Wide symmetrical lenses on Fuji GFX bodies: the case of the Schneider-Kreuznach APO-Digitar 35/5.6

[rdeloe]

On the subject of shims, it was @Ben730 who put me on to Misumi shims. Misumi makes all kinds of shims in standard sizes, and will do semi-custom. I say "semi-custom" because often you can only get close to exactly what you need; sometimes close is enough though. The shims for Copal 0 are off-the-shelf standard sizes. I had custom ones made for my Mamiya G 50mm and Mamiya N 65mm lenses so that I could shim them more precisely. It was well worth the money.

For Copal 0 lenses, I ordered CIMRS 20-35. The CIMRS code is for stainless steel. The two numbers are outer diameter and inner diameter. I bought 10 packs each of 0.1mm, 0.05mm and 0.02mm.



With this combination, I can cover 0.02mm, and then 0.04mm up to any thickness in 0.01mm increments. In this spreadsheet I stopped at 0.25mm, but the pattern repeats so there's no upper limit as long as you have enough shims.



The shims that Schneider-Kreuznach uses are made of brass, and have a smaller outer diameter. They fit a bit better, but they are also easier to damage, especially the thin ones. Mangled and deformed shims are a problem because you're defeating the purpose of accurate spacing.

I haven't shimmed a lens yet where I had any issues putting the Misumi steel shims under the front cell despite their being a bit wider than is ideal.

 

[rdeloe]

@usm asked earlier in the thread for a bit more detail on housing a Schneider-Kreuznach APO-Digitar 35/5.6 XL-102 in a Schneider-Kreuznach B-0 housing. Here you go.

A B-0 housing uses the Copal 0 standard. Any cell designed for Copal 0 housings and shutters will go into a B-0, assuming that it physically fits. The aperture scale may not match, but the aperture can be adjusted for the focal length (more on that below).

Here's a B-0 housing with the enlarger lens cells that were installed in it when I bought it. The male thread on the rear is the same as you'll find on a Copal 0 shutter. It takes Copal 0 thread cells on front and rear.



I don't need a housing with a shutter, so I move the cells from lenses that came into shutters into B-0 housings. Here's what an APO-Symmar 100/5.6 looks like in a B-0. My APO-Digitar 150/5.6 is in a B-0 too, which makes for a very compact long lens.



The housing accommodates any front cell that fits inside this space. The cell needs to screw into the front cell threads and allow the aperture ring to turn freely.



Look at the shape of this front cell from a Digitar 47mm. Below the mounting threads is a straight-sided section that is the correct width to fit inside a B-0 housing. It's tall enough for the next level (the front of the lens) to clear the aperture ring.



Unfortunately, the front cell of the APO-Digitar 35/5.6 XL-102 has a very flat profile. It fits inside a Copal 0 shutter, or a Compur 0 shutter with the dust cover removed. It does not fit inside a B-0. The threads on the front cell don't even make contact with the threads in the housing.



However, if you're willing to shorten the height of the aperture ring, it works. You need to get it to look like this. The key is to not make it so thin that you cut into the f-numbers; at that point it's too thin to grasp easily. The solution is to shorten it to just above the numbers, and then add a concave profile.



You can do the job with hand tools. It's tedious but with a flat file and a half-round file plus coarse and fine sandpaper, you can complete this modification. If I had to do this again, I'd buy a drill press sanding disk to get it down to the numbers. If you do it by hand, the technique I recommend is as follows:

• Remove the aperture ring from the housing and clean off the grease so metal filings don't get stuck in it (see below for how to do this).
• Put the flat file on your workbench. Clamp it down at the end if you can.
• Turn the ring upside down and holding it tightly, make five passes up and down the file. Rotate a quarter turn and repeat. Keep doing that until you get it to the right height.
• At this point, experience regret that you didn't get a drill press sanding disk.
• Once it's even and down to just above the numbers, hold it half-on, half-off your workbench (so that half the hole is exposed), face up (in the direction shown above), and using the half-round side, carefully start cutting the concave face. The angle is right if the tip of your file is going through the hole in the centre without touching the edges. File, rotate a bit, file, rotate a bit, etc.
• When you think you are close, sand it with fine sandpaper, remove all the filings from the part (every last particle if you can), and then fit the ring back onto the housing loosely. See if you can screw the lens cell all the way down (without shims) and turn the aperture ring freely. If you can, you're done except for a bit more sanding to make it look nice and smooth.

