How LF lens board thickness effects optics of lens rear element?

windhorse · May 10, 2021

I would very much appreciate if anyone can help me understand how using a slightly thicker than standard lens board may effect the optical performance of the rear element of my LF lens?

The lens is a 90mm Rodenstock Grandagon f4.5 in Copal 1 shutter with a large rear element that screws on to the thread on the rear of the front element and is held in place on the lens board with a standard retaining ring.

The lens board is a 3D-printed copy for Copal 1 shutter of the original Copal 0 lens board for my legacy LF camera that I had made by a colleague. We opted to make the 3D-printed lens board a little thicker than the original, as it is made of synthetic material not metal, to ensure it was strong enough to support the big lens.

Fortunately, the lens board fits the camera and I have no problem mounting the lens and securing the retaining ring on the 3D-printed board. But I notice that, due to the extra thickness of the 3D-printed lens board, the rear element of the lens does not screw in as far on the thread of the front element as it does when the lens is attached to the original lens board.

The result is that the retaining ring still screws on enough to hold the lens quite firmly on the lens board - but the rear element of the lens ends up positioned 1 to 2mm further back on the thread of the front element with the extra-thick 3D-printed lens board than on the original slightly-thinner lens board.

I am no expert so am concerned about how this slight difference in distance of the rear element from the front element of the lens may effect its performance?

AgX · May 10, 2021

The two lens cells (front and rear) must be screwd together (via the shutter) until they fit neatly.
If a board is that thick that either the board or the retaining ring keeps the cell to be mounted this way, either of them have to be machined to make the cells fit.

I doubt that a home-printed board has true flatness anyway, but fellows with practical experience may make me wiser.

windhorse · May 10, 2021

Thanks for this interesting observation - good to know!

Checking this, I am surprised to notice that when the front and back cells of the lens are screwed completely together with the Copal shutter and retaining ring in the middle but no lens plate, then the maximum gap left between the retaining ring and the flat back of the Copal 1 shutter also appears to be very slightly less than the thickness of the original oem lens plate for the camera I am using (Cambo X2-Pro). So even with the original oem X2-Pro lens plate, the two lens cells appear not to be screwed 100% completely together.

Perhaps my eyes are not as good as they used to be? But if my observation is correct, then one explanation could be that the X2-Pro was designed to be used with more modern lenses and shutters that have a longer thread, giving more space for the lens board and retaining ring than the norm when my old lens was made?

The original X2-Pro lens board appears to be about twice as thick as the old lens board that I used for years with this lens on a Graflex Crown Graphic.

Fortunately, with 3D-printing it is now possible to print a slightly thinner lens board than the original oem Cambo X2-Pro lens board - so I can mount both cells of this old lens neatly on the X2-Pro, even if it was not what the manufacturer had in mind (-:

So far I have no reason to doubt the flatness of the 3D lens board. The material used also seems to be quite strong enough to support this heavy XL-sized lens. But I am not an expert, so look forwards to learning more.

jimgalli · May 10, 2021

Besides the usual jamb nut that most Copal 1 shutters have you can also occasionally find a "flange". With a flange you mount the flange itself with fasteners onto the front of the lens board and then you can change your design so that where the rear element engages there's a chamfer so that you don't have a physical collision between rear element and plastic board. Plus the advantage of a single lens board you can quickly mount and dismount any Copal 1 shutter and mount another one. You can then use the same board for the 90mm and change to a 305mm G-Claron in less than 5 minutes if needed. What's happening is the rear element on the 90 can't seat in the shutter thread because it's physically impacting the jamb nut before that happens. Ebay sellers may have flanges or contact me and I can scrounge one up.

ic-racer · May 10, 2021

Lens separation affects flatness of field (curvature of field).

AgX · May 10, 2021

Schneider in their more modern datasheets got cross sketches of front- and rear-cell mounted to a shutter, with all threads and flanges. These sketches should make clear how the two cells must be mounted. These could be taken as hint at least for lenses from other manufacturers I guess.

windhorse · May 10, 2021

jimgalli said:
Besides the usual jamb nut that most Copal 1 shutters have you can also occasionally find a "flange". With a flange you mount the flange itself with fasteners onto the front of the lens board and then you can change your design so that where the rear element engages there's a chamfer so that you don't have a physical collision between rear element and plastic board. Plus the advantage of a single lens board you can quickly mount and dismount any Copal 1 shutter and mount another one. You can then use the same board for the 90mm and change to a 305mm G-Claron in less than 5 minutes if needed. What's happening is the rear element on the 90 can't seat in the shutter thread because it's physically impacting the jamb nut before that happens. Ebay sellers may have flanges or contact me and I can scrounge one up. View attachment 274157

I think I get it - but am a bit slow (most likely due to lack of coffee). Do I understand right that the "jamb nut" you mention is the proper term for the circular band that screws on the rear thread to lock the lens and shutter to the lens board that I have been refering to as the "retaining ring"?

