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03-12-2011, 07:33 PM   #1
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Is this the correct difference between 135mm and 200mm focal lengh?

I just got a superzoom. I never have telephoto before. So can not make comparsion. I just want make sure if my lens is fine. I just feel the difference is so small. Thanks!


135mm



200mm



Last edited by liukaitc; 03-12-2011 at 07:42 PM.
03-12-2011, 07:56 PM   #2
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As you focus closer, the focal length of many zoom lenses decreases. In your example your zoom most likely does not extend to 200mm because you're very close to the subject. Repeat the test outdoors on a distant subject.
03-12-2011, 08:03 PM   #3
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Super-zooms often use a design that only reaches its rated focal length at infinity focus. That may be what's happening here. Since you're inside, the lens is probably closer to its minimum focus distance. Go outside tomorrow and test it there.

In real-world use, no one usually notices the difference. It can be a little surprising to see the focal length used data in EXIF; the number is not adjusted automatically for distance.
03-12-2011, 09:16 PM   #4
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Yeah, as I move between 135 to 200. The date all shows as 200mm.....
Anyway my lens is 28-200. I find below 135mm, the difference is big between every focal scale on the lens. but from 135 to 200. the difference is so small.
Maybe this is the nature? I do not know much about this aspect of knowledge.

03-12-2011, 09:57 PM   #5
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QuoteOriginally posted by liukaitc Quote
Yeah, as I move between 135 to 200. The date all shows as 200mm.....
Anyway my lens is 28-200. I find below 135mm, the difference is big between every focal scale on the lens. but from 135 to 200. the difference is so small.
Maybe this is the nature? I do not know much about this aspect of knowledge.
The wider you get, the more significant a difference any mm makes. Conversely, the more tele you get, the less remarkable the difference between one focal length and another.
03-16-2011, 06:08 AM   #6
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It really makes sense when you think of it this way, the difference between 18mm and 36mm is 18mm's and double the original focal length, so roughly double the amount of "zoom". The distance between 36mm and 54mm is still 18mm, but is only 50% more than the original focal length. To get double the amount of zoom from a 36mm lens you would need a 72mm lens. That explains why the "zoom" is a lot more pronounced at the wider end of the lens
03-16-2011, 10:32 AM   #7
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QuoteOriginally posted by liukaitc Quote
I just got a superzoom. I never have telephoto before. So can not make comparsion. I just want make sure if my lens is fine. I just feel the difference is so small. Thanks!
-------------
The apparent size of a feature on equal sized prints should be proportional to the focal length. Size.one/size.two = focal.length.one/focal.length.two


Actually it is fairly close to what it should be. I measured the apparent width on the wall switch plate in the two photos: 8.19mm & 10.38mm. The ratio of these two sloppy measurements is within 17% of the ratio of the lens' nominal focal lengths (as it should be).

Last edited by newarts; 03-16-2011 at 11:07 AM.
03-16-2011, 10:51 AM   #8
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Thanks, everyone!

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