Bellows extension

Discussion in 'Large Format Cameras and Accessories' started by Terry Bowyer, Nov 2, 2008.

  1. Barrie B.

    Barrie B. Subscriber

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    Greetings , Buy some Black and White film ,load up some double darks and take some images, you will enjoy the process. However shpould you decide to ' come in close' for some images I recommend you make one of these :-A quick Disc, invented by Philipp Salzeber,you can read off the exposure compensation at any time without having to do any maths. You can download one to print out on yor computer. :- http://www.salzgeber.at/disc/ Enjoy.
    Barrie B. Melbourne Australia.
     
  2. Mateo

    Mateo Member

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    this can be easy if you want it to be

    if you got a 5.6 inch lens and you extend the bellows to 8 inches you need to add a stop
    if you got a 5.6 inch lens and you extend the bellows to 11 inches you need to add 2 stops

    if you got a 210mm lens and you extend 320mm you ain't gonna be far off if you add 1 stop
    if you got a 210mm lens and you extend 450mm you ain't gonna be far off if you add 2 stops

    see the pattern, forget about inverse square law this is quick and dirty and if you can be more accurate you have some super calibrated lenses and need to calm down
     
  3. jnanian

    jnanian Advertiser Advertiser

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    exactly!
     
  4. RodKlukas

    RodKlukas Member

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    Bellows factor.

    to calculate easily get a small tape measure:

    Measure the bellows extension from lens board to image plane(ground glass).

    Take the focal length of your lens in inches and the extension of your bellows in inches. ie 210mm lens = 8" lens roughly.
    bellows extension of 16"or 16.5".
    Now using the 8" and 16" measurements think of the 8 and 16 as F-stops.
    So the difference between F8 and F16 is 2 stops and that is your compensation.
    This is most easily calculated or taken care of by changing your ISO down this amount or in our example, 2 full film speeds. So if using ISO 100 you would simply change your ISO to 25 and you have your answer.
    Takes much less time to do than to tell you.
    Rod
     
  5. Mark Sawyer

    Mark Sawyer Member

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    ...or just measure the bellows extension and the diameter of the aperture through the front element, then divide the former by the latter. This gives you the actual f/stop at the externsion and aperture your camera and lens are set for.

    There are quite a few simple methods, but the books all tell you to divide the square of this by the square of that, which seems unnecessarily complex.
     
  6. Doremus Scudder

    Doremus Scudder Member

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    Since this is a sticky thread and since I do not see my particular solution (which I thought was standard procedure...), here goes.

    1. Figure out bellows extension factors for all your lenses for a set of exposure factors (I use 1.5, 2, 3, 4, 6, 8, 12, 16, and 20, dropping the higher ones as the bellows length is exceeded).

    2. Make a chart and carry it with you in the field, along with a small tape measure.

    3. When you need to apply a bellows extension factor, measure your bellows extension, look up the factor in the chart for the lens you are using and apply it as you would a filter factor.

    Voilá! Done and no fussy calculations needed in the field.

    FWIW, I'm attaching the bellows extension factor tables for my lenses for up max bellows extension 16 in/400mm in both inches and mm. Maybe they will be of use to someone.

    Best,

    Doremus Scudder
    www.DoremusScudder
     

    Attached Files:

  7. raizans

    raizans Member

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    i have a different question about bellows extension. how do you calculate the extension needed to focus at a certain minimum distance? if i had a 240mm lens, how much extension would i need to focus .7m away, for instance?
     
  8. jp80874

    jp80874 Member

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    Terry,

    If all this hasn’t made you want to go back to 35mm lets now start on the Scheimpflug Rule.

    John Powers
     
  9. John Koehrer

    John Koehrer Subscriber

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    That's just so fun to say isn't it? Scheimpflug, Scheimpflug.
     
  10. Ian C

    Ian C Member

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    Bellows Draw for a Given Diaphragm-to-Object Distance

    If you could restate your question then it then it can be answered after a fashion.

    In a second semester university physics course the material takes a brief look at optics. It doesn’t go into great depth and doesn’t consider more than 2-element systems. Most of the section on optics involves a theoretical “thin” lens that is thin relative to its diameter and is usually a single-element double convex lens like you’d see in a magnifying glass with each surface the same spherical radius.

    I seem to recall that for the purpose of some calculations a symmetric compound-element lens is roughly equivalent to a thin double convex lens whose single element is centered at the plane of the diaphragm. With this simplifying idea it’s possible to calculate something similar to what you asked.

    The Thin Lens Formula is:

    1/f = 1/p + 1/i

    where f = focal length of the lens, p = distance from the object to the center of the lens, and i = the distance from the image to the center of the lens.
    So

    1/i = 1/f -1/p = (p-f)/fp

    Then

    i = fp/(p-f)

    If by “0.7 meters away” you mean 0.7 m from the object to the plane of the diaphragm, then p = 700mm. You said that f = 240mm, so now we can calculate i, the distance from the film plane to the plane of the diaphragm.

    i = 700mm*240mm/(700mm-240mm) = 365.22 mm.

    To test the calculated prediction I used a 240mm f/5.6 Rodagon enlarging lens to project the image of the lit filament of a 60-watt light bulb onto a piece of white mat board. The lens was placed with the plane of the diaphragm approximately 700mm from the center of the bulb. I measured the distance from the plane of the diaphragm to the mat board where the position of the board gave the sharpest image of the filament. It’s somewhere between 355mm and 375mm so that suggests that the calculated value is close to the actual diaphragm-to-image distance. With a proper optical bench I could have refined the measurement.
     
