EF-S lens focal length [duplicate]

Question

Possible Duplicate:
Does my crop sensor camera actually turn my lenses into a longer focal length?

If I have an EF-S lens with a focal length of 10mm on an APS-C camera (i.e. with sensor crop factor of 1.6x) does it mean that the focal length becomes 16mm? Or since an EF-S lens can ONLY (without any tweaking of lens/camera) be attached to an APS-C Camera, is the focal length really still 10mm?

If 10mm in lens = 16mm in Camera, why specify a 35mm equivalent on the lens since it can only be used on APS-C Cameras?

Answer 1

If the lens effective focal length is 10mm, it's 10mm, no matter what. On APS-C sensor it covers area similar to what 16mm lens on full frame sensor would cover, but the lens is still 10mm on focal length, and thus any lens with focal length of 10mm would cover the same area on same sensor, as long as the lens is wide enough.

In other words: EF lens has larger glass area because it needs to work on larger sensors. But on APS-C sensor camera, it does not matter if the lens is EF or EF-S - the focal length is what matters.

EF lens on EF-S camera just has glass area that is not used, but the focal length is physical measure of the lens, it does not change by installing.

The reason why the 35mm equivalent length is specified is to give common reference point on how large area is covered. Because people are more familiar with focal lengths on 35mm sensor/film than angles in 3D space.

Answer 2

The focal length is always the same (so in your example, the 10mm lens always has a focal length of 10mm). But on an APS-C camera the sensor, whilst the same distance from the lens as in a full-frame camea, is 1.6 times smaller. Hence the resulting photo is effectively a crop of just the middle part of the picture.

The 35mm equivalent lengths are just given as a convenient reference point for people who are familiar with full-frame 35mm cameras (such as film SLRs). It helps someone moving from a full-frame SLR to a cropped-sensor DSLR to figure out which lenses will give the same effective focal length they're already used to.

Answer 3

There are some very good points already covered in the answers explaining the focal length and why in some situations, especially for the cameras with very small sensors, the 35mm focal length equivalent is provided. I just want to add one point to this part of your question:

If 10mm in lens = 16mm in Camera, why specify a 35mm equivalent on the lens since it can only be used on APS-C Cameras?

You are right that EF-S lens cannot be used on a full frame camera, but you need to remember that full frame FE lenses can be used on a crop sensor body.

If the number provided on the ef-s lenses was referencing 35mm equivalent focal length it would cause even more confusion. For example the 50mm ef-s lens would cover different focal length that 50mm ef.