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Thursday, April 21, 2011

Image Magnification Change Subject to Object Distance

Occasionally, I saw people complained about the change of image magnification when they shot objects at different distances, no matter if the lens in use is a prime or a zoom. Recently, I have come across this quick experiment come with some illustrations, which I found to be clear and useful:-

http://forum.xitek.com/showthread.php?threadid=614523
(Google translation to English Here)

The test was done with a Nikkor 18-70DX zoom lens.

Look at those posted comparison pictures, they are self-explanatory and we should know.. (EXIF preserved.)


(at 70mm, focused to Infinity)


(at 70mm, focused to Close Distance)


(at 50mm, focused to Infinity)

In fact, the effective focal length and image magnification of the "70mm" at close distance is about the same as that of the "50mm" at infinity! :-o Interesting enough? :-)
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Zebooka's avatar

Zebooka · 728 weeks ago

That's because they don't know about what is focal length and other features as floating lens groups etc.
3 replies · active 728 weeks ago
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RiceHigh's avatar - Go to profile

RiceHigh 110p · 728 weeks ago

"They" mean Nikon? :-o ;->
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Zebooka's avatar

Zebooka · 728 weeks ago

No, "they" means users, that don't understand why FOV is increasing in such situation.

That's because of focal lens decreasing in such zoom lenses while focusing closer (because of floating rear group).

For classical fixed lens without floating groups and focusing by moving whole lens assembly FOV is decreasing while focusing closer, because rear focal length is increasing.

:)
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RiceHigh's avatar - Go to profile

RiceHigh 110p · 727 weeks ago

I know that the second phenomenon that you describe is correct. But for the first one, could you elaborate a bit more? Thanks.
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Mistral75's avatar

Mistral75 · 728 weeks ago

This phenomenon is called "focus breathing" and is quite common with lenses, in particular internal focussing lenses.

Only cinema lenses are completely exempt of it and that explain part of the price difference between photo lenses and cinema lenses.
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Denis's avatar

Denis · 728 weeks ago

The phenomenon is indeed common. Try Pentax DA* 60-250 and you will see the same "focus breathing". Its effective focus length at ~3 m is about 170mm, as far as I remember (I tested it few months ago). It is not always related to price. A cheaper Tamron 70-200 f/2.8 is almost free from this defect.

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