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Thread: How to Calculate Bellows Extension correction for a true Telephoto lens?

  1. #1
    Drew Bedo's Avatar
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    How to Calculate Bellows Extension correction for a true Telephoto lens?

    I have a 380mm Telephoto lens that I would like to use on my Zone VI 4x5. Infinity focus is at about 8".

    I am not sure how to figure the exposure correction for bellows extension with this lens.

    My most frequently used lens for outdoor field work is a 150mm (6 inch) lens. With this lens, an extension beyond infinity of three inches ( nine inches total) requires one full stop of additional exposure . . .or 1/3 stop per inch. This works quite well outside with changing light and a cold wind blowing.


    So how can I use this approach with the 380mm/15 inch telephoto? Seven and 1/2 inches beyond infinity would be one full stop Do I measure from infinity focus point at 8 inches of bellows draw?
    Drew Bedo
    www.quietlightphoto.com
    http://www.artsyhome.com/author/drew-bedo




    There are only three types of mounting flanges; too big, too small and wrong thread!

  2. #2

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    Re: How to Calculate Bellows Extension correction for a true Telephoto lens?

    Quote Originally Posted by Drew Bedo View Post
    I have a 380mm Telephoto lens that I would like to use on my Zone VI 4x5. Infinity focus is at about 8".

    I am not sure how to figure the exposure correction for bellows extension with this lens.

    My most frequently used lens for outdoor field work is a 150mm (6 inch) lens. With this lens, an extension beyond infinity of three inches ( nine inches total) requires one full stop of additional exposure . . .or 1/3 stop per inch. This works quite well outside with changing light and a cold wind blowing.


    So how can I use this approach with the 380mm/15 inch telephoto? Seven and 1/2 inches beyond infinity would be one full stop Do I measure from infinity focus point at 8 inches of bellows draw?
    Drew,

    The "formula" you are using for your 150mm lens works only coincidentally and approximately, but well enough for your purposes.

    With your telephoto, and using your approximate approach, you just need different numbers and, most importantly a different starting point.

    Assume that whatever extension required for your telephoto lens at infinity focus is 380mm (or 15 inches) regardless of what it really is. You say your lens focuses at infinity at "about 8 inches," so take that as your starting point. An additional extension of 15 inches from your 8-inch starting point would then be 23 inches measured, but for the purposes of exposure calculation you would consider it 30 inches and a two-stop adjustment. To be clearer: just add 7 inches to your total bellows extension measurement and calculate from there.

    If you want the "halfway" point, you would add 7.5 inches to your starting point (8+7.5=15.5) for infinity focus and then mentally add another 7 inches (15.5 plus the phantom 7 inches = 22.5 inches and a one-stop adjustment.) Or you can just measure from your infinity focus point, whatever is easiest for you. You can estimate smaller increments from there using 2.5-inch increments for each 1/3 stop.

    I have to point out, however, that your method is just a practical working method based on an approximation. A doubling of extension from the infinity position does indeed need a two-stop adjustment, but the "halfway point" that you are working with is not really accurate (but close enough for most purposes). The true intermediate point (i.e. the one-stop adjustment point) is really the focal length multiplied by the square root of 2.

    For example, for a 6-inch lens, you'd multiply 6 by 1.414.... and get approximately 8.5 inches, not the 9 inches you are figuring from. But, no matter, it's close enough.

    For the 15-inch lens, the numbers are 15 * 1.414... = approx 21.2, not the 22.5 I used above. Still, it's close enough for most purposes.
    Finding the exact 1/3-stop positions requires more math.

    Bottom line, you can apply the method you already use by doing what I described above. If you really need full exposure, add a third or two-thirds of a stop to what you figure and you should be fine.

    Best,

    Doremus

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    Re: How to Calculate Bellows Extension correction for a true Telephoto lens?

    Drew, extension is measured from the infinity focus position, i.e., where the front standard sits when the lens is focused at infinity.

    The magic formula for exposure compensation given extension is independent of focal length. It is in two steps:

    (1) magnification = (extension/focal length) - 1 Extension and focal length have to be in the same units, such as mm, cm, m, inches, feet, rods, furlongs, miles, light years, ...

    (2a) for a symmetrical lens, exposure compensation in stops = magnification + 1

    (2b) for an asymmetrical lens facing normally, exposure compensation in stops = ((magnification/pupillary magnification) + 1) Pupillary magnification is (diameter of exit pupil, seen from the rear/diameter of entrance pupil, seen from the front). This is your tele lens.

    (2c) for an asymmetrical lens reversed, exposure compensation in stops = (1/pupillary magnification)*(1 + magnification * pupillary magnification)

    There's no mindless rule of thumb for asymmetric lenses. You have to measure pupillary magnification and do the calculations. But you only have to do it once.

    If you want a really easy way out, use a 4x5 Horseman Exposure Computer. This is an averaging exposure meter that slips in like a sheet film holder. For most subjects, with a long lens a 2x3 Horseman Exposure Computer is a 4x5 adapter will give good enough results with a lens normal or longer, can give underexposure with lenses shorter than normal. This because it doesn't see the outer part of the 4x5 frame, where falloff with a lens shorter than normal can be problematic.

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    Drew Bedo's Avatar
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    Re: How to Calculate Bellows Extension correction for a true Telephoto lens?

    Doremus: Thanks that is a great explination. I had not learned the square root of two times the focal length to get the one stop distance. I will work with that to pre-figure the correction.

    To make this clear in my mind now: to get to the one stop distance (or 1/3 stop positions) I must measure from the telephoto's shortened ( or Telephoto) infinity focus distance. Do I have that right?

    And yes, I do understand that the method I have described is an on-the-fly sort of thing. I use it for non-macro imaging.
    Drew Bedo
    www.quietlightphoto.com
    http://www.artsyhome.com/author/drew-bedo




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    Drew Bedo's Avatar
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    Re: How to Calculate Bellows Extension correction for a true Telephoto lens?

    Dan: Thank you for the detailed explanation of how to do this when it needs to be done meticulously.
    Drew Bedo
    www.quietlightphoto.com
    http://www.artsyhome.com/author/drew-bedo




    There are only three types of mounting flanges; too big, too small and wrong thread!

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    Re: How to Calculate Bellows Extension correction for a true Telephoto lens?

    Quote Originally Posted by Drew Bedo View Post
    Doremus: ... To make this clear in my mind now: to get to the one stop distance (or 1/3 stop positions) I must measure from the telephoto's shortened ( or Telephoto) infinity focus distance. Do I have that right? ...
    Yep, that's basically it. I was trying to simply apply what you already do to your telephoto lens so you didn't have to change methods. FWIW, I simply made a bellows-extension table for all my lenses and carry it in my exposure-record notebook. When I need, I measure extension and consult the table. My way is pretty accurate, but likely takes more time than your method.

    Best,

    Doremus

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    multi format
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    Re: How to Calculate Bellows Extension correction for a true Telephoto lens?

    Doremus
    thanks for this explanation !
    makes total sense
    john
    enjoy your coffee

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