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05-09-2011, 07:39 PM   #1
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When 200mm isnt, really...

So... I have a 50-200mm dal kit lens, and I got my hands on a 18-200mm promaster, which is a rebadge of the tamron lens of the same specs.
Thing is, the view through the 200mm end of the kit lens is a good deal closer than the 200mm end of the 18-200mm... see below:

as opposed to:

and, just for kicks, here's both of them at 50mm (or 48mm on the promaster - couldnt quite get it to 50mm...)


Is this a result of some immutable law of optics im unaware of?

I look forward to your wise and learned responses.


05-09-2011, 08:39 PM   #2
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I believe focal length is measured when focused to infinity. As you focus closer, the FL changes, at least on some lenses, and that change might be different from lens to lens.
05-09-2011, 08:50 PM   #3
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I saw a post about the Nikon 18-200 fully extended being only 135mm at close focus distances, so it's a common thing. Supposedly internally focusing lenses are more prone to this.
05-10-2011, 01:00 AM   #4
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It is far from unusal that FL decreases at close focusing distances. This depends upon the specific lens designs (e.g.: Internal Focus and/or Fixed Rear Elemements) and thus, may even happen with certain primes. See for example Dojidar Dimitrov's Pentax K-Mount Page:

Focal Length Experiments

As another example I have measured the actual FL of my Tamron SP 35-80 mm CF Macro lens (Model 01A) at closest focusing distance and at the 80 mm macro setting. I find the actual FL to be 55 rather than 80 mm - see "Example 1" at the bottom of this page:

Last edited by Stone G.; 05-10-2011 at 01:07 AM.
05-10-2011, 02:59 AM   #5
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i guess, you try both of your lense, with your Dslr on a tripod.

it's a bad idea

to measure well that kind of thing, it is not the Dslr that need to not move, it's the part of the lense that is the closest to the subject.

i guess the 2nd lense come out much more than the first one.
05-10-2011, 06:11 AM   #6
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The previous posts regarding focal length are correct. focal length is only measured at infinity focus.

The posts are also correct that depending upon design, specifically internally focused lenses and fixed rear element lens designs, the focal length can change considerably when focused close.

This is especially true of super range zooms, i.e. lenses with more than 5-10 x focal length ranges.

It is also true of many zooms which can do macro, as you will find that the maximum magnification ratio is only at minimum focal length (somewhat counter intuitively)

I started a post for people to report the true focal lengths of Internally focused lenses at minimum focus, but it sort of died.
05-10-2011, 04:12 PM   #7
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thanks, all. i guess i just wanted to make sure that there wasnt something wrong with the lens.
as regards the note about internal focus making a difference, that explains why a different lens i tried, a 28-210mm, also seems much closer at 210mm than the kit lens is at 200mm.


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