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04-18-2014, 10:17 AM   #1
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Is your 200mm really a 200mm

I just got a Vivitar 200mm f/3.5 M42 lens. I wanted to test and see how it performed against my other 200mm lenses (mostly zooms). So I setup my tripod and took a few shots of a melting paw print on my front porch at roughly 7' away.

Once in Aperture I noticed something right away... Some lenses at 200mm had the same FOV as my 135mm prime (and the DA* @ 135).

I always thought and was always told that even the new "for digital" lenses had their focal length measured as the 35mm equivalent even though the actual length will be 1.5x larger. A 200mm will be a 300mm on a crop sensor and a 50mm will be a 75mm on a crop sensor...

But after those test shots, my Sigma and Tamron zooms at 200mm both look like my 135mm Deitz (film era) and not my 200mm Vivitar... and 135mm x 1.5 = 202mm. Are these lenses focal length measured for APSC?

The DA* seems like it is correct if you compare to the film era prime (135mm)

I can understand seeing some slight variation between brands and lens series but look at the shots below...

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04-18-2014, 10:25 AM   #2
Zav
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The focal length is the same regardless of the size of the sensor. 200mm is 200mm. And the focal length is given at infinity, closer, it can vary a lot.
04-18-2014, 10:37 AM   #3
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Internal focus lenses do not magnify as much as external focus lenses. As the front element moves out on an external lens to bring the image in focus, magnification increases, just like you'd added an extension tube. On internal focus lenses the front element doesn't move and there is no additional magnification. Nut short at infinity with the from element drawn all the way back and 20mm will be the same on both types of lenses.

I've posted similar threads here
https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/242576-da-6...0-vs-a400.html
04-18-2014, 10:39 AM   #4
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Thread moved. Once again, the General Talk section is for non-photographic discussion only.

04-18-2014, 10:48 AM   #5
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QuoteOriginally posted by Zav Quote
And the focal length is given at infinity, closer, it can vary a lot.
Sounds about right. I just did a test run focussed at infinity and all the 200 are very similar.
04-18-2014, 11:48 AM   #6
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QuoteOriginally posted by Zav Quote
The focal length is the same regardless of the size of the sensor. 200mm is 200mm. And the focal length is given at infinity, closer, it can vary a lot.
And the winner is...Zav!

This is a hidden flaw for many internal focus zooms. Some get "shorter" (wider FOV) as they focus close. I wish FOV at various focus distances was part of the test suite used in lens reviews, but I can't see that I have ever seen that done.


Steve

Last edited by stevebrot; 04-18-2014 at 12:46 PM.
04-18-2014, 12:51 PM   #7
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QuoteOriginally posted by stevebrot Quote
And the winner is...Zav!

This is a hidden flaw for many internal focus zooms. Some get "shorter" (wider FOV) as they focus close. I wish FOV at various focus distances was part of the test suite used in lens reviews, but I can't see that I have ever seen that done.


Steve
I just pulled out my vintage Tamron 70-150/4, my new Sigma 17-70/2.8-4, and my Helios 44M 58/2 and compared all three at about 3 feet distance. The Sigma at 70mm had the same FOV as the 58mm Helios while the external focus Tamron at 70mm was significantly narrower.


Steve
04-18-2014, 01:00 PM   #8
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As already stated, focal length is only measured at infinity, and internal focusing lenses modify focal length as they focus, shortening the focal length, this was first noticed with the K28/2 which has a fixed rear element, the forerunner of internal focusing

04-18-2014, 01:10 PM   #9
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Folding cameras had front cell focusing back in the 1930s.
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