Also, the flange seems to be a good solution - providing it is less thick than the lens board it is designed to replace, right?

Jim Jones · May 10, 2021

While using new technology to fabricate items such as lens boards is tempting, the experience gained from decades of using old methods often produces better results. Early lens boards were often made from three pieces of wood glued together so the outer two counteracted any tendency for the middle piece to warp. When properly done with ideal wood this works well, but ts not nearly as strong as building up or milling down the boards from plywood. When building up a lens board, make the center hole of the back layer enough larger than the hole for the lens to clear the retaining ring. In lens boards milled down from plywood, a router mounted on a table makes fast work of forming a circular rabbet to provide clearance for the lens retaining ring.

windhorse · May 10, 2021

AgX said:
Schneider in their more modern datasheets got cross sketches of front- and rear-cell mounted to a shutter, with all threads and flanges. These sketches should make clear how the two cells must be mounted. These could be taken as hint at least for lenses from other manufacturers I guess.

Thanks for pointing this out. Now even I understand that the cells must be screwed neatly together.

jimgalli · May 10, 2021

Yes, jamb nut is just another word for retaining ring. Any mechanical solution that allows the area on the rear barrel to bottom out against the shutter solves the problem. Then and only then are you at the factory designed spacing.

AgX · May 10, 2021

Here such a sketch:

Nicholas Lindan · May 10, 2021

Another alternative is to make a lensboard for your camera that in turn takes cheap standard lensboards. Two common ones are Speed/Crown Graphic boards and Linhoff boards. They are both thin aluminum and so there is no problem mounting lenses. Check ebay.

I have also made lensboards from cardboard by laminating up sheets of of good quality cardboard (the board from AmPad Gold Fiber legal pads works a treat) with contact adhesive. I black them with whatever is handy and then apply a few coats of lacquer. They work for years.

grat · May 10, 2021

windhorse said:
The lens is a 90mm Rodenstock Grandagon f4.5 in Copal 1 shutter with a large rear element that screws on to the thread on the rear of the front element and is held in place on the lens board with a standard retaining ring.

Just to be clear, I presume you mean the large rear element screws into the thread on the rear of the shutter?

The lens board is a 3D-printed copy for Copal 1 shutter of the original Copal 0 lens board for my legacy LF camera that I had made by a colleague. We opted to make the 3D-printed lens board a little thicker than the original, as it is made of synthetic material not metal, to ensure it was strong enough to support the big lens.

Understandable, but probably not necessary. I printed a lensboard out of PETG at 30% infill, and it's strong enough to hold my Symar-S 210mm f/5.6 securely. Obviously, your Rodenstock will be larger, but it's more balanced as the rear cell is also large. If I wanted more strength, I'd raise the infill to 50%.

Fortunately, the lens board fits the camera and I have no problem mounting the lens and securing the retaining ring on the 3D-printed board. But I notice that, due to the extra thickness of the 3D-printed lens board, the rear element of the lens does not screw in as far on the thread of the front element as it does when the lens is attached to the original lens board.

That doesn't sound right. Unless you made the lens board about twice as thick, it shouldn't be pushing the retaining ring out far enough to interfere with the rear cell. Can you post a photo?

windhorse · May 10, 2021

Nicholas Lindan said:
Another alternative is to make a lensboard for your camera that in turn takes cheap standard lensboards. Two common ones are Speed/Crown Graphic boards and Linhoff boards. They are both thin aluminum and so there is no problem mounting lenses. Check ebay.

I have also made lensboards from cardboard by laminating up sheets of of good quality cardboard (the board from AmPad Gold Fiber legal pads works a treat) with contact adhesive. I black them with whatever is handy and then apply a few coats of lacquer. They work for years.

Good to know that lens boards made from laquered cardboard work for years, in which case it is likely that 3D-printed lens boards should also work fine, providing the design and care taken with printing done by a knowledgeable person familiar with photography.

Here are a few photos of the two 3d-printed lens boards designed and produced for me by a colleague in Germany.

The first photo shows the designs of the 2 plates he came up with - one, very slightly thinner than the original oem X2-Pro lens board, and another design for a thicker reinforced board with indents to fit the front mount of the camera.

This is the reinforced plate mounted on the camera:

And this is the modified design he suggests for the back of the reinforced plate to ensure it is not too thick to prevent the two cells of the lens from fitting neatly together:

If you have had sufficient patience to follow this, I would be interested to know what you think about these plates, which seem to me to be very good examples of what can be achieved with 3D printing?

grat · May 10, 2021

AgX said:
I doubt that a home-printed board has true flatness anyway, but fellows with practical experience may make me wiser.

