Optimal aperture position?

[E Dinkla]

Optimal position for the aperture in the optical path?

For me not just a theoretical question as I am thinking about a Frankenstein assemblage of a projection lens with an astro focal reducer, both without an aperture. In that process it will be easier to add an aperture mechanism in between both parts. Possibly not the best or even an optimal position but on the other hand adding a focal reducer to a lens with a given aperture position does not sound as best or optimal either.

I expect that the lenses we buy have the aperture at an optimal spot in the optical path. Optimal as floating elements/groups in zooms and primes will narrow the choices for its position. Lenses where the aperture is not at the best spot will show focus shift with changed aperture settings and in worst cases vignetting. The way the aperture acts on changing CA phenomena will be influenced as well. Near the MFD of a normal lens the focus shift is usually more noticeable.

I do not see much mention of focus shift or vignetting with the use of focal reducers while the combination of focal reducers with a wide variety of lenses must show some issues. Anyone with better information on that subject? Or experience?

Am I seeing things that do not exist, on a totally wrong theoretical path?


Met vriendelijke groet, Ernst
No photographer's gear list is complete without the printer mentioned !

 

[aidaho]

Optimal position for the aperture in the optical path?

For me not just a theoretical question as I am thinking about a Frankenstein assemblage of a projection lens with an astro focal reducer, both without an aperture. In that process it will be easier to add an aperture mechanism in between both parts. Possibly not the best or even an optimal position but on the other hand adding a focal reducer to a lens with a given aperture position does not sound as best or optimal either.

I expect that the lenses we buy have the aperture at an optimal spot in the optical path. Optimal as floating elements/groups in zooms and primes will narrow the choices for its position. Lenses where the aperture is not at the best spot will show focus shift with changed aperture settings and in worst cases vignetting. The way the aperture acts on changing CA phenomena will be influenced as well. Near the MFD of a normal lens the focus shift is usually more noticeable.

I do not see much mention of focus shift or vignetting with the use of focal reducers while the combination of focal reducers with a wide variety of lenses must show some issues. Anyone with better information on that subject? Or experience?

Am I seeing things that do not exist, on a totally wrong theoretical path?

Met vriendelijke groet, Ernst
No photographer's gear list is complete without the printer mentioned !

I think you may find this interesting: http://forum.mflenses.com/canon-fd-4-5-400mm-vs-novoflex-5-6-400mm-t-t80838.html#1528109

Here is another thread, in which OP tries to put an aftermarket aperture, does not listen to advice and it works backwards for him: http://forum.mflenses.com/viewtopic.php?t=80955

TL;DR: Lens must be designed for aperture. Aperture will reduce vignetting, ca and increase sharpness only if the lens was made for it.

Otherwise your mileage may vary a lot. In the real world there probably will be a lot of results that are somewhat inbetween of extreme outcomes.

 

[E Dinkla]

Optimal position for the aperture in the optical path?

For me not just a theoretical question as I am thinking about a Frankenstein assemblage of a projection lens with an astro focal reducer, both without an aperture. In that process it will be easier to add an aperture mechanism in between both parts. Possibly not the best or even an optimal position but on the other hand adding a focal reducer to a lens with a given aperture position does not sound as best or optimal either.

I expect that the lenses we buy have the aperture at an optimal spot in the optical path. Optimal as floating elements/groups in zooms and primes will narrow the choices for its position. Lenses where the aperture is not at the best spot will show focus shift with changed aperture settings and in worst cases vignetting. The way the aperture acts on changing CA phenomena will be influenced as well. Near the MFD of a normal lens the focus shift is usually more noticeable.

I do not see much mention of focus shift or vignetting with the use of focal reducers while the combination of focal reducers with a wide variety of lenses must show some issues. Anyone with better information on that subject? Or experience?

Am I seeing things that do not exist, on a totally wrong theoretical path?

Met vriendelijke groet, Ernst
No photographer's gear list is complete without the printer mentioned !

