Bellows shorter than lens' focal length

Discussion in 'Exposure Discussion' started by Darryl Roberts, Oct 22, 2018.

  1. Darryl Roberts

    Darryl Roberts Member
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    I'm aware that with a bellows extended past the lens' focal length, exposure must be increased.

    If the bellows is decreased below the focal length, should it be decreased?

    Thank you.
     
  2. RalphLambrecht

    RalphLambrecht Subscriber
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    No.there is no lens extension requiring an exposure decrease. only a lens focused at infinity needs no exposure increase.At that point, the lens extension is'0'.any additional lens extension will focus the lens closer than infinity and will require an exposure increase.equations covering the exposure increase are readily available online.
     
  3. Chan Tran

    Chan Tran Member
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    If the OP would move the lens closer than infinity position then nothing is sharp but I do think he would gain a bit of light intensity.
     
  4. OP
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    Darryl Roberts

    Darryl Roberts Member
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    I'm asking if the 90mm lens is pulled back to 70mm worth of bellows, do I decrease the exposure?
     
  5. kmg1974

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    Difficult to say without info on the lens. Maybe you can simplify by assuming :
    For an area of 900 mm^2 you need an exposure of x (lens at infinity ). Most likely your lens will not cover 900 mm2 at bellows extension less than focal length. So now the light would be spread over less than 900mm2, so my guess is that yes you would have to decrease exposure
    basically what poster 3 said
     
    Last edited: Oct 22, 2018
  6. kmg1974

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    ok, maybe it can be explained better with typing on computer:
    1. the amount of light going through a lens is X.
    2. that light X is spread over an area A (pi*r2), where r is the radius of the circle.
    3. increasing the area the lens covers (i.e. focusing closer than infinity) increases the total area to A^1. the increase is the extension squared (as the circle projected by the lens increases by the distance from the lens). but X stays the same regardless of A.
    4. if the lens is closer to the film plane, then X will be spread over less area. lets assume your 90 mm lens has 90 degree coverage. that gives you a circle of projection of 254 cm2. if you move it to 7 cm, the circle changes in size to 154 cm2. so, around 70% of the area covered by the lens at 90 mm.
    if we set X to be 100, then the intensity per cm2 is around 4 for a lens at 90 mm, and around 6.5 for the same lens at 70 mm
     
  7. Ian C

    Ian C Member
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    Regarding post #4

    Measuring the bellows length alone tells you nothing. The “bellows extension” must be reckoned from the second nodal point of the lens to the image plane (i.e. to the film plane).
     
  8. kmg1974

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    true, but it would mainly be in play if you are looking at very short bellows extensions. I.e. if you are off by 1 cm the extension factor will be a lot bigger if you are going from 3 to 2 (or vice versa) than if you go from 8 to 7
     
  9. Dan Fromm

    Dan Fromm Member

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    Darryl, you shoot 4x5. You have a 90 mm lens.

    I don't know which one, but since, if I'm not mistaken, there are no 90 mm retrofocus lenses for 4x5 it doesn't matter. When y'r 90 is focused to infinity the film plane to rear node distance will be 90 mm. The rear node will be very near the diaphragm.

    If you move the lens closer to the film plane -- reduce the extension -- then, as Chan Tran pointed out in post #3 above, the image the lens puts on the film plane will be out of focus. Focusing through infinity make no photographic sense.

    So your question is, to be polite, silly. Now tell us about the problem you're trying to solve.
     
  10. jim10219

    jim10219 Member
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    Yes, in theory. The reason for the difference in light is due to the inverse square law. Basically, the further you go out, the more the light is dispersed across an ever increasing image circle. So as the image circle expands, the intensity of light will be at any one point on the circle will be reduced. Basically, there will be fewer photons per square inch if you increase the square inches without increasing the number of photons. And the opposite would also apply. If you're shrinking the area of the image circle without changing the number of photons, then you'll get more photons per square inch. Whether or not it's enough to be worth calculating, is another story. You can look up the inverse square law to get the formula, its fairly simple to use. But that's the principle on which bellows extension is calculated, and yes, it does work both ways.

    Now, as others have stated, since all large format 90mm lenses that I'm aware of actually focus to infinity somewhere around 100mm (you'll have to look up the specs on your specific lens), you'll want to calculate the distance against the actual point of infinity focus, and not the labeled focal length on the lens. Also, as others have stated, you're not going to have anything in focus (which I'm assuming is what you're trying to accomplish here).
     
  11. kmg1974

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    i can see why focusing beyond infinity could be interesting with large format lenses. Most likely an artistic image with blurred shapes.... maybe as part of a multiexposure setup?
     
  12. Ian C

    Ian C Member
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    Here’s how to use bellows extension to determine exposure. First you need to know the position of the 2nd nodal point of the lens. It will be on or near the lens assembly. This position is fixed. You can determine the position of the 2nd nodal point easily by first focusing at infinity. Then measure from the film plane (front surface of ground glass on a view camera) forward the distance f.

    The 2nd nodal point is at the distance f forward from the film plane when the lens is focused at infinity. For any particular lens the 2nd nodal point is always in the same place. Call the 2nd nodal point P. [This might not apply to a zoom or varifocal lens]

    To calculate the exposure, first focus the camera as you want. Measure from the film plane forward to P. Call this distance E—the extension. Then the amount of additional exposure due to the extension is 2.885*ln(E/f). [ln is the natural logarithm function on a calculator]

    For example, if you use a 90mm lens and E = 150mm, then

    Exposure increase = 2.885*ln(150/90) = 1.47 stops. If the light meter read, say, 1/15 second at f/16 you’d need to increase the exposure by about 1.5 stops. You could double the time for a one-stop increase and open the aperture for the another one-half stop increase.

    This can be had with 1/8 second with the aperture halfway between f/11 and f/16. Or you could use 1/4 second and the aperture halfway between f/16 and f/22, depending on your depth-of-field requirement.

    The phase “bellows extension” really means the distance from the film plane to the first nodal point—not the bellows length alone.
     
  13. Doremus Scudder

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    At infinity focus, the lens' nodal point(usually about the middle of the lens in most designs; exceptions being telephoto designs and retrofocus wide-angle lenses) is the lens' focal length from the film. Shortening this distance (e.g., your 90mm lens 70mm from the film) will indeed increase the intensity of light according to the inverse square law. Unfortunately, there will be nothing in sharp focus since the lens is, in effect, focused "beyond" infinity. Unless you want an out-of-focus image for some reason, focusing shorter than infinity has no purpose whatsoever. Therefore, worrying about adjusting exposure for such a shot is moot.

    Best,

    Doremus
     
  14. bernard_L

    bernard_L Member

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    Exposure compensation is a little more complex for non symmetrical lenses. From the Kodak Professional Photoguide:

    CloseUpExposure1.jpg
     
  15. OP
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    Darryl Roberts

    Darryl Roberts Member
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    Thank you, I miscalculated my mm length the 7 3/16 bellows (Calumet CC-402) is approximately 183mm, I get it now.
     
  16. ic-racer

    ic-racer Member

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    Focusing beyond infinity does not get you much, except a blurry image. It does not get you to "Infinity and Beyond" if that is where you want to go...
    Screen Shot 2018-10-22 at 8.47.56 PM.png
     
  17. OP
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    Darryl Roberts

    Darryl Roberts Member
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    Thank you all for your insightful answers.
     
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