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Thread: Finding f-stops for remounted lens

  1. #1

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    Finding f-stops for remounted lens

    Any suggestions for best way to find f-stops for lens mounted in a different shutter?
    I just moved my Protar IIa from an almost worthless Compound to a modern shutter. I know I can send it off and pay a fortune to get it done,but there has to be a reasonable method for DIY. If nothing else, I will aim it at a constant bright area, mount it in the old shutter, check the brightness on the GG, then move it to the new shutter, and close the aperture until it reads the same. A log and tedious process.
    Thanks for your ideas.

  2. #2

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    Re: Finding f-stops for remounted lens

    Focal length measurement in mm. divided by apparent aperture measurement in mm. = f stop.
    Aperture measured from front of lens, through elements.

  3. #3

    Re: Finding f-stops for remounted lens

    Precompute the required apertures by dividing the focal length of the lens by the desired f/stop number - 5.6 - 8 - 11 -16 - 22 - 32 and so forth. Looking through the front of the lens (the glass elements must be in place) measure the diameter of the seen aperture. As you close down the aperture and each precomputed diameter is reached mark your shutter's position. Simple.

  4. #4
    C. D. Keth's Avatar
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    Re: Finding f-stops for remounted lens

    These guys hit it on the head. If your aperture isn’t very round, like an octagon, I just consider the average of the ‘flat to flat’ and the ‘point to point’ measurements and figure that’s very close.


    Sent from my iPhone using Tapatalk
    -Chris

  5. #5

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    Re: Finding f-stops for remounted lens

    I just use my light meter. First determine the light absorption of the gg/fresnel. Enter that as a filter factor. Point the camera at an evenly illuminated surface, use the microscope attachment and open the aperture all the way. Note the aperture on the meter and then close the aperture while noting each 1/3rd change on the meter. Now your lens is calibrated in 1/3rd stops. Just make your scale marked accordingly. No math needed. Just make sure that you adjust your meter to take any bellows factor into consideration. Or point the camera at clear sky for no bellows factor.

  6. #6

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    Re: Finding f-stops for remounted lens

    I know well how to compute f-stops by measuring the apparent aperture and applying the formula. I was looking for a method which is a little faster more practical. This Is a triple convertible lens.
    Bob seems to have the most practical method. I had considered a very similar method, and will use his suggestion.
    THanks to everyone for your assistance.

  7. #7

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    Re: Finding f-stops for remounted lens

    A similar approach, using a DSLR:

    https://www.largeformatphotography.i...Aperture-Scale

    Kumar

  8. #8

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    Re: Finding f-stops for remounted lens

    I have nothing to do with DSLR's, or the images they produce.

  9. #9

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    Re: Finding f-stops for remounted lens

    Quote Originally Posted by Jim Noel View Post
    Any suggestions for best way to find f-stops for lens mounted in a different shutter?

    Jim, let me tell the way I used several times to calibrate apertures, as I rescued many of my lenses from trash I ended swapping several shutters, also I upgrades several lenses by purchasing the cells alone...


    1) Prepare a "wall" (indoors is possible) that has an stable and uniform illumination

    2) Take a patron lens that has well calibrated apertures, mounted in the view camera focus at infinite.

    3) Point (with the patron lens) to the wall you prepared, use a luxometer to measure the LUX reading in the GG plane (GG removed may be better perhaps, id the patron lens has very different focal) in all apertures , you may sustitute the GG by a clear glass for total precision in the sensor position, you may use a $20 luxometer for that:

    Click image for larger version. 

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    4) Rest is easy, isn't it ? Mount the lens you want you calibrate, focus infinite, point to the approx. same point on the wall, ajust aperture to match the LUX reading in the patron lens for each aperture, and make the marks in the aperture scale.


    It is a transmission based aperture, not a geometric based aperture, but the newly calibrated lens will expose like the patron lens, speed accuracy apart.


    I have this one https://www.amazon.com/-/es/2000-Cue...504212&sr=8-45

    But the one having the cord for the sensor is more convenient, I got that one because it is takes 0.01 readings, which I use to calibrate film in LIRF conditions.


    Another choice is mounting a (say) Nikon F-Mount extension ring on a monorail camera lensboard, place that lensboard in the rear standard and mount an SLR on it, (so you don't need to touch a DSLR ) use the camera meter readings to match the reading of the apertures of the two lenses, the patron one and the one you adjust.


    You may also use a probe meter if you have one:

    Click image for larger version. 

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    ...but the numeric reading in the luxometer will allow a more accurate matching.

    _____


    Not necessary to tell you that the view camera has to focused at infinite which each lens to avoid the compensation factor... I point it because somebody else may use that procedure in the future...

    ____

    After doing that and calibrating shutter speeds exposures were totally perfect.

  10. #10

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    Re: Finding f-stops for remounted lens

    Beware of some older lenses that the iris opened up wider than the listed aperture for the lens. I have a 12 3/5" Protar Series VII which is listed as a f/7.7 optic. The iris actually needs to be closed down about one stop to give you that f/7.7 (starting) aperture. On my 5.9" No. 5 Periscope lens, its listed "maximum" aperture is f/14. On this lens the iris has to be closed more than one f stop from its maximum physical opening to give you its listed f/14 "maximum" aperture. Read that for the Periscope lens, the aperture could be opened up wider that its listed "maximum" aperture for "ease in focusing".

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