How to qualify am LF lens upon receipt from vendor

Discussion in 'Large Format' started by steve_kofol, Sep 9, 2003.

  1. I'm about to buy my first LF lens for a 4x5 from a retailer by mail.
    I'll be buying a Schneider APO symmar "L". The retailer has a 7-day
    return policy. My question is: how do experienced LF photographers
    qualify a lens for acceptance during this period? Should I, for
    example, be shooting resolution charts (as from, for example,
    www.normankoren.com)? If so, how do I decide what is "good enough"?
    Or should I do something less quantitative--like shoot some scenes I
    have shot in the past with rented lenses? Or, is there some other
    prudent action I should take during the retailer's 7-day grace
    period?

    What, if anything, do you do? And how satisfied are you with your
    procedures?

    Thanks in advance--Steve Kofol
     
  2. I shoot a couple sheets of Velvia (at ei 40). If it knocks my socks off, I'm happy.

    -j
     
  3. The resolution performance of the lens can be masked by ground glass misalignment. Tell us more about your camera. Did you buy it new or used? Does it have a Fresnel brightening screen or not? If not, was it originally designed to have a such a screen. If yes, was it originally designed to not have such a screen? All of these things can be clues to whether or not the gg has been messed with. If it has, all bets are off on lens testing. I witnessed many LF users repeatedly trading lenses in search for a sharper one, only to later discover their gg was so far off, there was no hope of ever making a sharp negative.
     
  4. Since this is an "L" lens I assume you bought it new. There are two separate issues: is the lens defective and do you like the lens? With a new lens I wouldn't feel the need to do the same checking for defects that I would with a used lens. I'd shake it a little to see if anything rattles, trip the shutter at each speed while listening to see if the speed progression sounds right, make sure the press release works, make sure the apertue ring operates smoothly (some stiffness is normal with a new lens), hold it at an angle to the light to make sure there aren't any obvious defects in the coating, then hold it up to a light and look through it. You'll be discouraged to see the amount of dust in there but don't worry, dust is normal even with a new lens. Just look for anything that appears strange or like it shouldn't be there - e.g. paint chips, fungus, etc. I wouldn't bother shooting a chart, you presumably have researched the lens before buying it so you already know things like resolution, image circle, etc., especially with Schneider which has a lot of detailed information about this kind of thing on their web page.

    Apart from defects, there's the question of whether you just like the lens or not. Does it fit in your pack, does it seem heavier or bigger than you anticipated, can you see the image on the ground glass o.k., etc. You could make a few photographs too but if you do that make sure you enlarge them to at least 8x10, preferably larger. The bottom of a coke bottle will make an acceptable snapshot size photoraph. It's highly unlikely that the lens will be unacceptably sharp or have other problems since it's presumably new and made by Schneider but you never know. The first batch of the Schneider 80mm Super Symmar XLs had a focusing problem.
     
  5. Mr Zeichner,
    I am inexperienced, but surprised by your response suggesting the ground glass would affect the lens performance! My camera is an old Arca-Swiss, probably from the '70s, and has been heavily "messed with". There is no Fresnel lens in the ground glass. All I know is that I have succeeded in producing an image with a rented Schneider lens mounted on this camera that I think has reasonably decent sharpness--but again, since I am learning on my own, no expert has seen my work. I would be pleased to hear more from you on this subject.
    Thanks,
    Steve Kofol
     
  6. Steve, unless your buying used, I doubt there will be much of a problem. On a used lens, the glass should be clear with no damage to the outer elements.Shine a light through and hold up to light. Shutter speeds should also be tested. The one second should be one second, if it sticks or is longer, the shutter needs service. Seems to me the biggest question should be is this the best focal length for you to suit your needs. Ideally you can learn and shoot with this one lens. Less gear is a big advantage in large format.
     
  7. Steve,

    Robert Zeichner has a definite point about ground-glass alignment: If the sharp image on the ground glass is in a different plane than the film, then there is no way to get a sharp image. Having the ground glass in a different place than the film usally happens when a camera has been modified.

    So, if you are already getting razor-sharp images from other lenses, your ground glass is probably in the right position. If your images are always on the soft side, it may be misaligned. You need to determine this before trying to test a new lens. A simple test is to do a close-up shot of a ruler laid out at a horizontal angle to the film plane. Focus on a specific spot (e.g. the 6" mark) and shoot wide open. If your developed negative shows the 6" mark as the sharpest, then the ground glass is properly aligned. If the sharp point is another than the 6" mark, you will have to change the position of your glass by adding or removing shims until the results are correct.

