Focal length vs ‘effective’ focal length

Lenses are always identified by their focal length. It’s not just a physical measurement, it’s an indication of the type of lens it is, for example, a wideangle lens, standard lens, telephoto and so on. In other words, the focal length of the lens gives you an indication of its angle of view.

Or at least it should. However, digital cameras use sensors of a number of different sizes, and a lens of a specific focal length will capture a different angle of view on all of them.

The idea of using the lens focal length as an indication of the angle of view goes back to the days of film, when 35mm cameras were by far the most common format amongst enthusiasts and experts, and applies to today’s full frame cameras (the exact 35mm equivalent). Users quickly understood that a ‘standard’ lens was around 50mm, a wideangle lens was 28mm or less and a telephoto lens had a focal length of 70mm or more.

This doesn’t work with the many sensor sizes in use today that are smaller than full frame. A 50mm lens might give a ‘standard’ angle of view on a full frame camera, but an APS-C sensor will capture a smaller part of that 50mm lens’s image. It will ‘crop’ the lens’s angle of view, hence why smaller sensors are often called ‘crop sensors’.

This is not a problem in itself, but it does mean that smaller sensors give a smaller angle of view for the same focal length lens, which makes that lens’s focal length effectively longer.

When you work out the maths, a 50mm lens on an APS-C camera actually gives the angle of view of a 75mm lens on a full frame camera. This is why camera makers and photographers will say that a 50mm lens has an effective focal length of 75mm on an APS-C camera.

You can work out effective focal lengths very easily using the camera sensor’s ‘crop factor’. For APS-C sensors, the crop factor is approximately 1.5x (for Canon APS-C cameras it’s more like 1.6x, but let’s not worry about the difference here). For the smaller sensors in Micro Four Thirds cameras, the crop factor is 2x.

Effective focal length table

Here’s a table that might make this clearer. There are three columns for three different sensor sizes. Each row corresponds to a specific angle of view, and columns show the focal length that gives that angle of view for three different sensor sizes.

You can use it another way. If you see a Micro Four Thirds lens with a zoom range of 12-40mm, for example, you can look along the row and see that this corresponds to a 24-80mm lens in full frame or 35mm camera terms – the lens has an ‘effective’ focal range of 24-80mm, even though it’s actually a 12-40mm lens.

Many APS-C cameras come with an 18-55mm ‘kit lens’. If you look up these focal lengths in the APS-C column then check across to the full frame column, you’ll see this is equivalent to a 28-82mm lens in full frame/35mm camera terms. The nearest equivalent kit lens for a Micro Four Thirds camera is a 14-42mm lens.

More experienced photographers will do these mental calculations quickly in their heads, but most will still carry out this full frame ‘effective focal length’ conversion, even when they’re using smaller format cameras.