I skipped over a few steps along the way, so now let me explain how you take one of these housings apart so you can work on the aperture ring.

Here's the front side of the housing again. Notice that thin ring around the central section? It's a springy C-ring that holds the aperture ring onto the housing. It's the only thing holding the aperture ring onto the housing. To remove the aperture ring, you need to remove the C-ring. It's a bit fiddly, but the easiest technique I've found is to use a fine tipped flat screwdriver. While pressing down with your finger on the ring so that it doesn't move, get the screwdriver underneath the side end of the ring on the side opposite to the one you're holding down. You're going to scrape things, but the scratches won't affect function. Once you get the tip under the C-ring, gently pry it up a bit and slide the tip between the ring and the raised area. Once it gets above the raised area, it's easy to lift it over.



Once you have the C-ring off, lift the aperture ring straight up. It fits snugly over that raised area in the middle that is the mounting surface for the front cell. Careful because there will be a lot of grease on the bottom of the ring.

There are a few important things to notice in this next picture.

• In the inside of the housing, you'll see a copper coloured part that is held down with screws. There's a notched area on the left. This is supposed to have a raised section with a raised ball that forms a spring that creates the clicks when you turn the aperture. On this copy, the previous owner snipped it off to "de-click" the lens. Very annoying, but the person who sold it to me neglected to mention that.
• On the right side of the housing at around the 4:00 PM position is the aperture lever.
• The bottom of the aperture ring has a series of notched teeth covered in grease. These are the stop and half-stop positions that should be engaging the spring with the ball-shaped indentation.
• The ring also has two large raised areas with a flat section in between. That's where the aperture lever fits.

Here's a closer view of the bottom of the underside of the aperture ring with a better view of the section that engages the aperture lever.



To put it back together once you've modified the ring and adjusted the aperture (see below), you simply reverse the above process. Don't forget to put some appropriate grease on the notches for the click mechanism. This is the original grease Schneider used. It's nothing fancy -- just a heavy general purpose grease that will stay put.

...Continued

 

[rdeloe]

...Continued from above post

Except for the aperture blades, which I don't much like (five blades only and a weird shape), I really like these housings. The best feature of all in my view is that regardless of focal length, they are all the same. There are minor design differences that you can't see from the outside, and they can have different f-numbers on the ring, but Schneider made one housing that it used for all the lenses it mounted in that housing, regardless of focal length.

How is this possible given that different lenses need different sized openings? It works because of their clever and simple design. Here's how:

1. Using a fine slot screwdriver, loosen the five screws one full turn.
2. Are you feeling an urge to undo them completely, lift off the copper plate to see what's inside, and then lift off that central section that has the mount surface. If so, put your screw driver down and stare at a spot on the wall until the feeling goes away. If you remove those parts you risk knocking the aperture blades out of place, and putting them back is a frustrating puzzle.
3. Adjusting the aperture is extremely simple, but knowing how big to make the hole is trickier (details on that below). To adjust the size of the maximum aperture, you need to move the lever to about where you see it in this picture, then while holding the lever, twist the middle section that has the mount surface until the aperture is the right size for your lens. Don't let the lever move while you're doing this. If the middle section doesn't rotate, you may need to loosen those five screws a bit more.
4. Once the aperture hole is the right maximum size for your lens, tighten down two of the opposing screws, move the aperture lever to the middle of the open area, and then carefully fit the aperture ring on so that those two big notches I drew your attention to are on either side of the lever. Did you remember to grease the notches that catch the spring that makes the click?
5. You'll know you set the opening correctly if you can turn the aperture to the f/5.6 setting, and the aperture opens up to the correct maximum opening for your lens. If it is too small or too big, repeat the above until it's right.
6. If you got it exactly right, lift off the aperture ring, tighten down all five screws (snug, but don't force them), and put the ring on again as you just did.
7. Once the aperture ring is in position, put the C-ring back on. You can press it on with your finger. It is bendy but be gentle. It will snap into place and should be flush to the inside base of the aperture ring part.

If you discover a mistake, it is easy enough to repeat this process until you have it right, but you're going to scratch up the inside of the housing each time you lift off the locking C-ring, and you may bend the ring. Be careful and try to get it right initially.



