Before the rush

Before the rush
by evan-pak

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In the specifications of Canon 550D, about the lens the following is mentioned

Lens Mount      EF/EF-S
Focal Length    Equivalent to 1.6x the focal length of the lens

Now this is really confusing. Why is the focal lenth 1.6 times the focal length of the lens?

I thought the focal length of the lens was the focal length of the lens!

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up vote 11 down vote accepted

This is just their ultra confusing way of dealing with a crop factor field of view. It just means that a lens mounted on the 550D will have a field of view equivalent to 1.6 times narrower than the field of view of the same lens on a full frame.

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Check for more information about crop factor on…. – rfusca May 16 '11 at 2:50
It's too bad that they word it so poorly. The US site, at…, says "35mm-equivalent focal length is approx. 1.6x the lens focal length", which is better (if still not great). – mattdm May 16 '11 at 2:55

@rfusca's answer is totally right. I just wanted to make this part completely clear. You say:

I thought the focal length of the lens was the focal length of the lens!

and in short, you are right and they are wrong, confused, and confusing. Canon's web page should be fixed.

The focal length of the lens is a physical property and can't be changed. The smaller-format sensor does reduce the field of view given by a lens of a focal length on a larger format — that's where "crop factor" comes in. See What is the difference between focal length and crop factor? for a bit more, and What is "angle of view" in photography? for a hands-on visual explanation of how this all works.

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So here's another way to look at it. A 50mm lens on a camera with this 1.6x situation looks like an 80mm lens on a camera without (like my old film SLR). So when you buy a Zoom that goes to 300mm it looks like 480mm on a traditional SLR. The CMOS is smaller on this "1.6" camera: .625 smaller than a 35mm size piece of film - which these lenses are typically sized for - a carry over from the old days.

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The markings on the lens are correct for a full-frame camera. So if you had a 5D, 5DMkII, or any of the 1x series cameras (or of course, a film SLR such as the EOS 3...) then a 50mm lens will indeed be a 50mm lens.

However with the advent of consumer digital SLR's, many of them have sensors that are smaller than the standard 35mm size. This means that it only 'sees' the central area of what the lens sees. Therefore is, in effect, magnified. This is where the magnification factor comes into play.

Hope that helps.

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