How critical is distance between two lens element?

Discussion in 'Large Format' started by chi_cheung, May 21, 2004.

  1. I have a 19 inch red dot Artar in barrell. I like to fit the two
    elements into a compound shutter. My question is how critical is the
    distance between the two pieces. would 4 to 5mm make a big difference
    in image sharpness?
     
  2. That distance is very critical to maintaining the aberration corrections in the lens. People often mention that unit focusing is sharper than front element focusing in smaller formats; they're measuring a difference introduced by moving the front element about 2 mm away from the other elements in the lens. Depending on the design, you are also likely to change the focal length of the lens.

    Best bet if you want the lens to perform as designed is to send lens and shutter to S. K. Grimes and let the experts there fit things together.
     
  3. Hi,

    I found that increasing airspace between front and rear cells is not critical with artar-type designs. Increasing air space with these lenses (artar, apo ronar, apo germinar etc.) may be a means to optimize corner performance at infinity. The increase/variation of air space may be up to several millimeters with your focal length, you will have to refocus, but performance will be ok. So just give it a try with the compound shutter !

    Best regards

    Joerg
     
  4. In addition to the spacing between the cells, the distance from the aperture to each cell can be important. How critical these dimensions are depends on the lens design. Some lenses (not Artar types) are made with spacer washers to adjust the separation -- these washers are a small fraction of a mm thick.

    Someone with the skill and equipment to machine an adapter to connect the threads of the lens cells to the threads of a shutter, or to machine new thread on the lens cells, should be able to hold a much tighter tolerance on the cell spacing than a few mm. If the fitting is sloppy and changes the spacing, you might get lucky and improve the performance for distant objects, or, if the change in spacing is too much, or in the wrong direction, you might make it worse.

    If you are looking for an easy way to have a shutter with your barrel Artar, you might consider mounting it in front of a shutter. Much less precision is required this way. This will require a larger shutter than center mounting. One shutter that is frequently used for this purpose is the Packard.
     
  5. I haven't mounted any process camera lenses into shutters (yet) but I have some experience taking lens elements for view camera lenses and getting them into shutters that I found separately.

    My rule of thumb is to measure from front rim to back rim of the lens in the barrel before dismantling, preferably with a dial caliper. (These are available very inexpensively these days. I picked one up at a hardware store -- Swiss made -- for little over $20. I use it all the time, don't know how I got along without it. That one's in millimeters. I picked up a Chinese-made, all-metal one, that measures inches, from Harbor Freight, I think also under $20. I'm all set....)

    Then, if the lens elements screw into the shutter, measure again. If the measurement from front rim to back rim is the same, then you're 99% certain to be good to go.

    Early on, there was no standard for shutter/lens thread dimensions. Sometime I think in the '40s or '50s, by some miracle of communication (probably as a result of frustration) the shutter and lens manufacturers settled on standards. So the chances are good that a fairly modern lens will fit in a fairly modern shutter of the correct size.

    I've put a 300mm Symmar convertible into a Compound -- even found an aperture scale for it! Also a 240mm Symmar convertible into an old Compur. Fits like a glove, and the spacing on both are just right.

    One more thing: Process lenses are optimized for *very* close focus. I used to operate these big cameras. I routinely made shots anywhere from 25% to 400%, but more typical was between 75% and 300%. I did read somewhere some months ago that taking a process lens and changing the spacing a little bit will bring it more into line for shooting at infinity focus. I just can't remember for sure if you decrease or increase the spacing, but I believe that you decrease it. (If a spacer is present in the barrel, try removing it and see how the lens does at infinity, especially at the corners, compared to with the spacer. This might answer that one.) But the increment of spacing adjustment certainly isn't on the order of 4 or 5mm -- it would probably be in tenths of a millimeter.

    In any event, if your front-to-back measurement is okay, then check the elements one at a time, see if they're close to being the same distance from the diaphragm as they were in the barrel. If the front-to-back measurement checks out, odds are the spacing at the diaphragm is spot on too.

    THEN, put it on the camera, give it some front rise, and see how the corners look! (Of course I forgot to mention -- put it on the camera while it's still in the barrel, give it some front rise, see how the corners look!) Compare this test with how it looked in the barrel, and that'll tell you all you need to know.

    You can pull all kinds of crap with a lens and things will still look great towards the center of the image. But the truth of whether or not your lens mounting experiment worked will be told at the corners of the ground glass, especially with some front rise. If you've still got a good flat field focus, and it covers as well as -- or better than! -- in the barrel, then you've got it made.

    Hope this helps. Good luck!
     
  6. This is an excellent question I have not seen posted before. I do not know about other focal lengths but I thought this was as good as place as any to mention the effect on performance of the 65mm f8 with respect to element spacing. My personal experience trying to mount an SA into a different shutter has been that even 1.5 to 2mm difference means a huge loss in depth of field outside the middle 3" x 3" area. That centre area remained perfectly focussed but the outside would be completely unfocussed, there may have been excessive light fall off as well but I did not test this properly at the time. Good luck.
     

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