Confused about lenses on small sensor SLRs

[devin]

OK, so I am a little confused about the whole relationship between lenses and "small sensor", specifically 1.6 crop SLRs. For example, I have a 50mm EF lens. When this lens is attached to my Rebel XSi, it's not the same as if a 50mm EF lens was attached to a full frame camera, right?

So, if I get the 10-22mm EF-S lens, will this lens perform exactly like a 10-22mm EF lens would perform on a full frame camera? Also, Tamron markets a lot of their lenses (DiII I believe) for 1.6 crop cameras. So If I buy, for example, the Tamron 17-50mm f2.8, will this lens actually work just like a 17-50mm would on a full frame, or do I need to "adjust" the numbers to get my actual focal length?

The way I understand it, you lose out on some wide angle with a 1.6 crop SLR, right?

I hope somebody can help! Thanks guys! :)

[kgosden]

Yes, you get to do the math for your particular camera. This is usually true even for lenses that have no full frame camera to attach to such as the 'crop only' lenses or those for my own 4/3rds series cameras. Part of the reason for this is that companies like Tamron make the same lens for 1.5, 1.6 and full frame cameras and they want you to know that these are the same basic lens.

[Prospero]

Part of the reason for this is that companies like Tamron make the same lens for 1.5, 1.6 and full frame cameras and they want you to know that these are the same basic lens.

This has nothing to do with it.
The only reason why this is the case is that the focal length of a lens is fixed. The focal length of a lens does not depend on the camera the lens is mounted on. A 50mm lens on a camera with an APS-c sensor has the same focal length as a 50mm on a Full Frame Camera.

The thing which does change for different sensor sizes is the field of view / angle of view a lens will give you. A 50mm lens on a full frame camera has a much larger field of view than a 50 on an APS-c camera.

Using the crop factor you can calculate which focal length you would need on a full frame camera to get the same field of view as on the camera you are using.
So, if you are using a 50mm lens on a APS-c camera, you would need a 75mm lens on a full frame camera to get the same field of view.
This goes for all lenses, even lenses that can only be used on crop cameras.

[David Metsky]

So, bottom line, you always have to multiply it out when you have a crop sensor SLR. All lenses use the true focal lengths, which are independent of what camera body they are designed for.

[devin]

Aha. Thanks David. That's what I was wondering. That must be why they make the 10-22 for the EF-S platform. A fairly common wide angle for a full frame camera would be, say a 17mm. 17mm/1.6=10.625. So you need approximately a 10mm on the APS-c to get the same effect as a 17mm on a full frame camera. Good to know.

[DonSchap]

Here's a pictorial representation of this discussion. Perhaps, it may offer a little more thoroughness to the discussion.

43844

Same lens w/FF & APS-C

[devin]

Wow, that is really quite a difference there!

[David Metsky]

Wow, that is really quite a difference there!
It's roughly 1.6x, but you knew that. :)

[devin]

It's roughly 1.6x, but you knew that. :)

Indeed :cool: