Process lenses

[Pere Casals]

Reread what I wrote. Apo-Nikkors are either dialyte types (symmetrical) or tessar types (asymmetrical). Both are quite good at distance. Process Nikkors are 4/4 double Gauss types and are poor at distance. Not similar at all, unless you regard all lenses with four elements as essentially the same. I don't.

Dan, I don't need to reread it, near I can recite your article from memory, it explains it very well. I was pointing external similarity, anyway now there is a (not cheap) Process-Nikkor 210mm at ebay and front element it is way more curved. Well, it covers 74º at f/22, and not 45º like the APO.

[Dan Fromm]

As I said, 4/4 double Gauss type and a lens not to buy or even accept as a gift.

[lenser]

Thank you for all the replies!

[Bernice Loui]

What metric or criteria would be used during the image making process to determine the amount of "unscrewing" the front lens cell to optimize image quality -vs- subject distance?


Bernice

About long distance subjetcs with process lenses, some ULF photographers do unscreew a bit the front cell (when it is possible) to get best peformance. Distance between cells is a parameter that can be optimized for particular subject distance, and also for corner vs center performance...

[Drew Wiley]

A common misunderstanding involves coverage. Published image circles for these kinds of lenses are generally related to precise apochromatic dot reproduction even at the corners of the field - a much stricter standard than what applies to general-purpose taking lenses. An unrelated consideration not mentioned yet is the character of the out-of-focus rendering. For example, I sometimes use a 360 f/9 Zeiss tessar process lens for its lucious bokeh, whereas my Apo-Nikkors render a relatively obnoxious harsh look in this respect.

[Bernice Loui]

Or the mis-understanding of illumination circle -vs- actual image circle of design criterial. More than a few "process" less have a significantly larger circle of illumination with "good enough" definition for photographic image making but not good enough for critical graphics process work.

There was a time not too long ago when precision grid films were made using the photographic process. These high precision grids could be more than a few feet long and high. Grid precision is measured in thousands of an inch from edge to edge and the line width cannot have only minimal geometric distortion and line width variation. Only high quality process lenses were used for this high precision photographic work. Similar would be monolithic semiconductor make making. These mask were once hand cut on rubylith that were up to 4 feet x 8 feet in size, then photographic reduction to produce microscopic sized photographic mask used in the semiconductor making process. Or color separation films of Red, Green, Blue of identical image size to make plates for color printing. These are a few of the extreme demand made on process lenses that are a world apart from the typical photographic image making lens.

Another area where these "process" lenses were commonly used was studio table top images. Rodenstock offerings of APO Ronar (later versions were multicoated) , and Goerz APO Artar and later Schneider APO Artar in shutter were very popular with studio table top image makers for their high definition the common Plasmant cannot match.

Not all "Process" lenses have poor out of focus rendition, it depends on the specific lens design, aperture shape and more.

Point being, there appears to be an obsession with lens image circle coverage while the other lens performance factors are often ignored, not considered or completely forgotten.



Bernice

A common misunderstanding involves coverage. Published image circles for these kinds of lenses are generally related to precise apochromatic dot reproduction even at the corners of the field - a much stricter standard than what applies to general-purpose taking lenses. An unrelated consideration not mentioned yet is the character of the out-of-focus rendering. For example, I sometimes use a 360 f/9 Zeiss tessar process lens for its lucious bokeh, whereas my Apo-Nikkors render a relatively obnoxious harsh look in this respect.

[Drew Wiley]

Even Apo El Nikkors are still being mfg for tech applications, but only in fixed aperture for designated magnifications. It's been about 25 yrs since I've heard of a 360 Apo El being sold for enlarging 8X10 film, and it went for $11,000. Once in awhile a 105 or 210 still turns up, covered for high-end repro of paintings with scanning cameras. But utter overkill for ordinary use. Apo Nikkors are only a stop slower and 1/20th the cost.

[Pere Casals]

What metric or criteria would be used during the image making process to determine the amount of "unscrewing" the front lens cell to optimize image quality -vs- subject distance?

Bernice

Hello Bernice,

For DIY the criterion is empiric, it consists in measuring resolving power at center, corner and mid while you unscreew. It is possible that image on Ground Glass (granularity) does not allow a perfect adjustment (with a magnifier) because GG do not show all possible sharpness. Of course a manufacturer may know a theoric increment in cells best spacing depending on focus plane distance.

Manufacturers like Rodenstock (for some models) were including a shim or more (or none) for some lenses, I guess they used fancy optical equipment to find optimal cell to cell distance for each unit, perhaps they also searched for good combinations of rear and front cells with compensating errors.



