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FORUMS Photography Talk by Genre Macro Talk
Thread started 27 Aug 2015 (Thursday) 01:47
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DOF effects of add-on diopters

 
Chris-R
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4 posts
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Aug 27, 2015 16:04 |  #16

Dalantech wrote in post #17684737 (external link)
So I shouldn't be taking images like this one (external link) :)

There's so much more to a good image than getting every little pixel razor sharp, and diffraction is just one aspect of many that can rob you of detail. Would I shoot a spring tail at F16? Nope, but then again I wouldn't shoot one at all cause I think that they're boring ;)

Quite - that's obviously a hopeless shot!  :p

I'm guessing we're ALL grizzly enough to have done it a lot with film cameras, probably much the same way I started, with a reversed standard lens on tubes. NO options whatsoever, you just had to stop right down. We've all got boxes full of Kodachromes ...


Things move on though, I notice there are more cameras which are nearly capable of producing a stack of say "4K" images in a very short period for slow-mo movies, which may be just about short enough to deal with the movement problem. The potential for live stacking, and therefore bigger prints revealng more detail, is there. Even now, clever people using cold weather or "unfortunate" (for the animal) methods for producing very sharp stacks are making the artistic but rather blurred pictures look second rate. If we can get the old "more to it" aspects AND decent sharpness, then good.


Archibald :

According to the MP-E65 manual, a marked aperture of 7.1 is effectively f/21.3 at 2x. At 2.3x, it is f/23.5 [=a*(1+m)]

Well good, they've got it right then :). It's just the formula I quoted above.
Canon is imho less confusing than Nikon. If you have a micro Nikkor at around say 1:0.7 and a short tube, you have no readout of any meaningful aperture at all. Unless it's an old micro Nikkor, then you have, unless it's a very old micro Nikkor 55 f/3.5... !!




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Archibald
You must be quackers!
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Joined May 2008
Calgary
Post has been edited over 2 years ago by Archibald.
Aug 27, 2015 17:26 |  #17

LordV wrote in post #17685135 (external link)
exactly correct- normal macro lenses or lenses with ext tubes obey the

Apparent aperture = set aperture * (1+M) where M is the magnification.

Gaining magnification from the use of diopters or reversed lenses does not follow this rule which was something I didn't know before seeing the post on dpreview.

I'm a dutiful follower of laws. Therefore I feel a need to obey the effective aperture formula.

As Chris-R said, it's the effective aperture of a lens that matters. And the effective aperture depends on the entrance pupil, not the physical diameter of the iris. The entrance pupil is affected by the optics in front of it. When a diopter is placed on a lens, the effective aperture gets bigger, because diopters are converging lenses. When I do this with a Canon 500D closeup lens on my 55-250mm lens (at 250mm), the entrance pupil swells to about double size. Hence a marked f/8 effectively becomes approx f/4, when focused on infinity.

So I'm thinking the a(1+m) formula works with diopter combos too, as long as we start with the correct value for a, which would be the marked aperture after taking the diopter into account.

The same must apply to reversed lenses and whatever combo one could concoct, but it becomes difficult to estimate the effective (infinity-focus) aperture. If one really wants, this could be determined by shining light through the lens from the focal point and allowing it to be intercepted at the front of the lens by tissue paper.

I'm still not fully satisfied with this explanation because there is this nagging voice in my mind saying DOF depends on the exit pupil instead of the entrance pupil, but I'm going to ignore that for now.


Pentax Spotmatic F with 28/3.5, 50/1.4, 50/1.8, 135/3.5; Canon digital gear
C&C always welcome.
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Lester ­ Wareham
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Joined Jul 2005
Hampshire, UK
Aug 30, 2015 03:40 |  #18

Yes my understanding is the dof difference is just down to the difference in effective aperture.

I used to often use a diopter lens for close up rather than tubes in the film days because of the lack of light loss.

I must say I quite like the Nikon system of setting the effective aperture rather than the physical aperture as in the Canon system, it would make it much easier to use manual exposure in macro work.

It would be handy if Canon let you set a CF to chose the setting mode, it is just arithmetic after all. However I suppose there may be an IPR reason Canon can't do this (ie Nikon hold a patent on the method or control interface aspect).


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Dalantech
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Joined Jul 2006
Bacoli, Italy
Aug 30, 2015 03:41 |  #19

LordV wrote in post #17684641 (external link)
...I did ask the person I regard as an expert on lens optical matters Rik Littlefield here http://photomacrograph​y.net/forum/viewtopic.​php?t=28518 (external link) and he confirmed the observed effects.

Brian V.

I respect Rik's knowledge, but in the interest of full disclosure he has a vested interest in perpetuating the "diffraction is the devil" mentality in macro because he's the author of Zerene Stacker...

Was banned on that site for a while because one of the focus stacking faithful told me that some day, if I was talented enough, that maybe I could learn to focus stack. The ban hammer fell when I responded that all technique, including the ones that I use, boil down to nothing more than muscle memory and mechanics and that anyone can learn to focus stack.


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DOF effects of add-on diopters
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