So now for the elephant in the room: how do you decide how big to make the hole? I use simple, empirical techniques:

• Do you have the housing the cells came in? If the aperture is correct in that housing, use it as a guide. Measure it as best you can. Study how the blades overlap the cells. Try to make the hole in the B-0 housing the same size. It's not that easy because the shape of the iris may be quite different, making it hard to compare.
• Do you have a lens of that focal length for the same camera? Set up the camera with the lens whose aperture you trust to be accurate against an evenly lit subject. Put the aperture on f/5.6. What is the shutter speed at f/5.6. Now put your lens mounted in the adjusted B-0 housing on the camera and set it to f/5.6 (same position, same light). Are you getting the same shutter speed? If not, you need to adjust the aperture in the housing to make it bigger or smaller.

Others may have techniques that work. I'd love to learn about them too (if they're simple).

I hope that helps and is reasonably clear. I'm happy to elaborate if I forgot something or was unclear.

Happy modding!

 

[usm]

Wow. Really a great description. That’s why forums like this are so important!
Thanks.

 

[rdeloe]

Here's another example of a slightly different cell spacing challenge. This is the lens that started it all for me: my Mamiya N 43mm f/4.5 L. This lens has a stellar reputation among people who shoot the Mamiya 7 camera. Like all the Mamiya G and N lenses for the Mamiya 6 and 7 rangefinder cameras, it has an internal shutter. Unfortunately, Mamiya decided that the default position of the shutter is closed. The shutter is electronically controlled from the Mamiya 7 body. (There is a Fotodiox adapter that will open the shutter and let you use Mamiya 7 lenses on a GFX camera, but because of the issue I'm discussing here, I'm not confident it works.)



If you don't like modifying lenses, this would be a good time to find another thread because this is about to get ugly.

You can use Mamiya 6 and 7 lenses as technical camera lenses, and they perform beautifully because they are superb optics. However, you have to be willing to permanently "hack" them. This involves either removing the shutter blades or locking them open, removing controls that stick out of the mount, and in the case of the Mamiya 6 lenses, re-mounting them. Mamiya 7 lenses use the same bayonet pattern as the Mamiya 645 system, so I was able to build mount adapters using Mamiya 645 extension tubes.

Another major quirk with the Mamiya N 43mm f/4.5 L is that it has a huge rear end. Unmodified copies have a metal cowl around the end that protects the bulbous rear lens element. When you're using one of these on a GFX camera, you can leave the cowl on, but the rear of the lens is deep inside the GFX body, so you get no movements. I removed the cowl and protected the rear cell with a functional but not pretty looking tape wrap. Before the tape wrap I had a neat looking metal collar, but I was hitting the inside housing on my GFX 100S enough times to leave marks. The tape wrap protects the end of the lens, and it protects the inside of the camera. Removing the cowl lets me shift 5mm -- which doesn't seem like a lot but has been plenty for me because I mostly use tilt and swing, with shift there to correct composition.



When I got this lens back from Bill Rogers, the Mamiya technician in Las Vegas who does the work for me, I put it on my camera, looked at the resulting image, and felt almost sick to my stomach. My very expensive lens was awful. It was sharp as a tack in the centre at f/4.5, but moving away from the centre of the unshifted frame, the image fell apart rapidly. Closing it down even to f/11 didn't help much. Bill is a pro and he tests every lens he gets for proper cell spacing before he starts work. He confirmed that this one was in spec.

It was a couple years ago so I can't remember exactly why I thought to try unscrewing the rear cell, removing the shim, and tightening the cell down to the mount. But I did. And it got a bit better. It didn't take long to find a cell spacing -- less than factory spec -- that gave me the image quality I'd hoped for. That was a huge relief.

Adjusting these lenses is fairly straightforward. My other Mamiya 6 and 7 lenses that have shims have them under the front cell. The way this one is built, that's not possible, so Mamiya put the shim under the rear cell.

I haven't checked what the thread is, but I discovered that Copal 0 shims are an almost perfect fit. I've shimmed it with Misumi steel shims previously, but I replaced them today because they stick out and are like little knife blades when compressed between the lens cell and the mount. I had to wrap it in more tape, and that was ugly. So today I re-shimmed, and fine-tuned in the process, using some leftover brass Copal 0 shims.



