're after a particularly shallow or broad depth of field, you can use the lens focal length and subject distance to help you. Choose wide angle lenses with larger f-numbers for a large depth of field, and go for telephoto lenses with the smallest f-number possible for a shallow depth of field with a blurred background. To further accentuate the shallow depth of field effect, move closer to your subject and ideally place the background as far away as possible. An extreme example would be macro photography where you may only be a few centimeters from the subject - at these distances it's easy to achieve a shallow depth of field even if the minimum f-number isn't that small. A quick word of warning though, if you're shooting at big f-numbers, remember they're letting in less light and will need longer exposures or higher ISO sensitivities to compensate, the former increasing the risk of camera shake or motion blur and the latter decreasing image quality. Always keep an eye on yuor shutter speed when you're adjusting the aperture.
To illustrate the effect of different f-numbers on the same subject I set up a still-life scene where the subject was about 1m away and the background ranged from several hundred meters to several Km. You can clearly see in the photos below how the depth of field increases as the f-number is increased. With the aperture wide open in the top left image, almost the entire background is blurred out of recognition - this is great if you want to isolate the subject and let it stand out, such as in a traditional portrait.
Subject at 1m photographed with different f-numbers.
Using Panasonic GX1 and Olympus 45mm f1.8 lens (90mm f3.6 equivalent on full-frame)
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f1.8, (equivalent to f3.6 depth of field on full-frame system)
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f4, (equivalent to f8 depth of field on full-frame system)
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f8, (equivalent to f16 depth of field on full-frame system)
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f16, (equivalent to f32 depth of field on full-frame system)
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If you read the captions in the example above, you may have noticed the full-frame reference creeping in again. This is because the effective depth of field is also affected by the sensor size. Luckily we can use the same crop factor as for coverage calculations, and once again everything is normally described in relation to a full-frame system. So to calculate the effective f-number in relation to full-frame, just multiply it by the crop factor.
So if you have a 50mm f2 lens and you mount it on a camera with an APS-C sensor, you'll need to multiply both the focal length and f-number by 1.5 times to calculate the effective coverage and the effective depth of field - so it would act like a 75mm f3 lens in terms of effective coverage and depth of field on a full-frame system. If you mounted a 50mm f2 lens on a Micro Four Thirds body, you'd multiply both figures by two to end up with a 100mm f4 lens in terms of effective coverage and depth of field on a full-frame body. Note the exposure is not affected by the crop factor, so an f2 lens will have the same exposure as another f2 lens regardless of the sensor behind it. But the effective coverage and the effective depth of field are.
This is important as many owners of cropped-frame cameras make the mistake of only applying the crop factor to the coverage and assuming the depth of field will just be the same as a full-frame system. For example a 25mm f1.4 lens on a Micro Four Thirds body may have coverage equivalent to 50mm on full-frame, and share the same exposures as a true 50mm f1.4 lens, but in terms of relative depth of field, it's not equivalent to a 50mm f1.4; instead it's equivalent to a 50mm f2.8 on a full-frame body. This is why it's harder to achieve a really blurred effect on systems with smaller sensors, although easier to achieve a larger depth of field. If you wanted to match the coverage and the effective depth of field as a 50mm f1.4 lens on the Micro Four Thirds system, you'd need a 25mm f0.7 lens, although this would have the benefit of faster exposures than a true 50mm f1.4 lens.
And now one final word of warning to those choosing small apertures and big f-numbers to deliver a large depth of field. Closing the iris too far can cause a detrimental optical effect known as diffraction where the image becomes softer overall. This is a catch 22 as for some photos you'll want a big f-number to achieve a large depth of field, but equally you don't want the image to be compromised in overall sharpness due to diffraction.
The trick is to find the right balance and only close the lens aperture by as much as you need to, rather than going overboard and risking too much diffraction. Again sensor size plays a role and you can normally avoid diffraction on full-frame systems at f-numbers of f11 or below. On APS-C systems you should try and shoot at f8 or smaller to minimise diffraction, and on Micro Four Thirds at f5.6 or smaller. Of course you can still shoot at bigger f-numbers if you like - indeed you may need to in order to achieve the desired depth of field - but beware the image may become a little softer as a result. Put it this way, I'd avoid f16 and f22 on smaller sensor formats.
Another gotcha involves dust marks on the sensor which become increasingly clear as the f-number becomes larger - another reason to avoid f16 and f22 unless you actually want to check how much dust has settled inside your camera. if you do want to check for dust, choose the largest f-number available on yuor lens and take a picture of a plain surface like a white wall or a completely blue sky. Any dust marks will show up as small dark fuzzy circles.
Focusing
Most modern lenses offer the choice of auto or manual focusing, although the rise of adapters for mirror-less cameras has seen a renaissance in manual focusing; companies like Samyang are also popularising manual focus lenses. But for the purposes of this section I'd like to discuss autofocus, as some lenses do it better than others.
Most modern autofocus lenses feature built-in motors to adjust the focus under command from the camera, although some older lenses may rely on a motor inside the camera body to do the work. The most common of these are older Nikkor lenses with AF as oppose to AF-S in their title. If you have an AF Nikkor lens, you'll need a body with a built-in motor for it to autofocus; if you mount it on a body without a built-in AF motor, it will become manual focus only. Nikon builds AF motors into its upper mid-range bodies and above, so there's no problem with the D7xxx or higher. But if you have an entry-level model like a D3xxx or D5xxx then it will not be able to autofocus with these older lenses.
