Lens conundrum


Froggy

Link Posted 10/03/2015 - 18:55
Whilst comparing images from my 50-135mm and18-135 lenses both set at 135mm I noticed that the field of view in each case varied enormously. can anyone give me an explanation or is there something wrong somewhere? Both were taken with the same K5.

johnriley

Link Posted 10/03/2015 - 19:05
The nominal focal length might be true at infinity, but may alter on focusing closer. This is a property of many zoom lenses. So, maybe both lenses change their focal lengths in different ways.
Best regards, John

McGregNi

Link Posted 10/03/2015 - 19:09
Thats interesting .... I do recall noticing occasionally, and wondering what was going on, that when focusing closely with the F35-70, that as I turned the focus ring I saw the edges of the frame move in and out slightly, and I thought I was somehow turning the zoom ring by accident.
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Last Edited by McGregNi on 10/03/2015 - 19:10

spinno

Link Posted 10/03/2015 - 19:23
aren't zoom lenses varifocal lenses?
David

petrochemist

Link Posted 10/03/2015 - 19:30
spinno wrote:
aren't zoom lenses varifocal lenses?

They can be, but can be parfocal too.
Varifocal lenses need refocusing when zooming, parfocal lenses keep the same focus at different focal lengths.
Mike
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Pentax:K5ii, K7, K100D, DA18-55, DA10-17, DA55-300, DA50-200, F100-300, F50, DA35 AL, 4* M50, 2* M135, Helicoid extension, Tak 300 f4 (& 6 film bodies)
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johnriley

Link Posted 10/03/2015 - 19:34
Even a prime macro lens may change focal length when focusing close, especially internal focus designs.
Best regards, John

Froggy

Link Posted 11/03/2015 - 13:01
Thanks for your comments everyone. You are spot on John. I tried the comparison again with both lenses focused at infinity and there was little difference which could be due to slight movement when the lenses were changed. It was nowhere the difference when close focused.
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