The pencil markings in this closeup of the lens where the rear cell meets the mount are indicators I made to help me keep track of where I was during the shimming and testing process. I used that rub mark on the lens cell as the indicator. In this picture it's pointing at position "E". To the left are positions C and D. The optimum position that balanced image quality across the whole shifted image circle ended up being half-way between the C and D marks.



Earlier in this thread I mentioned that extremely small differences have a significant impact on image quality. I evaluate the effectiveness of each shim position by shifting left and right at maximum aperture as far as I can against my Siemens Star wall and comparing the results carefully. In the case of this lens, position E was unacceptable, while C and D were trade offs between left-side and right side image quality -- a bit better on one side and worse on the other. Half-way between C and D gave me left and right sides at 5mm shift that a bit softer than was possible with C or D, respectively, but the two sides were equally good at the half-way mark.

It's always about trade offs in my experience. I will never use this lens shifted 5mm at f/4.5, so I can live with less than the very best possible image quality on the far shifted sides if it means that image quality across the whole image circle I can use is equally good with maximum shift at f/8 where I will use the lens. It's excellent at f/8 shifted 5mm from centre-focus, so I'm happy.

It's experiences like this that make me extremely skeptical when people describe a lens that ought to be good because it is a serious professional tool as not being good. Maybe it's just not calibrated properly.

At this point you might be wondering why I would even bother with a lens that can only shift 5mm on GFX. It's a fair question because I have an excellent 35mm that can shift 7.5mm and a superb 50mm that can shift 15+ mm. It's a couple things. First, I really like this focal length. I find 35mm to be quite wide on a 33mm x 44mm sensor. A 43mm lens is my "slightly wider than normal" lens. It has a very pleasing field of view. Plus the reputation of the lens is well deserved; it's a superb performer -- almost no distortion and wonderfully sharp. Despite that big rear element, I get as much tilt and swing as I need, plus a bit of shift to strengthen composition. On a non-BSI sensor, you get nasty lens cast, but if there's lens cast on my GFX 100S that has a BSI sensor, I'm not seeing it.




The other benefit of this lens is that Mamiya's choice to recycle the bayonet pattern of the Mamiya 645 system made it easy to build custom mounts for two usage scenarios. This lens mounts as easily on my Arca-Swiss F-Universalis as it does directly on the GFX camera with a custom adapter. I used an M65 focusing helicoid for the direct-to-GFX adapter, so not only can I use the lens directly, but also I get a closer minimum focus distance than people using it on Mamiya 7 cameras. There are times when it's just too cramped or awkward to set up the F-Universalis, so it's extremely useful to have a lens that can go directly on the camera; this one and my Mamiya N 65mm f/4 L fill that niche nicely.

 

[stevev]

Rob, would it be generally true to say that, prior to a shim adjustment, the true full potential of a lens when fully shifted (e.g. 20mm on my Actus), can be seen by refocusing when taking the shifted shots? I imagine that in some cases the issue may be field curvature rather than cell spacing, but I guess refocusing for the shifted shots may be a temporary workaround, for some lenses in either case?

To put that another way, I would like the shifted edges of my Nikkor SW 65mm lens to be sharper than they are, but refocusing does not seem to help. Would that necessarily rule out an attempt at shimming that lens in its Copal 0 shutter on the Cambo lens plate?

 

[jng]

Rob, would it be generally true to say that, prior to a shim adjustment, the true full potential of a lens when fully shifted (e.g. 20mm on my Actus), can be seen by refocusing when taking the shifted shots? I imagine that in some cases the issue may be field curvature rather than cell spacing, but I guess refocusing for the shifted shots may be a temporary workaround, for some lenses in either case?

To put that another way, I would like the shifted edges of my Nikkor SW 65mm lens to be sharper than they are, but refocusing does not seem to help. Would that necessarily rule out an attempt at shimming that lens in its Copal 0 shutter on the Cambo lens plate?

My understanding is the field curvature is affected by cell spacing, so if that's the culprit, focusing at some mid-point might be an acceptable work-around depending one's criteria for minimum sharpness. However I don't think this would mitigate issues of astigmatism imposed by the Fuji's thicker cover glass, but this is just a quasi-(un)educated guess on my part.

John

 

[rdeloe]

Rob, would it be generally true to say that, prior to a shim adjustment, the true full potential of a lens when fully shifted (e.g. 20mm on my Actus), can be seen by refocusing when taking the shifted shots? I imagine that in some cases the issue may be field curvature rather than cell spacing, but I guess refocusing for the shifted shots may be a temporary workaround, for some lenses in either case?

