Lens testing

Discussion in 'Medium Format' started by john_braud, Aug 19, 1998.

  1. I couldn't find if this was suggested before but there always seems to be much talk of lens testing techniques. All methods seem to use available light, varying shutter speeds and apertures while the camera is practically bolted down. My question is:

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    Why can't flash be used (which can be as short as 1/30,000 sec for most good flashes)in a darkened room? This speed would certainly eliminate any camera shake whatsoever and isolate the performance of the lens based on inherent sharpness.

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    Sorry if this has been suggested before but I wasn't able to locate if it was.

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    John Braud
     
  2. Yes, flash can be used. And it's a good idea IMHO.
    Check out the tests I did at:
    http://db.photo.net/bboard/q-and-a-fetch-msg.tcl?msg_id=0005l3
     
  3. I only test lenses this way any more. I lock up the mirror, open the camera shutter, move away from the camera and wait for at least 5 seconds. Then I fire the flash by hand off camera. I use the ratio setting on the flash for most F-stops, but for the smallest, I move closer to the target and use two to four flashes to get enough exposure. I use Tech pan for all tests.

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    I went to this method, when I found out that my P67 was too heavy for my then best tripod. I upgraded to a heavy bogen after that.

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    Camera shake, mirror flop, etc are eliminated to the maximum extent possible. I then view the negatives at max blowup on my elarger, and use a high magnification grain focuser to count lines.

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    I conducted a series of test like this with all of my filters to determine that lens quality was not compromised by any of my tiffen filters. I could detect no difference.
     
  4. Gene,
    I use the same method for just general shooting. If I have an exposure over a few seconds, I lock the shutter open and hold a black card in front of the lens and pull it away from the camera. Most MF reflex cameras have obnoxious shutter kick.

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    Sometimes it's good to try a comparison by hand holding the camera just to see how much shake there really is and how much it is degrading your images. I can't hold my RB67 with exposures much lower than 1/250 before I rapidly loose quality. "Lock and load buddy!"

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    Same experience with tiffen filters. I've used the Pro-Series for years and have never detected any loss of image quality.

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    //scott
     
  5. Scott;

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    I do all of the above in a totally darken room. No need to cover the lens.

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    I agree that hand holding (in my case a p67) below 1/250 is pushing it. I double the old rule about 1 over the focal length(which seems to work for 35mm), so if I have on a 55mm, I will not use less than 1/125 without support. For my 200mm, I use at least 1/500.

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    When I go to 1/125, I know not to count on getting a sharp 16x20.

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    I hear people say they can hand hold their Blad at 1/30, but I doubt it unless they are getting 6x6 inch prints.
     
  6. Gene,
    You forget that German engineers have learned to repeal the law of inertia and built it into their camera bodies That's why Hassies are so expensive and their advocates make such outrageous claims {grin}.

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    It's a shame the same technology can't be applied all MF cameras. It can't be that expensive. Of course the torque involved with keeping my RB stable during the mirror cycle would probably turn the camera into a gyroscope for a second.

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    //scott
     
  7. Dear John Braud;
    While I agree that your technique is sound; I would question your philosophy. What is the good of a very sharp lens if the body of the camera produces such intense vibration that the image is blurred from exposure? Unless you do all of your photography with flash heads in a darkened room (as a commercial photographer and assistant I can say that this is frequently the case) the test is not really that useful in the real world. I can't understand why a phtographer of landscapes, for example, would find such a test useful.
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    I have made frequent postings on the subject of lens testing; not all my views are popular to judge by the responses I have recieved.
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    stefan
     
  8. Testing with flash in a darkend room isolates the one variable, lens quality. If you don't know what the optics are capable of, then shooting in the field and evaluating the results does not tell you much if your pictures are not sharp. If your lens at f11 will cut say 90 l/mm, but you can't get an acceptably sharp 11x14 print, then you know at least that the lens is not at fault and you can look for another reaseon. If you shoot at f22 and your l/mm are 40, then you don't even bother to try and print above 8x10.(Real Experience with a real 6x7 lens)

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    This type of testing also culls out the bad ones real fast. Ive returned more than one used lens after this test and wins argeuments with clerks who see their comission going down the tubes. It also got me to stop buying used equipment!

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    In the field, when setting up a shot, having info on the performance of the lens can be a factor in how you set the shot up. Why worry about getting adequate DOF if your lens a f22 won't hold on an 8x10 piece of paper?
     
  9. Your comment "What is the good of a very sharp lens if the body of the camera produces such intense vibration that the image is blurred from exposure? " is valid, and a damn good point. I refer to this as "anechoic room studies". That's why I shoot mirror locked or "black card" as much as possible with my RB.

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    //scott
     
  10. Stefan,

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    The philosophy as you call it is an experiment to determine the sharpness of the lens and only that. In doing so you must eliminate as many variables as possible. If the result is a sharp picture while your available light, handheld pictures are blurry or not as sharp one might conclude it is something other than the lens at fault. The philosophy here is to determine the weak link in a chain of events that ultimately results in an inferior picture. How would you know if it is the body that is causing the blurry pictures as opposed to an unsharp lens if you don't accurately test the lens? A landscape photographer would find this test useful because he can have faith that the lens and back of his camera are capable of producing sharp pictures. Once he knows this he can look at other things to improve the sharpness of his photos.

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    John Braud
     
  11. Dear John Braud;
    Makes perfectly good sense to me.
    Thanks for the clarification.
    stefan
     
  12. Stefan;

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    We will expect a full report on tests of all of your lens in the near future!! I can send you a lens test chart that is easier to use than the Air Force chart.

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    Gene
     
  13. hi guys,

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    i have just a stupid question, hope you guy won't laugh, can we use those red laser pointer pen to test the len, like project it from the viewfinder through the
    len, like testing the how sharp your eye can focus.... or see the distance, sharpness, etc.

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    If i do that, do anyone know that those red laser is it going to kill my len coating or not? just wondering? you guys may laugh at me, but... no hurt to ask
    right.
     
  14. Using flash to illuminate the test target? This is exactly what Carl Zeiss, manufacturer of lenses for Hasselblad, Rolleiflex, Alpa, Contax, Sony, Arriflex motion picture cameras did recently, when they built a new lens testing room last year!

    Laser pointer and lens coating? According to sources inside the Carl Zeiss optical coating development lab usual laser pointers are in no way harmful to Zeiss T* coating.
     
  15. Kornelius:

    I'm glad to know that I have such a commanding lead in the development of lens testing technology, that Zeiss is now using my technique developed several years ago. Maybe a law suit in there some where!?!?!
     
  16. Gene: You didn't patent it, did you? :)
     

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