In the DIY realm, let me suggest 3 ways:

1)

Attach an eyepiece from a telescope or microscope (I use a x20 nikon eye piece from nikon microscope) in a lensboard, place that lensboard in the camera rear.

Place a USAF 1951 target at the intended distance, 1:30 magnification is good enough for distant subjects, from 1:30 you may not notice a change.

With rise-shift place the eye piece in the center, corner or mid of the image circle. Adjust tripod ball head to see the resolution target.

Check resolving power (group/element) for each half tour increment, check focus (belows extension) each time, as you unscreew you should check focus again.

This test do not check field flatness, as you get optimal focus for each reading, but it overcomes any camera alingment missmatch.

Then, if you know the good number of tours you also can know the shim thickness, just multiply the thread pitch by the number of tours.



2)

Another choice is using a DSLR in the back of view camera. Let me explain how I did some experiments for that.

I attached an extension ring (those for macro) ito a lens board, in that ring I place a Nikon D3300, this is better than a D3200 because the 3300 do not have the optical lowpass filter. A cheap D3300 has a high pixel density around 250 pix/mm IIRC.

Then the routine consists in placing a USAF 1951 target at the intended distance, for infinite distance optimization working at 1:30 magnification is enough. Then you unscreew half tour each time, then you have to measure resolving power (USAF 1951 group/element ) for center, mid and corner, so you use the camera rise/shift to place the camera sensor in each point, you check focus always before taking the digital shot, so you avoid camera alignment issues, as a view camera is not lab equipment. In this way you are not able to measure field flatness... but still you can find the best shim. You have a collection of digital images to find the right cell to cell distance.






3)

Another way, straighter, it consists in just taking series of shots with a sharp film (TMX, CMS 20...) , each with the front lens unscreweed more or less, but here you also have the effect of camera alignment, film flatness and film plane to GG matching.



Regards

[Bernice Loui]

Really, Really......


Bernice

Hello Bernice,

For DIY the criterion is empiric, it consists in measuring resolving power at center, corner and mid while you unscreew. It is possible that image on Ground Glass (granularity) does not allow a perfect adjustment (with a magnifier) because GG do not show all possible sharpness. Of course a manufacturer may know a theoric increment in cells best spacing depending on focus plane distance.

Manufacturers like Rodenstock (for some models) were including a shim or more (or none) for some lenses, I guess they used fancy optical equipment to find optimal cell to cell distance for each unit, perhaps they also searched for good combinations of rear and front cells with compensating errors.



In the DIY realm, let me suggest 3 ways:

1)

Attach an eyepiece from a telescope or microscope (I use a x20 nikon eye piece from nikon microscope) in a lensboard, place that lensboard in the camera rear.

Place a USAF 1951 target at the intended distance, 1:30 magnification is good enough for distant subjects, from 1:30 you may not notice a change.

With rise-shift place the eye piece in the center, corner or mid of the image circle. Adjust tripod ball head to see the resolution target.

Check resolving power (group/element) for each half tour increment, check focus (belows extension) each time, as you unscreew you should check focus again.

This test do not check field flatness, as you get optimal focus for each reading, but it overcomes any camera alingment missmatch.

Then, if you know the good number of tours you also can know the shim thickness, just multiply the thread pitch by the number of tours.



2)

Another choice is using a DSLR in the back of view camera. Let me explain how I did some experiments for that.

I attached an extension ring (those for macro) ito a lens board, in that ring I place a Nikon D3300, this is better than a D3200 because the 3300 do not have the optical lowpass filter. A cheap D3300 has a high pixel density around 250 pix/mm IIRC.

Then the routine consists in placing a USAF 1951 target at the intended distance, for infinite distance optimization working at 1:30 magnification is enough. Then you unscreew half tour each time, then you have to measure resolving power (USAF 1951 group/element ) for center, mid and corner, so you use the camera rise/shift to place the camera sensor in each point, you check focus always before taking the digital shot, so you avoid camera alignment issues, as a view camera is not lab equipment. In this way you are not able to measure field flatness... but still you can find the best shim. You have a collection of digital images to find the right cell to cell distance.






3)

Another way, straighter, it consists in just taking series of shots with a sharp film (TMX, CMS 20...) , each with the front lens unscreweed more or less, but here you also have the effect of camera alignment, film flatness and film plane to GG matching.



Regards

[Luis-F-S]

Really, Really......
Bernice

Guess some people have nothing better to do...............