When focal length cannot be detected... ?

Started Feb 11, 2014 | Questions thread
paulkienitz Veteran Member • Posts: 5,281
Re: When focal length cannot be detected... ?

Alex Sarbu wrote:

Symple wrote:

My new question(s) for the forum is regarding setting the focal length of lenses when specifically being used for macro and close-up work. The camera system is new to us, and we have chosen two lenses to use with a K-7 body, namely a Rokinon P 105mm macro, and a Tamron SP 35-80 zoom.

The lens focal length setting is confusing for me since my wife will be shooting somewhere in the 1:5 - 1:1 reproduction range with the Rokinon, and judging by its length it seems to be more like a 150-180mm when it is racked out for close-up work instead of its specified focal length for infinity focus.

There is no reason why that lens would increase its focal length when close focusing. There are lenses that are doing the opposite - the inner focus zooms - but obviously that's not the case.

Did you measure this focal length increase, and if so could you describe how?


A lens's "focal length" is really defined only for infinity focus. When focusing between two points a finite distance apart, any shortening of the distance on one side implies a lengthening on the other. I think it's something like, the reciprocal of the distance in front plus the reciprocal of the distance in back equals the reciprocal of the nominal focal length. So if you're using a macro at 1:1, the real distance from the lens's optical center to the image plane is like double the infinity focus length. And as far as SR is concerned, that needs to be taken into account.  But as others have said, SR isn't going to be very effective with macros anyway.  If it was a five axis system, it could do better, but ours is three axis.  (Which is still one more axis than the big brands offer.)

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