    All of this, however is a different issue than testing a new lens. Let us assume that your ground glass is correctly positioned and go on to the other considerations. First, as mentioned above, do you like the lens as far as size and handling goes. If not, send it back without further testing. If you do, then shoot some subjects with very fine detail. Take several shots, focusing carefully between each, develop the negs and examine them under the most powerful magnification you can. Microscopes are good, but I have found that using my Peak 40X enlarging magnifier with my enlarger head racked all the way up gives me more than enough magnification. Compare the detail on your test negs with other negs from lenses that you are happy with in the center, corners and edges. If there is significantly less detail and contrast, then the lens may be defective. If OK, then go ahead and purchase it. I know this is "unscientific" and down-and-dirty, but subjective comparison tests with with lenses of proven results are really pretty good, even though you can't quantify the results. If you have a test chart handy, by all means use that too. Keep in mind, however that test charts are never shot at infinity. If you plan on using your lens a lot at infinity focus, shoot some sharp horizons (trees and little branches on hills, etc.) as well.

    Good luck, ;^D)
     
  8. I think part of the premium you are paying for here (a brand new schneider lens) is not having to worry about this.
     
  9. As President Regan used to say, "Trust, but verify!" Things do go wrong at the factory.

    Not having an optical bench handy, nor the expertise to use one, I would be more interested in the performance of the shutter. I have a Calumet shutter tester, and this enables me to get all the data I want. Let's assume you have one of these. There are two characteristics in which I would be interested. To check the shutter, I would take about twenty measurements on each speed. Pick an f-stop, like about f16 or f22, that's reasonable for most LF photography. Then, I would evaluate the following.

    CONSISTENCY: Regardless of whether or not the shutter, on average, is at the correct speed, this evaluation determines the degree to which the shutter is firing at the same speed each time it's tripped. Based on data I've taken on all my used lenses, I would want the difference between the smallest and largest times in my 20 data points for each speed to be absolutely no greater than 1/5th stop. All speeds on almost all my 15 year old lenses meet this criteria. I rarely have them CLA'd. If "M" is the longest time and "m" is the shortest time for a given speed, you can determine the difference in stops by computing the quantity STOPS = [LOG(M/m)]/LOG(2). (You can use any log function on a calculator for this calculation.) This number should be no greater than 0.2 stops. If I were especially concerned about consistency, I would want this number to be no greater than 0.1. A large majority of speeds on my shutters meet this tighter criteria.

    ACCURACY: This is whether or not each speed is firing at the stated speed. Personally, as long as I know my shutter speeds are consistent, I don't care a great deal about accuracy. I want shutter speeds to be accurate enough that I have a full range of options in selecting shutter speeds. Beyond that, if I know how far off in stops my exposures are from that stated speed, then I can accomodate these differences by adjusting the aperture at the time of exposure. However, if you don't want to fiddle-faddle with these post-composition adjustments, then do the following for each speed. Take an average of the 20 measurements, let's call it "A". Let "T" be the stated speed. (e.g. 1/60th sec = 0.0166 sec.) Calculate STOPS = LOG(T/A)/LOG(2) for any log function on the calculator. I would want this estimate of accuracy to be in the interval [-0.1 stops, +0.1 stop]. That is, no greater than 0.1 stops off.

    If your speeds meet these criteria, then you can be reasonably certain that you will be within 1/5th stop of the actual speed each time you trip the shutter. Note that this doesn't account for the effect of cold on your shutter if you are shooting outside, etc. For myself, I would be concerned if I were off any more than this on any speed. If I decide to adjust for inaccuracy (second test above) each time I take an exposure, then I know that I will be within 1/10th stop of correct. I spoke with Schneider Optics technical support, and they didn't know their own specs for accuracy and consistency for the Copal shutters they sell with their lenses. I was a little surprised at this.

    If you don't have a shutter tester, I would say that this is a good investment. Once I collected data, I was pleasently surprised at how well my old shutters perform.

    If your camera g.g. is in excellent alignment, I'm wondering if one couldn't also check lines/inch using a chart. These charts come up on EBay on occassion. I've not done this, so I can't offer too much comment.
     
  10. With a new lens, elaborate testing or measurements shouldn't be needed. I give all lenses a careful visual inspection, including shining a light through the lens to look scratches. I operate the shutter at all speeds. With experience you can recognize major inaccuracies. With a new Copal shutter, the chances are that if it sounds right, the speeds will be accurate enough. With a new lens, I usually don't bother measuring the speeds. (With a used lens, I might measure the shutter speeds.) I take a few photos and examine the films with a loupe. I don't bother with test charts. A good test subject is a brick wall.