To put that another way, I would like the shifted edges of my Nikkor SW 65mm lens to be sharper than they are, but refocusing does not seem to help. Would that necessarily rule out an attempt at shimming that lens in its Copal 0 shutter on the Cambo lens plate?

It's times like this I wish I was an optics expert. I can answer your question, but only empirically, from experience.

The answer from my experience is "maybe". I agree with @jng (comment after yours) that the culprit is field curvature, and I think the cause is the thick cover glass. An optics expert would know -- but I have yet to go far wrong working from this assumption.

In cases where the field curvature is mild, you can often focus at the edges and use depth of field to get the centre acceptably sharp. This assumes we're focusing on a flat subject parallel to the sensor. If the curvature is equal, both edges should come into good focus at about the same time.

There are lots of cases though where one side comes into focus nicely, but the other doesn't, and vice versa. That's still field curvature I believe, but something else is going on. I have had good success fixing both of these problems, but sometimes the best I can do is get to a point where one side is very good and the other side is only good.

What's really vexing is when the field isn't a nice curve, but is actually wavy. I'm fighting this right now. I realized that I hadn't checked for the wavy field problem, and sure enough it's happening with my newly calibrated 43mm. At what I thought was the best spacing (between C and D), there's some waviness. A bit more tweaking suggests that the "E" position I marked on the lens is the right one. I'm going to check a bit more though because very small focus differences between shots also make a difference.

Now to your question: before I got my Mamiya 43mm and the light went on about this issue, I tried a Nikkor-SW 65mm f/4 and returned it because it was terrible. The centre was plenty sharp, but just like my Mamiya 43mm when I first tried it, the edges were awful and never got good. I just figured it was a bad lens and returned it. If I had one today, I bet I'd find that it has strong field curvature due to the GFX cover glass, and that it can be turned into a really good lens with proper shimming.

The catch is that if there are not shims in it already, you have to shorten the distance between mount surfaces. Rather than wreck your Copal 0 shutter by sanding it to accommodate shims, why not buy a B-0 housing and try to set it up with that? If it doesn't work, you haven't affected the resale value of the lens in shutter. From eyeballing pictures of that lens, I believe you can drop it into a B-0 without having to modify the aperture ring at all. You don't even need to adjust the aperture until you're sure you want to leave it in the B-0. Just figure out where on the housing you bought f/4 would be for a 65mm lens and you're good to go.

 

[rdeloe]

My understanding is the field curvature is affected by cell spacing, so if that's the culprit, focusing at some mid-point might be an acceptable work-around depending one's criteria for minimum sharpness. However I don't think this would mitigate issues of astigmatism imposed by the Fuji's thicker cover glass, but this is just a quasi-(un)educated guess on my part.

John

As I mentioned to Steve, I'm no optical expert! But I think you're on the right track. My only friendly amendment is that sometimes I've seen the curvature does not affect the two sides of the frame the same way. Focusing at the edges might not work well (unless it's minor curvature and there's enough depth of field).

 

[jng]

As I mentioned to Steve, I'm no optical expert! But I think you're on the right track. My only friendly amendment is that sometimes I've seen the curvature does not affect the two sides of the frame the same way. Focusing at the edges might not work well (unless it's minor curvature and there's enough depth of field).

Yes, it's interesting that field curvature can manifest asymmetrically. I wonder whether this just reflects some other underlying misalignment. But in practical terms, this would complicate choosing the right spot to focus on!

John

 

[rdeloe]

Yes, it's interesting that field curvature can manifest asymmetrically. I wonder whether this just reflects some other underlying misalignment. But in practical terms, this would complicate choosing the right spot to focus on!

John

That's a really important point John. I'm setting these up on a digital view camera where the front swings. I think it's perfectly aligned, but I know that being off by a very small amount is quite likely. Thus, it's entirely possible that I'm calibrating these lenses not only for GFX, but also for my specific F-Universalis is its current condition, using the current GFX 100S I own.

 

[stevev]

Thank you both. I should have phrased my question as ”would focusing each shot separately in a shifted series i.e. centre, left and right potentially overcome field curvature or obviate the need for shimming”? A temporary workaround for sure, but I get the impression that the answer remains maybe/not necessarily. I may look at the B-0 housing Rob, as you suggest.