I think you may find this interesting: http://forum.mflenses.com/canon-fd-4-5-400mm-vs-novoflex-5-6-400mm-t-t80838.html#1528109

Here is another thread, in which OP tries to put an aftermarket aperture, does not listen to advice and it works backwards for him: http://forum.mflenses.com/viewtopic.php?t=80955

TL;DR: Lens must be designed for aperture. Aperture will reduce vignetting, ca and increase sharpness only if the lens was made for it.

Otherwise your mileage may vary a lot. In the real world there probably will be a lot of results that are somewhat inbetween of extreme outcomes.

If I ever could implant an aperture on a Colorplan projection lens it would be in the center of that double gauss design, like the camera lenses that are related to it have it. It will vary with other optical designs and other enlarging factors.

I can understand that optical engineers have to evaluate the aperture size, position etc together with way more aspects of a lens design. Going back and forward to fit that into the lens mechanism and the design goals set, like for example the enlarging factor. That still does not imply that there is no optimal position possible for an existing aperture less lens. Optimal meaning; not the best in optical sense but fitting where possible near optical best.

Your links do not tell me much for the conditions I set in my request.

I am intrigued by whatever is possible for an aperture position with a projection lens + a focal reducer behind it. I would assume in between them, where best position might still be within the projection lens optics. Depending on the complexity of the projection lens, say triplet versus double gauss and the magnification aimed at. Plus, I am intrigued about the experiences with the shifted position of the aperture when a focal reducer is added to a normal camera lens.

Edit: If you read Brian Caldwell's article on the Speed Booster explains a bit more and illustrates what happens in ray tracing. For example the distance between the aperture position and the sensor actually decreases with the focal reducer in between mounted on the mirrorless camera if compared to the lens mounted on its native body.

Met vriendelijke groet, Ernst
No photographer's gear list is complete without the printer mentioned !

 

[AlexTG]

Most lenses have the diaphragm at about the optical center of the lens, which is conveniently located exactly the lens' focal length ahead of the image plane, i.e. film or sensor (by definition of the "focal length" iself). Of course, there are lots of exceptions, but that's a good starting point.

 

[aidaho]

Your links do not tell me much for the conditions I set in my request.

Your OP post does not contain any information required to make accurate guess.

If you want to be scientific, full raytrace of the combined optical scheme is required.

If you're after the least incorrect placement, lens scheme and parameters would be a start.

 

[Tons o Glass]

I think your thinking is sound... I think. ;-)

What projector lens are you working with? And what about optimal focal reducer position (I think the astro focal reducers can be used with spacers/extension tubes to increase the focal reduction ratio?)?

Regardless of whether the focal reducer is used or not, I think the ideal aperture position could be somewhere inside your projector lens - you've alluded to this in your post. If you have the tools and it's not too much of a financial burden if you ruin it, you may want to cut the lens in two then cut out a section for the aperture, or cut out a small slot for Waterhouse-type stops.

Finding the ideal position would require knowing the optical layout/measurements and the specifications of each element. With that information you could do some ray-tracing in software (can be $$$$$) or on paper (yuck). The ideal aperture stop position will be where where off-axis rays cross over.

If the projector lens (or its design) has a sibling meant for a camera, you can make an educated guess. If it's a very long focal length with a giant image circle, you may be able to get away with putting the aperture immediately after the rear element of the projector lens.

I'll be following the thread with interest and awaiting images of and from your creation! I tried out an unmarked astro focal reducer briefly with a lowly Barska 400mm/70mm before throwing the Barska in the garbage a few years back and the results were promising (it seemed to generally improve IQ on a camera (Fuji X-M1 - since sold).

 

[sensiblename99]

Incorrect aperture placement can cause flare/hot spot and distortion (iirc). Keep an eye out for both as you experiment.

 

[ProfHankD]

Optimal position for the aperture in the optical path?

Obviously, in front of the lens is optimal in the sense that it is most accessible.

Basically, there are multiple positions within, behind, and in front of most lenses that can be suitable places for an aperture stop. However, there are various differences between the different locations because rays don't get clipped the same way -- so, for example, some locations might result in better control of aberrations than others. It really takes either modeling the lens or physically testing it to determine the most viable aperture positions. A valid aperture stop position is one in which the opening does not cause vignetting (i.e., isn't a field stop); ironically, too large a hole is generally how you get vignetting.

