Crop factor question

[Photog82]

I thought I'd ask this here since a lot of the members are friendly and knowledgeable. I thought I understood the crop factor of my Canon 40D but I guess I don't as I got my hand slapped in a flickr discussion.

I have a Canon 28-135 lens and a Canon EF-S 10-22 lens among others. From what I understand, on a non EF-S lens, at 28mm, the camera crops in 1.6 cutting off the outer edges. I would always use this math: 28x1.6=44.8. I always though that the real world focal distance was 44.8.

Am I mistaken?

If I shoot a Canon EF-S 17-55 f/2.8 @ 55mm, the actual focal distance is 55, but if I shoot a Canon EF 24-105 f/4L @ 55, the actual focal distance is 88?

[HDNitehawk]

The crop factor would be a full frame body vs a crop camera comparison and has nothing to do with the lens. A 55mm lens is a 55mm lens regardless if it is EF-S or not.

If you do not own or use a full frame camera, it really is useless information for the most part.

Your 28x1.6 would be equal to the field of view of a 44.8mm on a full frame camera, not counting for small variations from lens to lens.

[Sean Setters]

You would apply the same focal multiplier to an EF or an EF-S lens. So a 17-55 f/2.8 IS EF-S lens would equate to a 27-88mm lens on a full frame camera.

[neuroanatomist]

Focal length is an intrinsic property of the lens, it has nothing to do with the size of the image circle or of the sensor behind the lens. I think you may be under the mistaken impression that for EF-S lenses, Canon somehow alters the focal length numbers to compensate for the fact that they are used on a crop sensor - that is not the case. 55mm on an EF lens is the same as 55mm on an EF-S lens - both would give identical framing on an APS-C camera, and both would give a wider angle of view, but identical framing on a FF camera (if the EF-S Lens could be mounted on one, that is).

[Always Looking]

I am just learing photography myself. I am not much of a fan of Wikipedia, but the illistration on the right side of this page makes sense to me. Hope it's ok to add links on this site.

http://en.wikipedia.org/wiki/Crop_factor

[Photog82]

I think I understand now, my thinking was correct, I just wasn't spelling it out correctly verbally. I read the wiki article as well as the article on this site and believe I better understand it. Thanks everyone.