 

[rdeloe]

Thank you both. I should have phrased my question as ”would focusing each shot separately in a shifted series i.e. centre, left and right potentially overcome field curvature or obviate the need for shimming”? A temporary workaround for sure, but I get the impression that the answer remains maybe/not necessarily. I may look at the B-0 housing Rob, as you suggest.

Ah, I see. I'm going to say probably not. Depending on the form of the curvature, you may end up with a set of pictures where far shifted left and far shifted right are good, but the opposite sides are a mess.

 

[rdeloe]

In another recent thread that touched on the issue of calibrating lenses, there was a nice summary of how Rodenstock does it for the HR lenses that can be adjusted on a Zeiss Calypso. What I'm doing with shimming seems quite crude in comparison, but after several hours of work today I did land in a good place with my Mamiya N 43mm f/4.5 L. I went off on a fine wild goose chase this evening, and ended up almost where I started: position "C".

This is a tricky process and you have to be patient and methodical. Every lens I've worked on responds differently to this kind of adjustment. Tonight's weird surprise was how wavy the field got when I went out a bit too far from the eventual ideal position. I was paying too much attention to the far shifted side and that got me into trouble. My procedure is to shoot the centre (first picture), then shift as far right as I can and then as far left. I set up so that the stack of three vertical Siemens Stars is the far-shifted edge.



Tonight I was pulled off course by how good the far shifted sides were looking as I added shims. But when I went back to the unshifted image, the left and right sides were weirdly soft even though the shifted images were fine. I also noticed that the items on my worktable were surprisingly out of focus. These are not big shifts, so I'm not quite sure what was going on, but it looks like a wavy field problem.

When I hit a wall like that, I go back to the beginning... This is where patient and methodical comes in. I started by looking for the shim spacing that gave me an excellent field across the unshifted image. Once I found that spacing, I started paying attention to the far shifted sides, but this time always keeping an eye on what was happening on the sides of the unshifted image. Long story short, I had almost the right spacing when I put this post up earlier today; everything I did after that was just making it worse in new ways. Sometimes that's how it goes.

This lens hits its best stride at f/8, but it's now really decent at f/4.5, even shifted. Contrast is a bit lower and it's a bit softer away from the centre, but Mamiya did an amazing job with this one.

 

[jng]

Rob,

Thanks for walking us through your process - this is all very informative. I certainly don't have your technical skills although experimenting with shims is within my admittedly modest capabilities. Your last post reminds me of the saying "better is the enemy of good enough" (been there, done that).

I'm not sure whether you covered this in prior posts, so forgive me for asking if you have: how do you assure parallelism between you sensor plane and your target plane?

Also, do you find that calibrations done in a lab setting - i.e., at modest focus distances - translate to far-to-infinity focus out in the field?

John

 

[rdeloe]

Rob,

Thanks for walking us through your process - this is all very informative. I certainly don't have your technical skills although experimenting with shims is within my admittedly modest capabilities. Your last post reminds me of the saying "better is the enemy of good enough" (been there, done that).

I'm not sure whether you covered this in prior posts, so forgive me for asking if you have: how do you assure parallelism between you sensor plane and your target plane?

Also, do you find that calibrations done in a lab setting - i.e., at modest focus distances - translate to far-to-infinity focus out in the field?

John

I'm glad this is helpful to someone John! We're such a nichey bunch that I'm trying to do my little part to keep the lights on.

It's definitely the case that "perfection is the enemy of done" (the version I've heard of your saying). In fact, I've concluded there is no perfection in this case. It's a complicated wide angle rangefinder lens, so I'm just grateful to get it to where it works well.

I don't have the ability to establish perfect parallelism. I measure and square up as carefully as I can. And then I double check against real world scenes.

Your last question is bang on. My process is really twofold. I always take it out into the world to check for distance performance. Sometimes calibration in my studio setup translates perfectly to longer distances, and sometimes it doesn't. This is another one of those trade off situations where you have to choose between close performance and far performance, because you can't have both.

 

[jng]

Your last question is bang on. My process is really twofold. I always take it out into the world to check for distance performance. Sometimes calibration in my studio setup translates perfectly to longer distances, and sometimes it doesn't. This is another one of those trade off situations where you have to choose between close performance and far performance, because you can't have both.

