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Thread: Bellows compensation for 14" lens on 8x10

  1. #1

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    Bellows compensation for 14" lens on 8x10

    Hello!

    I would like to know how to calculate, or if someone have already done this, a bellows compensation "ruler" for my 14" commersial ektar on 8x10 camera. Half stop intervals would be enough for me. For long now i have just added a stop or two for close ups, but would really like to be more accurate on my exposures from now on. So, help would be appreciated, since i got a bit confused while googling for an answer.

  2. #2

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    Re: Bellows compensation for 14" lens on 8x10

    try this:

    http://www.salzgeber.at/disc/index.html

    basically you put something in the field of view that you know the size of. Then by measuring its size on the ground glass you will be able to determine the magnification. From the magnification you can get the exposure compensation factor.

  3. #3

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    Re: Bellows compensation for 14" lens on 8x10

    You could make one pretty easily using this spreadsheet on the main LFInfo site. I find measuring bellows extension and using a premade ruler or simply doing the calcuation (Bellows Length/Focal Length)^2 much easier than messing around with discs that you end up leaving in the picture.

  4. #4

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    Re: Bellows compensation for 14" lens on 8x10

    Easiest way, but buy a cheap calculator:
    Focal length squared divided by bellows extension squared equals a factor less than 1. Multiply the film speed by that factor and you now have a film speed including the bellows factor. EG: 6" lens, 12" bellows, 36 (F) divided by 144 (BE) =.25 If the film speed is 100 X .25, the new film speed including bellows extension factor (BEF) is 25 (or t f stops). The reason this is easier is that when you don't have nice even numbers to deal with is may be difficult to figure it in fstops when the numbers are inverted, even though the result is the same.

    This is the reasonb, lets say that you have a 135mm lens and 13 inches of bellows. fist convert inches to bellows or mm to inches. When that is done with BE squared divided by F squared you have a factor of 6, now how much is that in f stops? If you invert the problem the answer is .165. Multiply a PXP film speed of 125 by .165 and the answer is a tiny bit more than ISO 16 (an amount not worth considering).

    By the way, if you convert everything to inches, the numbers are smaller or convert the mm to cm the numbers are smaller.

    Lynn

  5. #5

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    Re: Bellows compensation for 14" lens on 8x10

    thanks for the quick reply, does these numbers sound about right for 355mm lens? :

    435mm = 0.5 stop
    503mm = 1 stop
    562mm = 1.5 stop
    615mm = 2 stop

  6. #6
    Mark Sawyer's Avatar
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    Re: Bellows compensation for 14" lens on 8x10

    Quote Originally Posted by Lynn Jones View Post
    Easiest way, but buy a cheap calculator:
    Focal length squared divided by bellows extension squared equals a factor less than 1. Multiply the film speed by that factor and you now have a film speed including the bellows factor. EG: 6" lens, 12" bellows, 36 (F) divided by 144 (BE) =.25 If the film speed is 100 X .25, the new film speed including bellows extension factor (BEF) is 25 (or t f stops).
    This is not the easiest way, and for the life of me, I can't figure out why all the text books, manuals, etc. keep pushing that method.

    As usual, the easy way is the most direct way. Measure the bellows extension, and while you've got the ruler or tape measure in your hand, measure the aperture through the front element. Divide the former by the latter and that's your f/stop. Period. No squaring this, then squaring that, then getting a factor for reducing film speed or figuring the new f/stop.

    If you really want it to be easy, write the measurements of the f/stop diameters on your lensboard and put a scale on your camera bed or rail, (and for style points, do the division on a slide rule!)

    (The weird thing is, in all my time in large format, I've never met another photographer who uses this method. Do you folks all just love squaring numbers?)
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  7. #7

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    Re: Bellows compensation for 14" lens on 8x10

    damn, all these easy ways really seem confusing.. my intention was to put a simple guide ruler on the rail of my 8x10, i use primarily only one lens.. and my 8x10 stays mostly inside on my home studio.. so i just needed a quick and easy way to check my bellows correction. like it's on mamiya rz67.

  8. #8

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    Re: Bellows compensation for 14" lens on 8x10

    Quote Originally Posted by Mark Sawyer View Post
    This is not the easiest way, and for the life of me, I can't figure out why all the text books, manuals, etc. keep pushing that method.

    As usual, the easy way is the most direct way. Measure the bellows extension, and while you've got the ruler or tape measure in your hand, measure the aperture through the front element. Divide the former by the latter and that's your f/stop. Period. No squaring this, then squaring that, then getting a factor for reducing film speed or figuring the new f/stop.

    If you really want it to be easy, write the measurements of the f/stop diameters on your lensboard and put a scale on your camera bed or rail, (and for style points, do the division on a slide rule!)

    (The weird thing is, in all my time in large format, I've never met another photographer who uses this method. Do you folks all just love squaring numbers?)
    Mark,

    This is a typical way to use convertible protar lenses. Many times, the iris scale is in mm only. Otherwise, you would need 3 scales for a triple convertible lens and even more if you have more than 3 elements. And as you said, you have the bellows correction built-in.

    Pete

  9. #9
    Mark Sawyer's Avatar
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    Re: Bellows compensation for 14" lens on 8x10

    Quote Originally Posted by Pete Roody View Post
    Mark,

    This is a typical way to use convertible protar lenses. Many times, the iris scale is in mm only. Otherwise, you would need 3 scales for a triple convertible lens and even more if you have more than 3 elements. And as you said, you have the bellows correction built-in.

    Pete
    Interesting... I've never seen a Protar (or any other lens) marked in mm. But then, I haven't seen many Protars...

    You might need two scales for a double or triple convertible, one for having the front element on and one for it off, although I don't know whether the magnification factor would be significant...
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  10. #10
    wfwhitaker
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    Re: Bellows compensation for 14" lens on 8x10

    As an alternative to Mark's method that doesn't require long division, think of your relative bellows extensions in terms of "f/stops".

    For example:

    If you have an 8" focal length lens and the bellows extension at your desired focus is 11", you need to open up one stop to compensate because the difference between f/8 and f/11 is one stop. Works for other focal lengths, of course, and is independent of format. For a 14" lens, you could probably round to 16 (f/16) in most cases.
    Last edited by wfwhitaker; 12-Oct-2009 at 20:33.

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