This may sound like a big deal, but to be honest it's not. All of Nikon's recent lenses are AF-S models which feature built-in AF motors. All of those will autofocus on any Nikon body including the budget models. So the only thing you'll need to know is if you own an entry-level Nikon DSLR and want autofocus, just stick to the AF-S lenses.
Sticking with Nikon's terminology for just a moment, the S in an AF-S lens refers to it having a Silent Wave Motor for focusing, or SWM for short. Most SWM lenses will focus quickly and quietly.
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Moving onto Canon, all of its EF lenses have focus motors built-in, so there's no AF incompatibilities to worry about, but again some do it better than others. Canon lenses with USM in their title have special ultrasonic motors which are quicker and much quieter than non-USM models - it's roughly equivalent to Nikon's SWM system. More recently Canon has started offering lenses with Stepper Motors for AF, referenced by STM in their title. These are quieter for continuous AF and therefore more desirable when shooting movies, but it's only available on a handful of models and for still photos USM lenses are preferred overall. Staying with Canon's terminology for a moment, the company also offers a range of higher-end lenses denoted by the letter L for luxury. These all feature USM focusing along with build and optical quality that's superior to non-L models.
The equivalent technology for quick and quiet focusing from Sony, Pentax, Olympus and Sigma is called SSM, SDM, SWD and HSM respectively; note these names may only apply to their lenses designed for DSLRs as oppose to native mirror-less lenses from the same companies.
Speaking of mirror-less cameras most already incorporate quick and quiet focusing into their native lenses - by native I mean lenses designed for them. Note that most companies that sell both DSLRs and mirror-less cameras also offer adapters which let you use their DSLR lenses on their mirror-less bodies. Unlike third-party adapters for other company's lenses, these often boast the benefit of supporting autofocus. But beware, as DSLR lenses were designed to be focused by DSLR cameras, and when you mount them on a mirror-less body the autofocusing is normally much slower.
Finally it's worth mentioning internal focusing which as its name suggests takes place within the lens. This means the end section of the lens barrel doesn’t rotate while focusing, which is important for users of polarising filters.
Macro
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All lenses have a minimum focusing distance, below which they won't be able to deliver a sharp image. This can provide frustrating if you want to get really close to a small subject like a flower or an insect, but your lens just won't focus close enough. The answer is a macro lens which is designed to focus much closer than a normal lens, allowing you to fill the image with very small subjects.
Most dedicated macro lenses should be able to deliver life-size magnification, also known as 1:1, which means at the closest focusing distance the subject will be the same size on the sensor as it is in real life. If the subject is, say, 10mm long, then it'll be 10mm long on the sensor. Obviously how big this will be on your photo depends on the size of your sensor. If you have an APS-C sensor, it'll typically measure about
Beyond this, you'll find macro lenses available with different focal lengths, typically between 40 and 100mm. Now the important thing to remember is all of them should offer the same 1:1 magnification. The difference between them is how close you'll need to be to the subject to achieve this magnification. The longer focal macro lengths will achieve 1:1 magnification from slightly further away which might be preferrable if you can't - or don't want to - get too close to your subject; it can also minimise shadows cast by the actual lens itself. Shorter focal length macro lenses will achieve 1:1 magnification from slightly closer distances, which again may be preferrable depending on the subject and yuor choice of lighting.
Most macro lenses have short telephoto focal lengths which means they also double-up as good portrait lenses. Some may additionally feature optical stabilisation to help reduce the effect of camera shake, see below.
Anti-shake
Some lenses feature anti-shake facilities which allow you to typically handhold at shutter speeds three to four times slower than normal. This won’t stop a moving subject from blurring, but it can greatly reduce the effect of camera shake.
Lens-based anti-shake systems all work in the same way by detecting wobbles and adjusting a special optical element inside the lens to counteract them in real-time. The benefit of fitting it inside the lens is you’ll see the stabilising effect through an optical viewfinder, which can be very reassuring, especially at longer focal lengths.
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While stabilisation is most commonly employed on telephoto lenses, it can be equally useful on standard or even wide angle focal lengths. Regardless of the focal length, stabilisation will still let you handhold at shutter speeds three to four times slower than normal, so for wide angle, that gives you the chance to handhold some seriously slow exposures. Ideal if you want to
blur waterfalls and rivers. Stabilisation is also extremely useful when filming video handheld.
Stabilisation systems can however get confused in certain circumstances. If you’re panning the camera to follow the action (see my
blurring action workshop), the stabilisation could mistake it for a wobble and try and counteract the motion.
Some anti-shake lenses offer a panning mode which ignores horizontal motion and only stabilises vertically. Some of the latest models can even detect this motion and switch their mode accordingly. Older, or more basic anti-shake lenses won’t work with panning though and the feature should be temporarily switched off. Likewise if you’re using a tripod, you should switch the stabilisation off or the system could actually introduce wobbling.
Each manufacturer has a different name for anti-shake. Canon calls it Image Stabilisation or IS for short. Nikon calls it Vibration Reduction, or VR for short. Sony calls it Optical Steady Shot or OSS for short. Sigma calls it Optical Stabilisation, or OS for short. Tamron calls it Vibration Correction or VC for short. So if you want a lens with anti-shake, these are the letters you should be looking for in its name. Note that Sony and Olympus build stabilisation into their camera bodies (excluding Sony's NEX models) which means any lens you mount becomes stabilised, even if it's an old model from a different company mounted via an adapter.
