Confusion over abundance of contradictory information re: process lens optimization

[Paul Kinzer]

Before I even get started, I know that the best way to find the answer I might be looking for is to just try things out and see. But I'm curious about what folks who have already tried have found.

I own a 19-inch Apo Artar. I got it as a barrel lens. I just got an Ilex #4 shutter in the mail. The shutter works very nicely, though the Artar lens cells do not screw directly into it. I did not expect they would, though wouldn't that have been nice? Still, the way they don't fit is the best way: the cells are smaller than the adapters that are currently in the shutter (the adapters were for mounting the cells of a 9-1/2 inch Conley Series VI lens; anyone want them?). This means that I can simply use something -- layers of tape to experiment; thin strips of metal for something more permanent -- to center the cells in the adapters; and I can easily tap holes for set screws into the adapters to grip the cells. It will be easy, and I could do it with just my drill press. Or, eventually, once I practice, I can make better adapters for the adapters on the mini mill I recently bought (making entirely new, threaded adapters will not be possible with my skills or this machine).

But I'm writing this primarily because the cells don't fit in another way: the barrel the lens came in is 1.55mm thicker/taller than the shutter with its adapters. That will also be an easy fix, with a simple washer I could make or maybe just buy. My question is: how accurate do I need to be? In fact, do I even want to be? I've read that process lenses were sometimes optimized for infinity use by changing the distance between the cells. But I've read two contradictory statements: some commenters have written that, in order to make them optimized for further distances, rings were used to place the cells farther apart, but in a couple of places, I read the opposite (I have been skimming in lots of places, so don't have links; sorry). I wondered if the latter were just typos, or poorly written, but don't know.

Of course, I've also read that they take lovely images without any modification at all, so it would make sense to get as close as I can to the original.

But another thing I've read is that, at least with the dyalite type lenses, distance is not as critical as with other designs. So, even if I do try to get close, just how close do I need to be? I have some material I could make a shim out of that is 1.5mm thick (nominally; I haven't checked it yet). I imagine that being within 0.05mm (0.002") would be close enough! But what do I know?

I plan to use this lens for contact prints from a 14x17 camera I'm going to build. It's going to be a good year, I think, before the camera is ready, and I'm just curiously reaching out here. Maybe this could clear up some muddiness for others as well.

[LabRat]

OK, I'll take the easy part by saying that the best (at home) way to mount would be to use the existing barrel and figure the way to mount the shutter behind or in front of the lens... Without careful machining, you will loose that factory alignment, so keep it in the barrel for best results...

Steve K

[Leigh]

Re-mounting a lens is definitely NOT a trivial task.

One problem that never gets mentioned is cell rotation. It's quite common for the optical axis of a cell to NOT correspond to the physical centerline of the assembly. When the lens was originally assembled, the cell axes were lined up as best as was possible to achieve maximum performance. Unless you have a good optical bench, and experience doing this, you'll just screw it up.

The other more obvious problem is cell spacing. That must be maintained exactly if you expect it to work.

- Leigh

[Pere Casals]

...Artar lens cells... I plan to use this lens for contact prints from a 14x17

Hello Paul,

The 19" Artar may perhaps deliver some 50lp/mm resolving power on film, if you stop beyond f/32 then diffraction is what will limit resolving power.

... but as you are to contact print you only need 10 lp/mm as much, to get 7lp/mm on paper. Human vision won't see a difference if not using a magnifier. So yo have a wide margin...

Anyway if you want optimal results you may use a USAF 1951 (or other) resolution target to measure what practical resolving power you have http://www.imatest.com/docs/sharpness/

I'm pretty sure that 0.05mm shift won't make a measurable difference, this 1/15 of a tour (if it was a Copal 3 thread), when I did tests with that with my lenses I had to move way more to see a performance difference, but see it on your own: measure your thread pitch and unscreew to get 0.05mm difference, then see...

To measure lp/mm you can attach a nikon D3300 style dslr in the back of the camera, use shift-rise to explore the circle boundary. I placed a F mount extension macro ring to a lens board to get the camera in the back of the view camera, in fact the ring can also be placed off-center to reach better the circle boundary. Focus again before to taking another digital shot to overcome alignment.

But the way I'd recommend is to use an eyepiece in the back of the view camera (for example a $20 cheap microscope ocular, 10x or 20x, amazon, ebay...) also using a lensboard to place the ocular in the back of a monorail view camera. You would directly observe the resolving target or any scene with detail like if it was a x20 enlarged print: straight, cheap, fast and significative !



