When to use APS-C lenses instead of full-frame
There’s no denying the image quality produced by full-frame lenses, but when to use APS-C lenses instead of full-frame? Turn your lens around and you’ll see that lenses are designed with an imaging circle around the portion of the optic that mounts into your camera body. This image circle collaborates with your sensor, and as such lenses are designed to be used with specific sensor sizes. A full-frame lens is roughly equivalent to a 35mm frame of film, while an APS-C sensor is a little bit smaller. When you mount a full-frame lens on a camera with an APS-C sensor you will get what is called a crop factor. This means your camera’s APS-C-size sensor magnifies the scene to produce an image that will match the lens’s full-frame image circle. The effect is that a 50mm full frame lens mounted on an APS-C body with a 1.5x crop factor will capture a field-of-view that is the same as a 75mm on a full frame body. For Canon, this crop factor is 1.6x. For Nikon, Sony, Pentax, it’s 1.5x. For Micro Four Thirds, it’s 2x. So as you’ve probably figured out, if you have a full-frame lens with an image circle bigger than your camera’s sensor, your camera will only record an image from the middle of that imaging circle. This means you are capturing less of your scene when you mount a full-frame lens on an APS-C-sensor camera. For portraiture this isn’t an issue, but if you are shooting sport or wildlife or landscape photography, even, you sometimes want to capture as much of the scene as possible. So the main benefit of using APS-C lenses and third-party equivalents instead of full-frame is that these ‘crop’ lenses offer wider coverage for those times when you need to capture as much of the scene as possible. Another thing to consider is that you can sometimes get more flare in your images when using a full-frame lens on an APS-C camera body. This is because an APS-C lens has a narrower field of view, so if you’re shooting close to the sun the light has no physical path through the lens. This really only happens though when you’re shooting in very bright conditions or right next to a main light source. A word of warning: as reader Rob Bridden rightly pointed out on Facebook, attaching Canon EF-S lenses to full-frame cameras can cause damage to the mirror mechanism of your camera. This is due to the extra long baffle on the mount. This can be an expensive repair job, so it’s best avoided with EF-S lenses.