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Thread: Finding f-stops for remounted lens

  1. #11

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    Re: Finding f-stops for remounted lens

    Quote Originally Posted by Pere Casals View Post
    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:

    Attachment 202088

    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:

    Attachment 202089


    ...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.
    Interesting approach, thanks for sharing. What’s the advantage of using a luxometer vs a meter (since we already have one) for this application?

  2. #12
    Peter De Smidt's Avatar
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    Re: Finding f-stops for remounted lens

    Bob's method works great. I use a related method. I have an incident meter that has a magnet. I simply put the whole meter inside my camera. It reads to my phone in 1/10th stops, which is close enough for me.
    Please stop feeding the trolls.

  3. #13

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    Re: Finding f-stops for remounted lens

    Quote Originally Posted by Kiwi7475 View Post
    Interesting approach, thanks for sharing. What’s the advantage of using a luxometer vs a meter (since we already have one) for this application?
    If you use a meter (depending on the particular model) then you may spect a 1/3 stop precision when matching the apertures of the patron lens with the serviced lens, instead with the Luxometer you can nail a 1/1000 precision if you want, you have more precision than required, because the meter will give you 3 significative digits.

    Some meters like this Sekonik also tells LUX and works like a luxometer and sensor can be rotated to face the lens while taking the reading, but other meters don't tell such an accurate reading as fractions of stops are told.

    Click image for larger version. 

Name:	Sekonic_L-478D_Litemaster_Pro.jpg 
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ID:	202093


    The trasmission approach is the most precise, easy and effective for DIY. The geometric approach is simply worse if our priority is making accurate exposures. I tried both ways and compared, with a DIY geometric aproach you may spect 1/2 stops shifts in the middle apertures, and a very high error in the small apertures, even 1 stop, while with the transmission way you nail 100% the matching. Of course an optical lab has the equipment to adjust the aperture gemetrically with total perfection, but in the DIY realm you have to go to transmission way if you want good precision, in special for the smaller apertures, errors from the DIY geometric way are not acceptable.

    The Luxometer is also interesting to see how precise is our aperture scale, as lux have to follow the right progression, also we can check what effective fall-off we have at different apertures in the corners.


    Also I often use the Luxometer to measure fall-off on the enlarger's easel, the enlarger I use, a 138, can move the bulb x-y-z which has an effect on the fall-off, and of course I use it for film/paper calibrations with absolute H scale, so those $20 were quite well invested.

  4. #14

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    Re: Finding f-stops for remounted lens

    Quote Originally Posted by Pere Casals View Post
    If you use a metter (depending on the particular model) then you may spect a 1/3 stop precision when matching the apertures of the patron lens with the serviced lens, instead with the Luxometer you can nail a 1/1000 precision if you want, you have more precision than required, because the meter will give you 3 significative digits.

    Some meters like this Sekonik also tells LUX and works like a luxometer and sensor can be rotated to face the lens while taking the reading, but other meters don't tell sach an accurate reading as fractions of stops is told.

    Click image for larger version. 

Name:	Sekonic_L-478D_Litemaster_Pro.jpg 
Views:	7 
Size:	30.5 KB 
ID:	202093


    The trasmission approach is the most precise, easy and effective. The geometric approach is simply worse if our priority is making accurate exposures. I tried both ways and compared, with a DIY geometric aproach you may spect 1/2 stops shifts in the middle apertures, and a very high error in the small apertures, even 1 stop, while with the transmission way you nail 100% the matching. Of course an optical lab has the equipment to adjust the aperture gemetrically with total perfection, but in the DIY realm you have to go to transmission way if you want good precision, in special for the smaller apertures, errors from the DIY geometric way are not acceptable. Some are not aware...

    The Luxometer is also interesting to see how precise is our aperture scale, as lux have to follow the rights progression, also we can check what effective fall-off we have at different apertures.


    Also I often use the Luxometer to measure fall-off on the enlarger's easel, the enlarger I use, a 138, can move the bulb x-y-z which has an effect on the fall-off, and of course I use it for film/paper calibrations with absolute H scale, so those $20 were quite well invested.
    Thank you! Very useful and clear.

  5. #15

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    Re: Finding f-stops for remounted lens

    Quote Originally Posted by Kiwi7475 View Post
    Thank you! Very useful and clear.
    One thing else, with the transmission way you have to take the reading in the center of the image circle, to avoid fall-off influence in the reading.

    Regards

  6. #16

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    Re: Finding f-stops for remounted lens

    Quote Originally Posted by Pere Casals View Post
    One thing else, with the transmission way you have to take the reading in the center of the image circle, to avoid fall-off influence in the reading.

    Regards
    Actually, if you have a modern Linhof - one made after the 3, then you can use their Focus/Metering Bellows. This attaches to the gg frame and, on older ones, had a 2x magnifier built in. The later one had two 2x magnifiers screwed together.
    The bellows eyepiece accepted a Gossen with the microscope attachment. This allowed metering an area the diameter of a quarter on any point of the gg or, by extending the bellows out, the entire gg. To use the later version for metering you unscrewed the upper magnifier.
    Of course, for this to work at any point of the gg, you have to have the fresnel on the gg installed.

  7. #17

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    Re: Finding f-stops for remounted lens

    Quote Originally Posted by Bob Salomon View Post
    Focus/Metering Bellows.
    The great thing of metering in that way is that we don't even need an aperture scale to have perfect exposures...

  8. #18

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    Re: Finding f-stops for remounted lens

    Quote Originally Posted by Pere Casals View Post
    One thing else, with the transmission way you have to take the reading in the center of the image circle, to avoid fall-off influence in the reading.

    Regards
    Personally am able to remove the GG and fresnel from the equation by using a Horseman Exposure Meter 45. It slips in as a film holder does and measures an area 4 5/8" x 3 1/2". Actually preferred it to a Sinar Six when I was doing a lot of Photomicrography and Photomacrography and shooting 4x5 Chromes. It was interesting to mount it on the back of my 4x5 and mount my lenses on the front all focused at infinity facing a constantly lit white wall. All f/16s are not the same! Pretty much the same for modern lenses, but on some of my pre 1900 lenses, you would easily see a variation of plus or minus 1/2 f/stop. A brass Petzval I had used warehouse stops (home made by its previous owner), they were beautifully machined from brass but were all off by a whole f/stop, of which I always had a hunch of.

  9. #19

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    Re: Finding f-stops for remounted lens

    I used a combination of Bob and Greg's ideas to complete my task. I then compared the diameter of the openings with the results of computations and the few legible markings on original shutter.
    Thanks to all who contributed and got me thinking of the quickest and most accurate method of solving my problem.

  10. #20

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    Re: Finding f-stops for remounted lens

    Quote Originally Posted by Greg View Post
    All f/16s are not the same! Pretty much the same for modern lenses, but on some of my pre 1900 lenses, you would easily see a variation of plus or minus 1/2 f/stop. A brass Petzval I had used warehouse stops (home made by its previous owner), they were beautifully machined from brass but were all off by a whole f/stop, of which I always had a hunch of.
    Of course, coatings changed that game, mainly after WWII. Single coated have a little loss, and a multicoated lenses require a lot of gropus to notice a loss


    Quote Originally Posted by Greg View Post
    A brass Petzval I had used warehouse stops (home made by its previous owner), they were beautifully machined from brass but were all off by a whole f/stop, of which I always had a hunch of.
    Yes... 3 groups are 6 uncoated surfaces, so we may expect 0.9^6 = 0.53 , to near exactly one stop less, what you say. An uncoated petzval transmits 50% of theoric light power !


    ... but a MC plasmat may not loss a 2% of the light, I guess.

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