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by evan-pak

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Not that it's very important but I'm wondering why doesn't a Canon 18-200 mm lens have visible change in framing between 170 and 200 mm? Why is it declared as a 200 mm lens if it stops zooming at 170-something millimeters?

Or is it something subtle that I'm missing? 30 mm on a lens is usually not negligible so I'd also like to know is it the same throughout the focal range or is it less noticeable at telephoto end?

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up vote 1 down vote accepted

The focal-length of lenses is measured while focused at infinity. If you were inside a store you likely focused at less than infinity and got a different focal-length than 200mm at the long end. The closer you focus, the more likely you are to be off.

At infinity, there should be a small (probably 7% as @mattdm computed) but noticeable difference. Otherwise the lens was most likely defective.

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You are absolutely right, the fact that I wasn't using it outside was the problem. When I focus to infinity, there definitely is a noticeable difference but when inside (thus focusing too close), the difference isn't noticeable right away but there is some change, which I confirmed with Photoshop (it's around 3 to 4%). But I still don't understand why it seemed to be "zoomed more" with a 70-200 mm lens, I'll have to check that again. Well, thanks for your help, I'm used to prime lenses so I got confused when using a lens without the need to move physically. :D – user7264 Jan 21 '12 at 10:07

Are you really seeing no difference, or is it just a slight difference? Take a look at this chart from this tangentally-related answer (thanks to Nick Bedford):

fov chart

It's dramatic at the beginning, but flattens out at higher focal lengths.

The basic relationship between dimensional field of view and focal length is linear. That means 30mm is a huge difference between, say, 15mm and 45mm, because the linear view is cut to a third. But when you go from 170 to 200, that's just a change of ¹⁷⁰⁄₂₀₀, or 85%. That means 7.5% off each edge, and that's not very much at all.

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Very slight change near 170 and after that I don't see any change or am not observant enough. Perhaps I'll use difference blending mode in Photoshop to compare the photos. Regarding the graph - yeah, basic physics, I guess... What a dope... But how would you then explain the fact that a 70-200 mm lens has a lot smaller (meaning, more zoomed) framed area when used at 200 mm than my lens at 200 mm. I tried the 70-200 mm lens at a camera store just to see the difference between them, though, but it is still quite observable. Why wasn't this lens just named 18-170 mm? – user7264 Jan 20 '12 at 19:42
@rlesko: That's pretty surprising! Are you sure? Take your lens into the camera store, try them both with your camera. It doesn't make any sense to me, either, that they'd be different. – khedron Jan 20 '12 at 19:47
Well, I did try it. I'll go tomorrow again to check it with a tripod. BTW, I don't have a full frame camera but I don't think that matters, 200 mm focal length should be the same on any lens if sensor size doesn't change. – user7264 Jan 20 '12 at 19:51
It is possible that your lens is defective. It's also often the case that the labelling of focal length involves some rounding or even outright exaggeration, but you should see some difference. – mattdm Jan 20 '12 at 19:55

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