Stacked+Reversed magnification & IC ?

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jvanhuys
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Stacked+Reversed magnification & IC ?

Post by jvanhuys »

Hi everyone.

Just curious if anyone with experience stacking lenses could elaborate on the resulting magnification of a 4x net magnification stack, when reversed. Would it definitely result in a 1:4 magnification and what about the image circle size? A 4x image will obviously fill a full-frame sensor most of the time, but can the same be said when it's reversed? (let's exclude microscope objectives if possible, due to the tiny IC).

What is the effect on CA when reversing?

Example setup would on Robert O'Toole's website, where he used a Schneider Makro Symmar 120 5.6 + Schneider Xenar 28mm f/2 for a 4x magnification.

chris_ma
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Re: Stacked+Reversed magnification & IC ?

Post by chris_ma »

I don't have practical experiences with stacked lenses, but to get the discussion going I going to start with a thought based on basic opticals laws and then let others correct me :)

since light rays are reversible, yes any optical system that has a 4x magnification will result in a 0.25x (or 1:4) magnification if you swap it around. it's easy to imagine this if instead to reverse the lens, you put the camera sensor where the object was and the object where the sensor was.
if you do this, you'll also see that the image circle on the original orientation was nothing more then the boundaries that could be captured at the object, so the IC is 4x as large as the circle of the visible object.
again if you reverse it, the IC will simply be four times smaller then the original one.
the problem is, that with stacked lenses it's probably hard to predict how large the IC in the original orientation exactly is other then measuring it.

CA is a bit more complicated, but logic suggests that if you don't have any CA in the original orientation you won't have any in the reversed orientation (simply reverse the light rays). if there are CA, it seems impossible to predict what happens upon reversing the lens other that there still will be CA. it seems to me that they should be smaller because of the smaller magnification, but that could well be an incorrect assumption .
chris

rjlittlefield
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Re: Stacked+Reversed magnification & IC ?

Post by rjlittlefield »

I do not recall ever seeing a detailed study of CA with reversed optics.

But chris_ma's analysis sounds right to me. Expanding on that...

Another term for lateral CA is "chromatic difference of magnification". That model, combined with reversing the direction of the light, suggests that reversing the optics will retain the same relative amount of CA but reverse its direction, for example red outside green at 4X would correspond to red inside green when reversed to 0.25X.

Longitudinal CA should work similarly, except that the direction won't reverse. If red focuses farther than green at 4X, then it should also focus farther than green at 0.25X.

--Rik

jvanhuys
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Re: Stacked+Reversed magnification & IC ?

Post by jvanhuys »

My goodness this is all so fascinating. Thanks guys.

Makes me seriously scratch my head about what kind of image parameters I can setup by stacking my Componon-S 80mm forward-mounted and facing the subject, with my Printing Nikkor 95mm f/2.8 reverse mounted behind it. Makes me want to butcher something together tonight... I don't have an adapter though.

I need to go study how this is all done. I do have a few SM2 tubes lying around...but would want to screw stuff in ideally.

I'm expecting the Componon-S with it's 1:20 to 1:1 range being able to fetch the subject a few meters away, then the Printing Nikkor will magnify the central portion of the Componon-S which has less CA issues, yielding a 2x version of whatever that is, and then the A7Rm4 will make sweet love to all that jazz.

This is how I see the outcome in my head of course, but would love any ideas to Frankenstein with from you guys.

chris_ma
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Re: Stacked+Reversed magnification & IC ?

Post by chris_ma »

Robert would be the person with a lot of first hand experience here, from what I remember though is that a lot of very high end lenses don‘t do too well with stacking.
The macro symmar 120 and the meijro 90 are good choises if i recall correctly, but you should be able to find much more detail on his website and on this forum.
chris

JKT
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Re: Stacked+Reversed magnification & IC ?

Post by JKT »

jvanhuys wrote:
Thu Jan 26, 2023 3:48 am
My goodness this is all so fascinating. Thanks guys.

Makes me seriously scratch my head about what kind of image parameters I can setup by stacking my Componon-S 80mm forward-mounted and facing the subject, with my Printing Nikkor 95mm f/2.8 reverse mounted behind it. Makes me want to butcher something together tonight... I don't have an adapter though.

I need to go study how this is all done. I do have a few SM2 tubes lying around...but would want to screw stuff in ideally.

I'm expecting the Componon-S with it's 1:20 to 1:1 range being able to fetch the subject a few meters away, then the Printing Nikkor will magnify the central portion of the Componon-S which has less CA issues, yielding a 2x version of whatever that is, and then the A7Rm4 will make sweet love to all that jazz.
I suspect from your description that you may need to think about it a bit more. Usually with stacked lenses the magnification is calculated as ratio of focal lengths. That works at the optimal situation where light rays are parallel between the lenses. In your case that magnification would be a bit over 1. The other end of your plan is something else entirely. I doubt anyone has tried how it would work and what would be the required lens positions. Usually distance between lenses increases vignetting and your case would likely end up quite extreme if working at all.

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