F-number on reversed lenses

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LordV
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Joined: Thu Nov 22, 2007 10:28 am
Location: UK

F-number on reversed lenses

Post by LordV »

If I reverse a lens does the F-number mean the same thing in terms of DOF and diffraction. I assume it does not necessarily mean the same thing if the entrance and exit pupil are different ?

This question came up as someone thought the DOF of a reversed canon kit lens at a set aperture was thinner than other normally mounted macro lenses at the same set aperture and magnification.

Brian V.
www.flickr.com/photos/lordv
canon20D,350D,40D,5Dmk2, sigma 105mm EX, Tamron 90mm, canon MPE-65

rjlittlefield
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Post by rjlittlefield »

For all lenses, the effective f-number means the same thing in terms of DOF and diffraction.

But lenses vary in how the effective f-number depends on the f-number that is set on the lens, especially when reversed and extended.

This does depend on the pupil ratio. See FAQ: What is "pupil ratio" and why would I care?

--Rik

LordV
Posts: 1571
Joined: Thu Nov 22, 2007 10:28 am
Location: UK

Post by LordV »

rjlittlefield wrote:For all lenses, the effective f-number means the same thing in terms of DOF and diffraction.

But lenses vary in how the effective f-number depends on the f-number that is set on the lens, especially when reversed and extended.

This does depend on the pupil ratio. See FAQ: What is "pupil ratio" and why would I care?

--Rik
Thanks for the answer Rik :)
Brian v.
www.flickr.com/photos/lordv
canon20D,350D,40D,5Dmk2, sigma 105mm EX, Tamron 90mm, canon MPE-65

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