Nothing has "true" flatness, but a well-calibrated printer should have tolerances (both accuracy and precision) in the 0.1mm range. The printing surface is usually an aluminum plate, or borosilicate glass-- both of which are pretty flat to start with. Adding a metal shutter with a retaining ring on the other side will usually improve that flatness if you tighten it down a bit.

There are issues that can come up with printing-- flow rate and nozzle height have to be pretty accurate. Too low a nozzle height and the first layer "skips", which will take two or three layers to clear up. Too much flow rate, and your lines are a bit thick, and the nozzle will drag through them on the next layer. This can cause plastic to stick to the nozzle, and eventually, it falls off "somewhere", leaving a bump that will need to be cleaned (although they frequently get knocked over in the infill area, and disappear).

Materials are important as well-- ABS has to be very evenly heated, or it tends to warp, and even without that, it tends to shrink a bit. Nylon's worse, but it's flexible enough I wouldn't use it for a lens board. PLA and PETG are pretty well behaved-- PLA produces very rigid parts, but they're susceptible to heat. PETG is slightly more flexible, but far more durable.

My Wista board is 1.9mm thick and my chinese made Linhof style boards are 2mm thick. I just measured my 3D printed version, and it's 2.1mm thick-- but checking the model it was printed from, it's 2.1mm thick as well, so well done to my printer.

windhorse · May 10, 2021

grat said:
Just to be clear, I presume you mean the large rear element screws into the thread on the rear of the shutter?

yes

grat said:
That doesn't sound right. Unless you made the lens board about twice as thick, it shouldn't be pushing the retaining ring out far enough to interfere with the rear cell. Can you post a photo?

When I bought the preloved X2-Pro it came with an original oem Cambo X2-Pro Copal 0 lens board, which is about twice as thick as the old metal lens boards I had used for years on my Graflex Crown Graphic. Not being very knowledgeable I did not imagine that the thickness of the lens board could be a problem, as long as the lens could be screwed tightly enough on the lens board that it didn't fall off in the middle of a portrait session, a phenomena that perhaps could be best described as "bush photography"

grat · May 10, 2021

windhorse said:
If you have had sufficient patience to follow this, I would be interested to know what you think about these plates, which seem to me to be very good examples of what can be achieved with 3D printing?

You have flat overhangs on both sides of the proposed board-- this makes it slightly more irritating to print-- supports will be needed to keep it from sagging, and supports always mean more cleanup on the final print.

windhorse · May 10, 2021

grat said:
You have flat overhangs on both sides of the proposed board-- this makes it slightly more irritating to print-- supports will be needed to keep it from sagging, and supports always mean more cleanup on the final print.

Thanks for this useful insight that I will pass on to my colleague in case he is not already aware of this, though looking at the difference between the first (draft) and second (finished) photos I posted he seems to have done a very careful cleanup of the boards he sent to me.

AgX · May 10, 2021

grat said:
Nothing has "true" flatness.

Yes, already at writing I was aware that I was very much simplifying.

Anyway, I had in mind reports of printed parts, especially "flat" parts, warping during printing. Thank you that commented on this.

windhorse · May 10, 2021

grat said: ↑
Nothing has "true" flatness.

what about the earth?

grat · May 10, 2021

AgX said:
Yes, already at writing I was aware that I was very much simplifying.

Anyway, I had in mind reports of printed parts, especially "flat" parts, warping during printing. Thank you that commented on this.

It is a problem-- I've thrown out prints for exactly this reason. With ABS in particular, as the different layers cool at different rates (and as I mentioned, ABS tends to shrink slightly), warping is common. Even PETG, my preferred material, can warp, especially if the print adhesion isn't good enough towards the outer edges. I usually have to dial in the print temperature and the bed temperature for each batch of PETG I get.

Still, because it's relatively thin, a lens board is something that's fairly easy to print flat reliably.

grat · May 10, 2021

windhorse said:
grat said: ↑
Nothing has "true" flatness.

what about the earth?

Depends on what you mean-- if you mean is it a flat disk, well, no. If you're asking how "flat" the surface of the earth is compared to it's volume, it's probably about the same as a slightly beat up lens board. With a known maximum deviation of about 20km (Everest vs. Mariana Trench) vs the average diameter of 12,700km, that's a very small amount of bumpiness (0.2%?).

I've seen more beat up lens boards than that.

jimgalli · May 10, 2021

:rolleyes: we gone

windhorse · May 10, 2021

thanks to everyone who contributed to this thread - as well as getting my question answered, I learned a lot

How LF lens board thickness effects optics of lens rear element?

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