If at all possible, I prefer the front mounting of a Waterhouse stop. It's easy to mount/unmount them on unmodified lenses and they are easy to make very precisely the size and shape desired, whereas an iris is really a pretty challenging precision build. My Waterhouse stops are generally cut on a programmable paper cutter, laser, or CNC router/engraver; larger aperture ones also can be 3D printed.

 

[Ching-Kuang Shene]

Optimal position for the aperture in the optical path?

For me not just a theoretical question as I am thinking about a Frankenstein assemblage of a projection lens with an astro focal reducer, both without an aperture. In that process it will be easier to add an aperture mechanism in between both parts. Possibly not the best or even an optimal position but on the other hand adding a focal reducer to a lens with a given aperture position does not sound as best or optimal either.

I expect that the lenses we buy have the aperture at an optimal spot in the optical path. Optimal as floating elements/groups in zooms and primes will narrow the choices for its position. Lenses where the aperture is not at the best spot will show focus shift with changed aperture settings and in worst cases vignetting. The way the aperture acts on changing CA phenomena will be influenced as well. Near the MFD of a normal lens the focus shift is usually more noticeable.

I do not see much mention of focus shift or vignetting with the use of focal reducers while the combination of focal reducers with a wide variety of lenses must show some issues. Anyone with better information on that subject? Or experience?

Am I seeing things that do not exist, on a totally wrong theoretical path?

The optimal location of the aperture mechanism is the center of the optical system. Missing this requirement would cause distortion (i.e., barrel and pincushion). In general, focus shift is not an issue, but vignetting is. And, don't mention the cat-eye issue caused by clipping the light path in one way of the other. Just look at those symmetric designs, the aperture mechanism (or the stop) is always at the center. You get focus shift is usually due to a rather poor lens design that fails tome all lights meeting at the focal point. In their, this can go on and on. But, to avoid the criticism for being to theoretical, let me stop here. My suggestion is: to ensure good results, please find the center of the optical system and place your stop there. Period.

CK

 

[E Dinkla]

Optimal position for the aperture in the optical path?

Obviously, in front of the lens is optimal in the sense that it is most accessible.

Basically, there are multiple positions within, behind, and in front of most lenses that can be suitable places for an aperture stop. However, there are various differences between the different locations because rays don't get clipped the same way -- so, for example, some locations might result in better control of aberrations than others. It really takes either modeling the lens or physically testing it to determine the most viable aperture positions. A valid aperture stop position is one in which the opening does not cause vignetting (i.e., isn't a field stop); ironically, too large a hole is generally how you get vignetting.

If at all possible, I prefer the front mounting of a Waterhouse stop. It's easy to mount/unmount them on unmodified lenses and they are easy to make very precisely the size and shape desired, whereas an iris is really a pretty challenging precision build. My Waterhouse stops are generally cut on a programmable paper cutter, laser, or CNC router/engraver; larger aperture ones also can be 3D printed.

The projection lens that I have is the Leitz Elmaron 150mm 2.8 for 6x6 slides. Basically a Triplet but 4 elements 3 groups with a two element negative group in the center. Trioplan related, more resemblance to old Hektor + Thambar designs though. Lens diagrams for these types show that an aperture can be in front of the rear element and close to that element. I was thinking of a Waterhouse stop too with two openings in the slider, one keeping 2.8, the other closing to best performance. Should be based on trials. However it will be easier to add the waterhouse stop behind the rear element and in front of the focal reducer that can also be fitted within the projection lens barrel. Milling two slits in the barrel of the Elmaron at the rear of its optics is possible. 3D printed parts to secure a lightight fit for the two apertures Waterhouse slide. The Antares 2" x0.5 is a focal reducer option, a 4 element would be better. Main difficulty is finding them secondhand and with a wide aperture.