Inter cell spacing can be adjusted for different reasons:

- Best average overall performance.

- Center vs Corner performance

- Close vs Distant subjects

In general a process lens (optimized for close objects) "may" work slightly better for distant if you unscreew a bit a cell to increase cell spacing, IIRC some ULF shooters did that.


For remounting, as you have a mini mill, take a black Teflon (or Nylon or PE 1000) rod and make your barrel. The hole in the barrel has to be some 0.1mm smaller than the cell outer thread, then the cell will screew in but making the thread while you screew, you can polish the barrel inside until the cell screews in with moderate force. In the barrel's hole, the first (some) 4mm of the hole can be wide enough to allow the cell enter easily, without screewing, in this way those 4mm works centering the cell before you press and screew to make the thread, again, the inner size of the hole has to be the right to screew in with moderate force while remaining secure, you can add a fastener to block it. This way worked just perfect for me.



Regards

[Paul Kinzer]

Thanks for the comments!

I realize the lens will not be perfect when I'm done with it. But that's okay: it was not perfect when I started. I'm sure it has been taken apart in the past, and I had to completely disassemble it to get it cleaned up. I have worked with telescope objectives, and so I know that alignment of the elements is critical for optimal performance. But as I said in another thread, telescopes need to be able to magnify their images up to hundreds of times. A camera lens does not. This lens was designed to take razor sharp images at 1:1 distances. I won't be taking 1:1 images with it (well, I don't think I will), and I will be stopping it down far enough that diffraction will effect image quality, too. I am aiming for 'good enough'. This is all experimental, and that's much of the fun for me.

Still, I did ask for comments, and I appreciate them. I'll follow Pere's advice for checking performance with an eyepiece (I have more than a dozen telescope eyepieces, and access to many more). I had already been thinking of using 'self-threading' plastic as an adapter material. It's nice to have someone confirm the possibility.

[Pere Casals]

checking performance with an eyepiece (I have more than a dozen telescope eyepieces, and access to many more).

One thing I forgot... the eyepiece will work ok when exploring the circle center, if you go to the boundary of the circle (to the side) you will need to use a swing to point to the lens and to see. This won't you allow to check projection flatness, you may need to check focus again... but for the rest it's a very nice way to see what a lens does.

The telescope ep should work ok...

Oculars can be infinite focused or at a distance, but this is not important for the center of the image you view in the ocular, and this is where you check the quality.

Regards

[Jim Galli]

If your original intent was to make visual art, you're on a rabbit trail going nowhere. Lens designers and opticians have the knowledge and tools to adjust distances for predetermined specifications. I buy stuff other people built and employ it (badly) in my pursuit of artistic expression. Sometimes I abuse and abandon original designs, but my only criteria is what is on the ground glass. Still, there's nothing wrong with fiddling with stuff, if you're having a good time fiddling. Carry on. But it takes a gazillion dollars and a lot of math to do what the optical engineers do. A Conley 9 1/2" Series VI is a fine old lens.

[Paul Kinzer]

Even the Conley was not in its original setting: The aperture scale was paper, with the numbers written by hand.

I agree about the rabbit hole leading nowhere. I like that. Though I tend to think more that it can lead anywhere, but that I cannot make it go just exactly where I want it to. Since I don't really know where I want it to go (or at least don't have the skills to get myself there), that's more than okay with me. If it leads to pleasing (to me) photographic results, that's all I ask.

Sorry for the tortured metaphor!

[Pfsor]

"This means that I can simply use something... to center the cells in the adapters; and I can easily tap holes... It will be easy,....
But I'm writing this primarily because the cells don't fit in another way:
That will also be an easy fix,
My question is: how accurate do I need to be?"

"But I've read two contradictory statements:...I wondered if the latter were just typos, or poorly written, but don't know.
Of course, I've also read that they take lovely images without any modification at all,..
But another thing I've read is that, at least with the dyalite type lenses, distance is not as critical as with other designs.
But what do I know?
It's going to be a good year..."

Yap, a good year, an easy one... The reason you read contradictory statements on the forum is perhaps that it is just full of those who see it, well, so easy...
A good and easy year 2018 to you!

[Joerg Krusche]

Symmetrical lenses of dialyte type .. artar, apo ronar etc. .. are acc. info from people that were in the know .. not critical as for spacing .. slightly increasing for infinity performance at moderately closed diaphragm may even be a good idea .. stopping down to 22-32 .. has the same effect .. as I say that is what I was told by people who made such lenses ..