    If you use flash, you might want to verify the flash synch. Connect the flash to the PC terminal and point the flash at a wall. Hold the lens, set at maximum aperture, so that you can view the wall and trip the shutter. You should perceive the flash through the lens. The chances that the synch won't work are very low, but the test is very easy to do.

    I have returned new lenses only twice. Both times were due to problems with the Copal shutters. Perhaps the quality control at the factor has gotten worse. On one shutter, the speed ring was extremely diffcult to turn, so stiff that the lens would have certainly rotated in a lensboard, if I had mounted it. Another shutter jammed after a few dozen firings. Despite the quality control problems, good Copal shutters are very reliable -- I have never had any problem with a Copal shutter that passed initial inspection.
     
  11. got any fresh polaroids?
     
  12. Steve, Ground glass/film plane coincidence is of paramount importance if one wants to get sharp results. There is a common misconception that shooting at small apertures will make up for this kind of error, but that is simply not true. The gg needs to be in just the right place for everything to work right. Film holders all have small amounts of error, but that is not surprising since they are relatively inexpensive pieces of plastic and aluminum. Most of them will pass the ANSI spec of +/- .007" for 4x5. If the gg is .007" shallow, a film holder with the opposite problem will position the film .014" from the ideal plane. This is why the gg needs to be perfect or as close to it as possible...so the average film holder will not place the film out of the accepted range of tolerance.

    Now back to your camera. Suppose when new, your camera had a Fresnel brightening screen between the gg and the lens. Many cameras have been designed that way. The typical Fresnel is about .060" thick and because it behaves like a lens, it shifts the image passing throught it rearward by an amount roughly equal to 1/3 the thickness of the screen or about .020". The designers of the camera took all of this into account when machining the pads on which the viewing screen(s) are mounted. Now let's just say a former user didn't like Fresnels. Not everyone does, you know. So, they took it out and just replaced the gg without properly shimming it. That would put the gg .060" closer to the lens, (the thickness of the Fresnel that was removed) minus the .020" that the image was shifted rearward when the Fresnel was in place. That means the gg would need to be shimmed rearward by approximately .040" to put it correct plane. I just tested all of this theory, by the way, when I removed the Fresnel from my Graflock 4x5 reduction back on my Deardorff. I used my gg alignment test target to confirm proper position and after a couple of minor "adjustments" was able to achieve perfect results. The processed negatives matched what was seen on the gg exactly. (I left the camera set up on the target while I processed the film so I could make this comparison).

    So, that is why it is important to know how your camera was originally designed with respect to the gg and also why a gg test with an appropriate target will confirm its position. When that's correct, you'll be ready to make meaningful lens comparisons. I'll email my article on gg alignment to you with instructions for building a test target.
     
  13. Mr. Zeichner,
    That was a totally lucid explanation. Thanks. I eagerly look forward to your email, and will get out the appropriate machinist instruments to measure the relationship between my ground-glass plane and the film plane with my fuji quickload. I'm quite embarrassed this potential problem didn't occur to me.

    On the other hand, I do usually get reasonably sharp (to my inexperienced eye) positives when I have used this camera with a rented lens. However, my image-sharpness results are not consistent--suggesting I still need to improve my focusing technique.

    Thank you very much for moving my thoughts in this direction.
    Steve
     
  14. Just one followup for anyone else out there who might be listening: After the coaching in this thread from Bob Zeichner (and some additional coaching from Bob off line through direct email), I measured my second-hand Arca-Swiss to determine the relative position of the ground glass plane compared with the film plane of my camera. To my surprise I found a difference of 0.075"--much more than can be tolerated. Interestingly, until I measured the camera, I was oblivious to the discrepancy. Prior to repairing the camera, I had shot approximately 25 color reversal transparancies, and had not detected the problem from the quality of the images! Why had I failed to detect the problem, you ask? The answer is: lack of experience. I have never seen an example of an excellent, in-focus, 4x5 transparency.....I guess I need a teacher!

    But, being the sort who likes the experience of discovery, I'll probably plod along at my own, slow pace, learning as much as I can from books and from you all on this site. But, here is one example where (the frequently maligned) shooting of resolution patterns or (as Mr. Zeichner suggests) staggered dollar bills might be just the ticket to self-taught focusing.

    All I can say is: "Thank you all, and, especially, thank you, Bob Zeichner."

    Steve Kofol
     

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