The Leica Elmaron 150 mm f/ 2.8 Lens. Specs. MTF Charts. User Reviews.

allphotolenses.com

Rathenower Optische Werke Cinerectim 62,5/180 (Projection lens) - Page 2

Hello, Does anyone know anything about Rathenower Optische Werke Cinerectim 62,5/180 lens? As I can guess by simple test it's 180mm as the 2nd number suggests but I am not sure what does 62,5 stands in the name? Also rear elements look like of Petzval design but I cannot unscrew the...

www.largeformatphotography.info

An interesting discussion / project on a DIY focal reducer for M4/3 before the Speed Booster even appeared, I wonder whether Brian in that thread got the idea to market it, could be another Brian/Brain of course:

Using focal reducers with GH1/2 and old lenses? - Personal View Talks

Met vriendelijke groet, Ernst
No photographer's gear list is complete without the printer mentioned !

 

[aidaho]

Optimal position for the aperture in the optical path?

Obviously, in front of the lens is optimal in the sense that it is most accessible.

Basically, there are multiple positions within, behind, and in front of most lenses that can be suitable places for an aperture stop. However, there are various differences between the different locations because rays don't get clipped the same way -- so, for example, some locations might result in better control of aberrations than others. It really takes either modeling the lens or physically testing it to determine the most viable aperture positions. A valid aperture stop position is one in which the opening does not cause vignetting (i.e., isn't a field stop); ironically, too large a hole is generally how you get vignetting.

If at all possible, I prefer the front mounting of a Waterhouse stop. It's easy to mount/unmount them on unmodified lenses and they are easy to make very precisely the size and shape desired, whereas an iris is really a pretty challenging precision build. My Waterhouse stops are generally cut on a programmable paper cutter, laser, or CNC router/engraver; larger aperture ones also can be 3D printed.

The projection lens that I have is the Leitz Elmaron 150mm 2.8 for 6x6 slides. Basically a Triplet but 4 elements 3 groups with a two element negative group in the center. Trioplan related, more resemblance to old Hektor + Thambar designs though. Lens diagrams for these types show that an aperture can be in front of the rear element and close to that element. I was thinking of a Waterhouse stop too with two openings in the slider, one keeping 2.8, the other closing to best performance. Should be based on trials. However it will be easier to add the waterhouse stop behind the rear element and in front of the focal reducer that can also be fitted within the projection lens barrel. Milling two slits in the barrel of the Elmaron at the rear of its optics is possible. 3D printed parts to secure a lightight fit for the two apertures Waterhouse slide. The Antares 2" x0.5 is a focal reducer option, a 4 element would be better. Main difficulty is finding them secondhand and with a wide aperture.
http://allphotolenses.com/lenses/item/c_4586.html#prettyPhoto[pp_gal][lens]/1/
https://www.largeformatphotography.ihttp://allphotolenses.com/public/files/optical/2caec131cb96a2dfe33c4f5574346281.png nfo/forum/showthread.php?113984-Rathenower-Optische-Werke-Cinerectim-62-5-180-(Projection-lens)/page2

An interesting discussion / project on a DIY focal reducer for M4/3 before the Speed Booster even appeared, I wonder whether Brian in that thread got the idea to market it, could be another Brian/Brain of course:
http://www.personal-view.com/talks/discusshttp://allphotolenses.com/public/files/optical/2caec131cb96a2dfe33c4f5574346281.png ion/577/using-focal-reducers-with-gh12-and-old-lenses

Met vriendelijke groet, Ernst
No photographer's gear list is complete without the printer mentioned !

Is this the lens you are speaking of?

I see no good place for the aperture at all. The least bad one is likely as close as possible to the middle element, not sure which side of it. Second worst is the front element, as close as possible. Doesn't matter which side. The worst one is behind the lens.

Speedbooster trace might give us more options, but I can tell you right away: the gap between the lens and booster is the worst option here. Aperture behind the back element should work significantly worse than the one behind the first: it will cut into corners before it will cut the center.

This placement will induce strong vignetting.

