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12-12-2012, 09:35 PM   #16
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QuoteOriginally posted by vonBaloney Quote
Zoom focal lengths are at infinity. At short distances they can be quite a bit less. But this can also be a blessing as the design allows for closer focusing of the lens at longer focal lengths. For instance, the DA 18-250 tops out at around a true focal length of 180mm on anything but distance subjects. On the other hand, it can focus on stuff a foot away at any zoom setting...
The 18-250 gradually loses range as focal length decreases. I compared mine to a non-IF 55-300mm, and set it for 250mm:
- at 1 mile distance, the 18-250 FOV matched the 55-300 at 230mm
- at 30 feet, it matched 200mm FOV
- at nine feet, it matched 160mm FOV on the 55-300.

12-12-2012, 10:45 PM   #17
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QuoteOriginally posted by audiobomber Quote
Nope, strictly a property of internal focus. Internal Focus prime lenses lose range at short subject distances too, just not as much as zooms. The A*300mm for example is not IF and does not do this, the DA*300 does because it's IF.
Interesting concept that makes sense, sort of. Do you have a reference?

If I understand you correctly, you are saying that IF lenses intrinsically lose focal length as a function of focus distance. I understand how that might be so. Might it also be true that some non-IF zooms might suffer from the same issue, but for different reasons? *

The story I had always heard (going back some 30 years or so) was that lens makers were pressed to provide close focus capabilities in their zooms and did so at the expense of lower magnification. This was back in the days before IF was common. That is consumers were cautioned to use the reproduction ratio, not the close focus distance is the true measure of close focus capability.


Steve

* The reason I ask is that my DA 18-55 kit (a non-IF zoom) increases FOV on close focus...

Last edited by stevebrot; 12-12-2012 at 11:27 PM.
12-12-2012, 10:56 PM   #18
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QuoteOriginally posted by audiobomber Quote
The 18-250 gradually loses range as focal length decreases.
Might you be meaning the opposite? Closer focus (shorter range) --> less than expected magnification.


Steve
12-13-2012, 12:00 AM   #19
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This isn't the first thread concerning this subject. Try here:

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/144003-lens-fo...h-rip-off.html

12-13-2012, 05:00 AM   #20
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This issue was first noted with Pentax lenses with the K28/2 which uses a fixed rear element, the forerunner of internal focusing. The simple fact is, stated focal length in all standards used for test is only at infinity. All other distances a not monitored.

Super zooms show the worst of this, and also internally focused macros.
12-13-2012, 06:01 AM   #21
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QuoteOriginally posted by stevebrot Quote
Might you be meaning the opposite? Closer focus (shorter range) --> less than expected magnification.
What I mean is that the 18-250mm is not a 250mm lens when you focus on a subject 3m away; it's a 160mm lens. The Sigma 150-500mm is a 320mm lens when fully extended and focussed at 3m.
12-13-2012, 11:11 AM   #22
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Given that no 75-300 lens I own and I own 3 actually gets me to 300mm sans distortion and a lot of softness I'd say no, that really a 300mm is more like 200 something. There's always a certain sweet range and beyond that it really not spot on. I've never yet seen a lens that was at it's best at the longest part of it's focal length. It's always a bit before which is why I've always wanted a 400mm prime. I figured I'd get more like 300mm plus a bit more with one. I don't actually expect it to be sharp at 400, in fact I'd be stunned if it was.
12-13-2012, 11:21 AM   #23
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QuoteOriginally posted by magkelly Quote
...which is why I've always wanted a 400mm prime. I figured I'd get more like 300mm plus a bit more with one. I don't actually expect it to be sharp at 400, in fact I'd be stunned if it was.
If it was a prime, it would always be at 400 and it would either be sharp or not, eh? (Not counting effects of stopping down.)

12-13-2012, 11:52 AM   #24
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QuoteOriginally posted by magkelly Quote
Given that no 75-300 lens I own and I own 3 actually gets me to 300mm sans distortion and a lot of softness I'd say no, that really a 300mm is more like 200 something. There's always a certain sweet range and beyond that it really not spot on.
We were discussing field of view. Sharpness is an entirely different thing.
12-13-2012, 01:11 PM   #25
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QuoteOriginally posted by magkelly Quote
Snip............... I've never yet seen a lens that was at it's best at the longest part of it's focal length. It's always a bit before which is why I've always wanted a 400mm prime.
Get a Sigma APO70-200EX F2.8 ( non DG non macro version) it is extremely sharp not just at 200mm but also wide open. Probably optically the best sigma. I use it all the time at 200mm with a 2X TC
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