Lens logic: If many process lenses are symmetrical, can they be convertible (using 1/2), or does it depend on the design?

Murrayatuptown · Oct 16, 2022

Some old lens patents and catalogs discussed use of one cell of a symmetric(al) lens.

Some lenses that were convertible (i.e., Schneider Symmar (no extra letters) 210/5.6 are satisfactory for some people, are not, it seems for a larger number of people.

Symmetry of lens halves is often a design feature to manage aberrations. I know one terrible candidate for this experiment would be a Metrogon lens. Coma correction for that lens depends heavily on the symmetry of the cell pair.

So, are process lenses that can have one cell unscrewed generally usable as a longer, much slower lens, or is this really a case-by-case situation where the user decides usability?

Anyone have any good experiences doing this?

Thanks

Murray

Nodda Duma · Oct 16, 2022

Design symmetry corrects coma, lateral color, and distortion. Remove one of the lens groups and those aberrations will be unbalanced, causing image degradation.

The amount of image degradation depends on the specifics of the design and cannot be predicted.

Murrayatuptown · Oct 17, 2022

Thanks.

I suppose I can try.

Supposedly workable with half a Rapid Rectilinear, but that is simpler.

Camera is from 1900-1. Original lens was separated. It was smallish & maybe f/6.8 if same vendor as camera (ROC), so 'primitive' look won't be objectionable.

Murrayatuptown · Oct 17, 2022

To me (I shot pinhole-only for a few years, too).

juan · Oct 17, 2022

I have a triple convertible Rapid Rectilinear probably made by B&L before 1917. I find the two individual cells to be soft and lacking contrast, but usable. It probably depends on what you want to do.

Murrayatuptown · Oct 17, 2022

OK.

I'm bandaiding back together an as-is 12" process lens that looked like the process camera it was attached to was dumped out of a truck, then rained on.

Bent barrel at Waterhouse-style-slot (that had a pivoting shutter instead of stops), corroded aluminum threads, and badly rusted iris mechanism (leaves/blades probably brass).

Weighing least of evils after remounting misaligned rear cell in carefully centered M42 & M39 adapters. I can insert fixed stops a little behind (6 mm?) original position (original is inaccessible), in front of, or behind lens, or see what happens with half. There is something wrong with all of these solutions.

Interestingly, the individual cells are both positive and > 24" f.l. (gap between them in original spacing shortens to 12", but extra dim.

I will try fixed stop between them as it's the least wrong of the offenses committed.

Easy to be fooled by ground-glass checks, but at least I'll know by doing that whether to stop trying to resuscitate it.

Thank you

Murrayatuptown · Oct 17, 2022

Rain for 40 days & nights over 40 years.

nosmok · Oct 20, 2022

I once found that the front half of a fast (f/1.9 or so) 75mm Oscillo-Raptar CRT lens became a pincushiony- distorted (but not too bad at the center) 150mm lens that had no trouble covering 4x5 at infinity, unlike the whole lens assembly. It all depends on what you're looking for. FWIW, I find that some historic lenses designed to be convertibles aren't that great when you take out a group, either.

Donald Qualls · Oct 20, 2022

FWIW, my main lens for my Graphic View is a Schneider Componon 150/5.6, which is apparently an enlarging lens (very similar design criteria to process lenses). I've made images with only the rear group (makes it a 265/9-ish) that I found very acceptable, even in corners in a 1200 ppi scan (I haven't darkroom printed one of those negatives to date, but for prints to 16x20 the scan is a harsher test). Key is that the rear lens group is used in its original position on the film side of the shutter; apparently results are much worse (at least with a Componon) if you use the front group alone in its original position or put the rear group in the front shutter threads.

As noted, this can vary wildly depending on the optimizations and precise presciption of the lens, but it's been my understanding that most double Gauss type lenses can do this about as well as a Componon.

epatsellis · Oct 20, 2022

Donald, the Componon lens is very similar to the Symmar, both being a plasmat type design. If I recall correctly the cell spacing has been optimized for higher magnifications at less than infinity. They were ”the” setup for tabletop shooters that had at least half a clue.

Murray, my experience has varied, even within lens types. The early Symmars were touted as convertibles, and typically had both aperture scales on the original shutters. Obviously the casket sets of Dagors and the like were interchangeable, or able to be used singly. I’ve played with a few Artar lenses, half of a 19” illuminates 16x20 with room to spare.

Donald Qualls · Oct 21, 2022

epatsellis said:
the Componon lens is very similar to the Symmar, both being a plasmat type design. If I recall correctly the cell spacing has been optimized for higher magnifications at less than infinity.

Which is exactly what I'd expect for an enlarging lens. I can testify that the rear group does fine at considerable distance -- I've got a negative in which faces can be recognized from roundly a half mile away (photo of a scenic lookout from another vantage). And the whole lens is, IMO, superior to the (somewhat older to much older) Tessars I have in 135 and 150 mm. I tried converting the lens after being told about this similarity several years ago, just after I got it.

I have not tried the front group on its own, either in original position or reversed; what I've read suggests this is close to being a waste of time and film (as well as producing a focal length that might be too long for comfort on my Graphic View II).

Murrayatuptown · Oct 27, 2022

Thank you all for the responses.

Good to hear from DQ & EP! Been a while.

I am still hopeful I can match the cell spacing on this 12" Kodak Process lens rescue effort. They are 0.018" too far apart at the moment but I am asking a machinist to remove that much from the back of the plate the rear cell mounts to. I cannot achieve a Waterhouse stop position exactly where the inoperable iris used to be. One step at a time. Wide open is f/10.