--
I like to shoot with manual lenses. Here are some of my photos: https://www.flickr.com/photos/curry-hexagon/

 

[aidaho]

The optimal location of the aperture mechanism is the center of the optical system. Missing this requirement would cause distortion (i.e., barrel and pincushion). In general, focus shift is not an issue, but vignetting is. And, don't mention the cat-eye issue caused by clipping the light path in one way of the other. Just look at those symmetric designs, the aperture mechanism (or the stop) is always at the center. You get focus shift is usually due to a rather poor lens design that fails tome all lights meeting at the focal point. In their, this can go on and on. But, to avoid the criticism for being to theoretical, let me stop here. My suggestion is: to ensure good results, please find the center of the optical system and place your stop there. Period.

CK

I'm not sure this thing "center of the optics" really exists. Yes, due to how many (not all) lenses are, extra elements for aperture placement are usually somewhere around the middle of the optical block.

However, nothing (except common sense for the specific design purpose) prevents one from putting air-gapped concave surfaces elsewhere.

Nothing specifically forbids doing this twice, just for fun. Don't know if anyone needs two optimal apertures in one lens, but where is the will, there is a way.







Two air-gapped concave surfaces is a dead giveaway of the aperture-designed lens. They are there in order to allow aperture to cut into straight rays before it will begin cutting into oblique ones.

Projection lenses not designed for aperture do not follow "the common sense" at all. So cutting them in half and putting aperture there might not be the best idea.

Aperture in aperture-designed lenses is often somewhere in the middle, because the elements making lens suitable for aperture are there, not the other way around.

--
I like to shoot with manual lenses. Here are some of my photos: https://www.flickr.com/photos/curry-hexagon/

 

[Alan WF]

The stops in photographic lenses are designed to act as pupil stops.

What does an ideal pupil stop do? Loosely, as it closes, it reduces the amount of light reaching the focal plane in a way that is even over the whole focal plane.

To see the importance of the italicized phrase, consider putting a stop right in front of the sensor. As you closed the stop, it would indeed decrease the light reaching the focal plane, but in a way that is very uneven over the focal plane. Technically, it would be acting as a field stop rather than as a pupil stop.

Experimentally, how can you tell if a stop is acting as an ideal or close-to-ideal pupil stop? Look at the lens from the back. Look at what happens as you open and close the stop from different positions in the focal plane. The change in the amount of light reaching the focal plane should be about the same over the whole focal plane. So, you might try placing the aperture at different positions in your system and selecting the one at which it most closely shows this behavior.

Now, the pupil stops in photographic lenses are rarely completely ideal. Often, they have vignetting, so that as you open up the lens you lose some light in the corners of the field compared to the field center. However, they typically aim to smoothly and continuously vary the amount of light reaching each point in the focal plane, even if the variation is not identical in each point.

You can place an aperture stop before the first element. This will often work quite well, but will tend to have more vignetting. This is one of the reasons actual lenses have their stops other than before the first element.

Regards,

Alan

 

[E Dinkla]

Optimal position for the aperture in the optical path?

Obviously, in front of the lens is optimal in the sense that it is most accessible.

Basically, there are multiple positions within, behind, and in front of most lenses that can be suitable places for an aperture stop. However, there are various differences between the different locations because rays don't get clipped the same way -- so, for example, some locations might result in better control of aberrations than others. It really takes either modeling the lens or physically testing it to determine the most viable aperture positions. A valid aperture stop position is one in which the opening does not cause vignetting (i.e., isn't a field stop); ironically, too large a hole is generally how you get vignetting.

If at all possible, I prefer the front mounting of a Waterhouse stop. It's easy to mount/unmount them on unmodified lenses and they are easy to make very precisely the size and shape desired, whereas an iris is really a pretty challenging precision build. My Waterhouse stops are generally cut on a programmable paper cutter, laser, or CNC router/engraver; larger aperture ones also can be 3D printed.

