What is a macro lens? Magnification and minimum focus distance explained

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In our latest photography cheat sheet we answer the question ‘What is a macro lens?’. In our infographic we explain some of the finer points of using macro lenses and illustrate the effects different magnifications can produce with your subjects.

The official definition of a macro lens is that it should be able to reproduce a life-sized image of an object on the recording medium – in this case the image sensor.

True macro lenses offer a magnification factor of 1.0x or 1:1 at its closest focus setting. Fit one of these lenses to a DSLR like the Canon EOS 60D, and a standard UK postage stamp will fill the whole frame.

That might not sound particularly impressive, but when you consider that the 18Mp sensor in cameras like enable very large format prints, the potential for creating massive enlargements from shots of tiny objects is really quite astonishing.

What’s more, you can get still get very impressive enlargements of small objects using a macro lens that only offers 0.5x (or 1:2) maximum magnification.

In our latest photography cheat sheet we’ve illustrated the effects produced by macro lenses at different levels of magnification and how your subject will appear.

Simply click on the infographic to see the larger version of this cheat sheet, or drag and drop it to your desktop.

‘Flat field’ lenses

One feature that’s shared by all macro lenses is that they’re ‘flat field’ lenses. General-purpose lenses typically suffer from field curvature, so the point of focus will be at a slightly different distance towards the corners of the frame compared to the centre.

For example, the Canon EF 24-70mm f/2.8L-series zoom lens is quite notorious for this. In macro lenses, field curvature must be minimised as much as possible, so if you photograph a small flat object square on, such as the postage stamp we mentioned, the centre and corners of the stamp should all be sharp and in focus.

What is a macro lens’ minimum focus distance?

The minimum focus distance of macro lenses with the same magnification factor of 1.0x increases with longer focal lengths.

For example, it’s around 20cm for a 60mm lens, and as much as 48cm for a 180mm lens. These distances aren’t measured between the object being photographed and the front of the lens, but rather between the object and the focal plane; this is marked on the camera body towards its rear, in line with the image sensor.

Some macro lenses also have internal focusing, which can be a bonus as the front element doesn’t extend when you shorten the focus distance.

In other lenses, the overall length can almost double at the shortest focus setting.

For macro lenses that have shorter focal lengths, this movement can put the front element of the lens uncomfortably close to the object that’s being photographed.

In our chart below we’ve noted some of the typical focal lengths of different types of macro lenses and suggested the best distance from your subject at which to use them.

Coping with shallow depth of field

When you’re shooting at the minimum focus distance, depth of field is extremely small. For example, with a 100mm lens on an APS-C camera, it’s just 0.6mm at an aperture of f/2.8, so only areas that are within 0.3mm in front of or behind the focus point will be rendered sharply. Even at f/11, the depth of field is only 2.6mm.

Shallow depths of field aren’t too much of a problem when shooting flat two-dimensional objects, but things get tricky in 3D.

Because focusing is so critical, you’re usually best off switching to manual focus, so you can focus on exactly the part of the object you want to be sharp.

Fixing the camera in place and using Live View is helpful, as you can select a magnified view on the camera’s LCD for high-precision focusing.