One reason I ask is that a while back I had in my possession for evaluation a 150mm lens that checked out fine when shifted at ~10m ft distance, but totally fell apart when I took it out into the field to shoot landscapes.

In a story with a happier ending, thanks to some sleuthing and patient guidance from @Rod S. , I was able to turn a good copy of an SK 60XL into an outstanding one by shimming the equivalent of a quarter turn of the front cell (~0.12mm). The experience was eye-opening, to say the least. The biggest challenge at the time was finding a shim of the appropriate thickness (which I did, by pure dumb luck after hitting one dead end after another), so the source you identified is very much appreciated!

John

 

[Rod S.]

Rob, this entire thread has been fascinating to read and provides some valuable lessons.

In particular, you've shown that the performance of symmetrical wide-angle lenses is degraded on the Fuji GFX cameras, presumably due to the thicker cover glass over the sensor, and - crucially - that the performance can be restored by reducing the intercell spacing using inexpensive SK aperture units along with simple tools, a steady hand and patience.

 

[stevev]

And a thread about the importance of parallelism in respect of the standards on an Actus is here: Parallelism on Actus standards

 

[rdeloe]

It seems the story of my Mamiya N 43mm wasn't quite done. In my post from last Saturday, I was quite pleased about where things landed. Things got busy so I didn't have a chance to test it out at longer distances for a few days. When I finally had some time, I took it upstairs to a 2nd story window that looks out over my hill slope backyard and the fence that runs across the top. It's a convenient test target for checking alignment, field curvature, etc. because the fence line is parallel to the back of my house.



It's possible when judging cell spacing against a wall of Siemens Stars to overlook a bit of softness on one side relative to the other, but it's a lot harder against a real world scene like this. When I looked at the images I made on the computer, there was no denying that the right side was soft, and that it never became acceptably sharp with smaller apertures.

There are a lot of links in the chain that could have been the problem. Starting with the lens, there's incorrect shimming, or perhaps an unfixable problem like damage to the housing that puts the cells out of parallel. The lens mounts to the view camera with a custom lens board I built, and that board might have an alignment issue. View cameras do go out of alignment with time and wear, so a swung front standard was possible. The GFX camera mount itself might even be out of parallel.

All the non-lens links were easy to rule out. None of my other lenses showed soft right sides on the F-Universalis, which means the standard is not out of alignment. My Mamiya N 65mm shares the same board, and it's flawless from left to right -- so there's nothing wrong with the lens board. Unfortunately, that left only the lens itself as the culprit.

Long story short, I re-checked the shimming and made some small adjustments that created meaningful improvements. But even after all this re-calibration, the right side was still bad. I almost gave up at this point. It's not like the lens was unusable. I used it extensively in 2023 with a shim spacing that wasn't quite as good as the one I have now. None of the scenes I shot in 2023 were flat targets, so I was using movements all the time to adjust the plane of focus, and that masked the softness on the right that would have been apparent had I been shooting brick walls.

Thankfully, I remembered one more trick I hadn't tried. One of the features of the F-Universalis design that I prize is the ability to rotate the square lens board to any of four possible orientations. Sometimes I want the scale on the lens up, but mostly I want it on the left side from behind so I can peek around the camera and see the aperture position easily when the camera is at head height. All of my Mamiya 43mm adjustments had the lens board mounted with the lens scale "sideways", on the left side. However, I remembered that I have had lenses that I could use only in one orientation because only that orientation gave good results. When I mounted the lens board so that the aperture scale on the Mamiya 43mm was "up" instead of sideways, the softness disappeared.

This is a 100% view of the right-hand side of the fence line at f/4.5 (wide open). At left, the lens scale is "up", and at right the lens scale is on the left side (from behind the camera).



There's no such thing as a free lunch in this game, so of course I wondered "where did the softness go?" Logically, if it is on the right side of the image when the lens scale is on the left side, then rotating the lens 90 degrees clockwise should have put the softness on the bottom of the frame. The mystery -- and the good news -- is that the softness seems to have disappeared entirely. I checked against my Siemens Star wall, and the image is now equally sharp everywhere. It's not soft on the bottom.

As I mentioned earlier in the post, the image quality "chain" has a lot of links in it with my kind of setup. All I can say is that the simple action of orienting the correctly shimmed lens on the camera the "correct" way seems to have meshed with all the other links in the chain to produce the best possible result. It's bizarre, but I'll take it!