The projection lens that I have is the Leitz Elmaron 150mm 2.8 for 6x6 slides. Basically a Triplet but 4 elements 3 groups with a two element negative group in the center. Trioplan related, more resemblance to old Hektor + Thambar designs though. Lens diagrams for these types show that an aperture can be in front of the rear element and close to that element. I was thinking of a Waterhouse stop too with two openings in the slider, one keeping 2.8, the other closing to best performance. Should be based on trials. However it will be easier to add the waterhouse stop behind the rear element and in front of the focal reducer that can also be fitted within the projection lens barrel. Milling two slits in the barrel of the Elmaron at the rear of its optics is possible. 3D printed parts to secure a lightight fit for the two apertures Waterhouse slide. The Antares 2" x0.5 is a focal reducer option, a 4 element would be better. Main difficulty is finding them secondhand and with a wide aperture.
http://allphotolenses.com/lenses/item/c_4586.html#prettyPhoto[pp_gal][lens]/1/
https://www.largeformatphotography.ihttp://allphotolenses.com/public/files/optical/2caec131cb96a2dfe33c4f5574346281.png nfo/forum/showthread.php?113984-Rathenower-Optische-Werke-Cinerectim-62-5-180-(Projection-lens)/page2

An interesting discussion / project on a DIY focal reducer for M4/3 before the Speed Booster even appeared, I wonder whether Brian in that thread got the idea to market it, could be another Brian/Brain of course:
http://www.personal-view.com/talks/discusshttp://allphotolenses.com/public/files/optical/2caec131cb96a2dfe33c4f5574346281.png ion/577/using-focal-reducers-with-gh12-and-old-lenses

Met vriendelijke groet, Ernst
No photographer's gear list is complete without the printer mentioned !

Is this the lens you are speaking of?

I see no good place for the aperture at all. The least bad one is likely as close as possible to the middle element, not sure which side of it. Second worst is the front element, as close as possible. Doesn't matter which side. The worst one is behind the lens.

Speedbooster trace might give us more options, but I can tell you right away: the gap between the lens and booster is the worst option here. Aperture behind the back element should work significantly worse than the one behind the first: it will cut into corners before it will cut the center.

This placement will induce strong vignetting.

Well for most triplet and triplet related designs they decided to go for the second air gap and the aperture can be quite far to the rear in some:


left item is the plane glass with silver center spot of the soft focus thambar









The point is whether the addition of the focal reducer can alter the optimal position of the aperture for the total. And I doubt it will bring it forward in the triplet designs.


Met vriendelijke groet, Ernst
No photographer's gear list is complete without the printer mentioned !

 

[aidaho]

Well for most triplet and triplet related designs they decided to go for the second air gap and the aperture can be quite far to the rear in some

I've googled a triplet ray trace, and it's a bad news for aperture placement. It's definitely an afterthought and it's effectiveness won't match something like double gauss, specifically designed around one.

The second air gap is a bit more preferable since the aperture mech can be a bit smaller.

The point is whether the addition of the focal reducer can alter the optimal position of the aperture for the total. And I doubt it will bring it forward in the triplet designs.

This is an easy answer for aperture-designed lens: no. See my post above: aperture is placed where conditions were created for it by lens optical design. For a double gauss that would be an air-gapped concave surfaces. Since those ain't going anywhere whether focal reducer is present or not, aperture shouldn't go anywhere else either.

With triplets, I suppose there could be a small theoretical possibility of better aperture placement inside reducer itself, but just by looking into the FC patent, it's pretty obvious this won't be a viable option https://patents.google.com/patent/US5499069

So it's a "no".

 

[Ching-Kuang Shene]

The optimal location of the aperture mechanism is the center of the optical system. Missing this requirement would cause distortion (i.e., barrel and pincushion). In general, focus shift is not an issue, but vignetting is. And, don't mention the cat-eye issue caused by clipping the light path in one way of the other. Just look at those symmetric designs, the aperture mechanism (or the stop) is always at the center. You get focus shift is usually due to a rather poor lens design that fails tome all lights meeting at the focal point. In their, this can go on and on. But, to avoid the criticism for being to theoretical, let me stop here. My suggestion is: to ensure good results, please find the center of the optical system and place your stop there. Period.

CK

I'm not sure this thing "center of the optics" really exists. Yes, due to how many (not all) lenses are, extra elements for aperture placement are usually somewhere around the middle of the optical block.