  11. Mats_A

    Mats_A Member

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    Very interesting thread this.
    What if you are using a 35mm camera with a zoom lens for metering. Is it not so that the bellows factor is "factored" in when using an SLR reading? A zoom lense has variable max f-stop from min to max. So if I use a zoom lens to get about the same size as on the ground glass, will the metered value be usable on the folding camera?
    Or am I as per usual completely off?

    r

    Mats
     
  12. keithwms

    keithwms Member

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    That the field of view is the same, doesn't mean that the bellows factor is taken into account. The lens designs are very different, at least if we are comparing typical 35mm lenses to LF lenses, so lens-to-film distance is very different, and falloff can be very different too. There are Scheimpflug issues, differences in lens coatings and contrast etc. etc.

    In the limit of infinity focus, yes, you can get a good idea from 35mm metering, but for anything involving substantial bellows factor or movements etc.... usually not wise.

    If you are using a 35mm camera to meter, it's best to take that meter reading and then apply the bellows factor to that reading. I do this quite often for landscapes... again, for situations with large bellows factor, beware! The 35mm cameras have the advantage of spot as well as area and average and matrix metering, which all have their plusses and minuses. Plus, if you use a digicam to meter, you can also get colour temp and scene brightness range (a histogram) and a "proof" image too.
     
  13. Rick A

    Rick A Subscriber

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    Page 36 of my 1981 Kodak Master Photo Guide has the same friggin cool bellows compensation factor gizmo that is exactly like Jason Brunners. All you need is a two inch target to place on the subject, then measure the GG with the factor scale in the guide, and presto, instant extension factor.
     
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  15. photobizzz

    photobizzz Member

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  16. lloyd

    lloyd Member

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  17. Shadowtracker

    Shadowtracker Member

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    Most interesting discussion. I found all of the replies of interest, and no, didn't scare me back to 35mm... my brain may be twisted though. It's interesting to see the various methods of solving the same problem, much as artists of all sorts do in their work be that music, painting, photography or any other form. Now, I just have to find one that works and fits my aptitude.
     
  18. seadrive

    seadrive Member

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    Not sure if anyone said this already, but bellows extension factor is pretty easy, once you realize that the inches of bellows draw are in the same relation as f-stops. All you need to know is the length of the lens in inches, and how far out the lens is racked.

    For example, say you are using a 210mm lens, which is approx. 8 1/4 inches. Convert the inches to an f-stop, and round off to the nearest stop or 1/2 stop. In this case, your "base" exposure is f8. If your lens is racked out to 11", then you need to increase the exposure by one stop, the difference between f11 and f8. If you're racked out to 16", then it's two stops. For distances in between, approximate. Racked out to 13"? Well, that's f11 + a third of a stop or so. So you'd increase the exposure by about 1 and 1/3 stops.

    Close enough for government work!
     
  19. holmburgers

    holmburgers Member

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    Ok, not to be contrary, but I want to do this completely differently..... :D

    For me, since my mono-rail doesn't have a built-in ruler, I would find it much easier to just have a table for *subject distance*. I think I could accurately guesstimate subject distance better than bellows draw (image distance). Why does no one do this? Why is the sky blue?

    Having a table divided by 1/3 stops would also be nice. So, if I have a row of desired exposure compensations (like 1.33, 1.66, 2, 2.66, etc... all the way up to 16 let's say, a.k.a 4 stops) and I know my focal length (180mm), I should be able to calculate for subject distance, right?

    Can anyone do the algebra and concoct a singular formula? I was having to convert exposure compensation to magnfication and then to subject distance and at about 2 o'clock in the morning I was beginning to lose my patience.
     
  20. Ian C

    Ian C Member

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    Exposure Compensation Based on Focal Length and Subject Distance

    You can easily generate the table you want with a calculator with the natural logarithmic function and the following information.

    Let the number of additional f-stops of exposure required for bellows extension be x, f = focal length of the lens, and p = diaphragm-to-subject distance.

    Then

    x = 2*ln[p/(p – f)]/ln2



    Example 1: f = 180mm, p = 3 meters = 3000mm.

    Then

    x = 2*ln[3000/(3000-180)]/ln2 = 0.18 stops.



    Example 2: f = 180mm, p = 360mm

    Then

    x = 2*ln[360/(360 – 180)]/ln2 = 2 stops.


    You can easily generate a table for each focal length you use. I find it more convenient to simply program the calculator with the generic formula and enter f, p, and execute. The result is the exact additional exposure needed in f stops. The calculator is easily carried in a camera bag or coat pocket.
     
  21. holmburgers

    holmburgers Member

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    That's good, but how about a formula where p=_____. I think I have a learning disability or something when it comes to algebra. If only they had told me in high school that I would need this stuff in 10 years to figure out my photography exposures, THEN I would've paid more attention.... maybe. :wink:
     
  22. Q.G.

    Q.G. Inactive

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    Because at those distances you can guesstimate, you don't need compensation.
    At those where you do, guesstimate errors are big.
     
  23. holmburgers

    holmburgers Member

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    Yeah, but why is the sky blue?

    No, but seriously... someone could just as easily measure the distance, but it seems no one uses subject distance to figure bellows-factor. That's what I was wondering "why does no one do this?", not estimating distance.
     
  24. Q.G.

    Q.G. Inactive

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    Because it is much, much easier to measure bellows draw than subject distance.
     
  25. 2F/2F

    2F/2F Member

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    Rayleigh scattering. My dad taught me that when I asked that question. What a great dad! :D
     
  26. holmburgers

    holmburgers Member

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    Hahaha, well I didn't expect a coherent answer to that question, but I'll take it!
     
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