Both cells focus, but I don't have enough bellows for the >24" each single cell produces. This may end up on a 4x5 monorail or a 5x7 project, but despite all the probability it will not work for distance as a primary lens on a lightweight 8x10 camera, I'm at the all-about-the-lens-salvage point at the moment.

Plan B- I do have an old Symmar 210/5.6 convertible (370/12) in a shutter if it will fit the small board (4-1/4" square) requirement & Plan C- a 15" Ilex Process Paragon that will work.

But obviously I'm obsessed with this rescue first.

I have a couple other process barrel-mounters that are too large (wrongly thinking a 6" square board was the target) and too heavy (8x10 camera weighs only 6# so 3-4# lenses are not my first choices).

Patrick Robert James · Oct 27, 2022

I think most lenses will be decent at least. None will be great. You can stop way down, and don't forget to use a filter to correct for chromatic aberration. One thing about converting lenses to watch out for is the bellows draw. On the plus side, the image circle usually goes way up.

Donald Qualls · Oct 27, 2022

Patrick Robert James said:
One thing about converting lenses to watch out for is the bellows draw.

You don't need to worry about that unless you're focusing close. Otherwise, just recalculate the aperture stops for the new focal length (real convertible lenses will have two or even three aperture scales). My Componon 150/5.6 is a 265/9 with just the rear group, and no bellows factor needed.

This is easy to do, too -- measure the infinity focal length of the group you'll use (ideally in controlled conditions), measure the iris aperture, and divide. No magnification factor needed for the iris if you're using a rear group behind the shutter, just hold a millimeter scale up to the iris.

E. von Hoegh · Oct 28, 2022

Nodda Duma said:
Design symmetry corrects coma, lateral color, and distortion. Remove one of the lens groups and those aberrations will be unbalanced, causing image degradation.

The amount of image degradation depends on the specifics of the design and cannot be predicted.

^^ What he said^^.
My 150 Plasmat type Symmar (first Symmars were Dagors, and there were triple convertible Dagor - type Symmars as well) when converted is a decent portrait lens. For landscapes a green filter helps, but don't enlarge it very much. Unscrewing the front element of a Dagor type gives a useable but not great longer lens, AA used the rear component of a 9 1/2" Dagor to make that photo of the rock cliff across the lake, and it looks pretty good, sooo.... they're at least useable. One component of an Artar didn't look too bad on groundglass, but I never made a picture with it.

Donald Qualls · Oct 28, 2022

E. von Hoegh said:
AA used the rear component of a 9 1/2" Dagor to make that photo of the rock cliff across the lake, and it looks pretty good, sooo.... they're at least useable.

With that focal length, it was probably on 5x7, whole plate, or 8x10 -- meaning for normal viewing it has almost no magnification needed (even a 16x20 print from 8x10 is only 2x). That helps a good bit.

E. von Hoegh · Oct 28, 2022

Donald Qualls said:
With that focal length, it was probably on 5x7, whole plate, or 8x10 -- meaning for normal viewing it has almost no magnification needed (even a 16x20 print from 8x10 is only 2x). That helps a good bit.

IIRC it was whole plate or 5x7, the complete 9 1/2 will cover 8x10 with not much movements.

Vaidotas · Oct 30, 2022

Some process lenses are not symmetrical and won’t work as convertible, apo tessars and triplets.
However Jim Galli managed somehow use rear group of longish tessar for fuzzilator.

choiliefan · Oct 30, 2022

Patrick Robert James said:
...and don't forget to use a filter to correct for chromatic aberration.

What type of filter corrects chromatic aberration?

Patrick Robert James · Oct 31, 2022

choiliefan said:
What type of filter corrects chromatic aberration?

Any color filter really. A red for example doesn't correct chromatic aberrations, it just blocks the other wavelengths of light that don't focus at the same point thus making your image sharper.

choiliefan · Oct 31, 2022

Thank you, makes sense.
I was thinking of CA in terms of color film, so guessing there's no miracle cure for that.

Donald Qualls · Oct 31, 2022

choiliefan said:
I was thinking of CA in terms of color film, so guessing there's no miracle cure for that.

Sure there is. Tricolor.

You just shoot three exposures through tricolor filters; each will have only a single color band (you may need to refocus for each exposure, however) and then combine the results either in darkroom or in software.

Of course, you don't really need color film for this...

DREW WILEY · Oct 31, 2022

Here we go again - if it worked for AA, it's good enough for me. Maybe. Ever seen some of those classic old 8x10 negs of his enlarged more than 2 times? - they can look rather funky compared to today's lens and film expectations. Besides, convertible lenses were deliberately designed to be used in ways process lenses are not.

If it's the rock and cliff face I'm thinking of - yeah, a historically significant image - but nowadays one could do better with even 35mm film in terms of image sharpness and less graininess.

choiliefan · Nov 2, 2022

Donald Qualls said:
Sure there is. Tricolor.

You just shoot three exposures through tricolor filters; each will have only a single color band (you may need to refocus for each exposure, however) and then combine the results either in darkroom or in software.

Of course, you don't really need color film for this...

Like those gloriously vivid Technicolor movies of yore.

Donald Qualls · Nov 2, 2022

Or like the first color photos, made in the 1890s.

Lens logic: If many process lenses are symmetrical, can they be convertible (using 1/2), or does it depend on the design?

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