If you are not sure about this, then IMO you should go to your local (or university) library and get an optics (or better photography lens) book and search for stop, aperture, entrance pupil and exit pupil in the index of that book. In fact, what I said is the most basic concepts in lens optics that should be covered in many lens and geometric optics textbooks. Even though you have searched many many places, I am not sure whether you have found the right and correct information. Here is a Cornell U link and here is Sidney F. Ray's Applied Photographic Optics , a popular but not very technical textbook.
As for your other points, Ray's book has nearly all the answer, including a Triplet design with aperture stop, entrance pupil and exit pupil. Is is not difficult to compute the center of optic in a simple lens, even by hand calculation.

CK

 

[fferreres]

Optimal position for the aperture in the optical path?

For me not just a theoretical question as I am thinking about a Frankenstein assemblage of a projection lens with an astro focal reducer, both without an aperture. In that process it will be easier to add an aperture mechanism in between both parts. Possibly not the best or even an optimal position but on the other hand adding a focal reducer to a lens with a given aperture position does not sound as best or optimal either.

I expect that the lenses we buy have the aperture at an optimal spot in the optical path. Optimal as floating elements/groups in zooms and primes will narrow the choices for its position. Lenses where the aperture is not at the best spot will show focus shift with changed aperture settings and in worst cases vignetting. The way the aperture acts on changing CA phenomena will be influenced as well. Near the MFD of a normal lens the focus shift is usually more noticeable.

I do not see much mention of focus shift or vignetting with the use of focal reducers while the combination of focal reducers with a wide variety of lenses must show some issues. Anyone with better information on that subject? Or experience?

Am I seeing things that do not exist, on a totally wrong theoretical path?

Met vriendelijke groet, Ernst
No photographer's gear list is complete without the printer mentioned !

I think you may find this interesting: http://forum.mflenses.com/canon-fd-4-5-400mm-vs-novoflex-5-6-400mm-t-t80838.html#1528109

Here is another thread, in which OP tries to put an aftermarket aperture, does not listen to advice and it works backwards for him: http://forum.mflenses.com/viewtopic.php?t=80955

TL;DR: Lens must be designed for aperture. Aperture will reduce vignetting, ca and increase sharpness only if the lens was made for it.

Otherwise your mileage may vary a lot. In the real world there probably will be a lot of results that are somewhat inbetween of extreme outcomes.

 

[ProfHankD]

If only it was easy to add an aftermarket apodizing filter instead of changing the aperture (which would also be nice).

Actually, it isn't hard.

There are just two problems:

First, the smoothness of the shading of the apodizing filter has a strong impact on the bokeh quality. Basically, unless it's smooth at the level of the wavelength of the light, an apodizing filter adds a texture by diffraction. This is why the Minolta/Sony/Laowa lenses all use smoked+clear glass in a glued optical flat to make their apodizers. Then again, some people like the textures -- think about the old Fuji "sink strainer" apodizing filters. For example, laser printing the following appropriately-sized on transparency material works, but the bokeh will look a bit grainy and the imperfect optical properties of the transparency material also degrade the image a bit:



Second, any internal vignetting will clip the OOF PSF shape exposing a sharp edge. This is the really hard part to get right, and it's why the Sony STF 100mm is so impressive: it has virtually zero vignetting. The only way to deal with this on a typical lens is to keep reducing the size of the apodized aperture until the base lens doesn't vignette. A typical 50mm f/1.4 might need to be stopped down to f/2.8 or slower before vignetting isn't a problem, so you'd leave the iris wide open but size the apodizing mask for an outer diameter corresponding to f/2.8.

Anyway, it really does work this way, and the nature of the apodizing mask actually tends to make the aberrations induced by suboptimal aperture placement less severe... so the front placement usually works quite well.

 

[E Dinkla]

With the Elmaron's resemblance to the Thambar design a bit of element shifting + some different sized spot filters at the front should create a similar soft focus. No aperture needed then. $6000 upgrade.

https://www.leica-camera.blog/2017/10/17/thambar-m-12-290-mm/


Met vriendelijke groet, Ernst
No photographer's gear list is complete without the printer mentioned !

 

[E Dinkla]

Without grasping all, this PDF was a nice introduction to a Triplet design as well:

https://www.willbell.com/tm/ChapterB.3.pdf


Met vriendelijke groet, Ernst
No photographer's gear list is complete without the printer mentioned !