There are many more reasons why a camera with different size sensors may be suitable for an individual photographer other than the influence on the apparent focal length of a lens but it is this which is often the deal breaker or decision maker and photographically, it is the area where people see the fundamental differences. As a result it is probably the key characteristic which requires the photographer’s understanding.
Most importantly it doesn’t physically increase the focal length of a lens. Optically a 300mm lens will always be a 300mm lens regardless of whether it is made for cameras with a cropped sensor or full frame.
What changes is the effective field of view seen by the camera sensor. An APS-C sensor is physically smaller so the field of view is smaller than that of a camera with a full frame sensor. It is much like looking at something through two different sized windows.
Take a look at the illustration below. The green and red represents the differences in the fields of view between a 35mm sensor and APS-C. Note, it isn’t exactly to scale, just an approximate illustration.
Lenses which are designed for cropped sensors are physically smaller than their 35mm equivalents because they require a more narrow channel for the light to pass through. You may still be able to use a lens designed for a APS-C camera on a full frame camera if you can set up the sensor to only use the smaller image area. If not you will get a very obvious lens vignette around the edges. However, some manufacturer’s dedicated APS-C lenses will not physically mount.
To calculate the effective field of view of APS-C cameras, multiply the focal length of the lens by the crop factor.
Fujifilm X mount cameras have a crop factor of 1.55x, this means my 100-400mm lens has an effective field of view of 155mm to 620mm. Nikon DLSR APS-C cameras have a crop factor of 1.5x. This means a 200mm lens has an effective field of view of a 300mm lens. Canon APS-C cameras have a crop factor of 1.6x and Olympus who use a ‘four thirds’ system have a crop of 2x.
There is no real answer! Best at what?
I favour the APS-C system as the lenses are significantly smaller and usually lighter. There are other effects on depth of field and high ISO quality, but that’s probably another future blog post!
Ultimately it comes down to individual use. There is a commonly held misconception that 35mm is for professionals and APS-C is for amateur photographers. It’s a ridiculous way to look at it, we use whichever works for us and this is based on budget, how it fits with existing gear and approach and style of photography.
This blog post was originally written in July 2015 and updated in May 2020.