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Thread: Do you need addtional exposure for longer focal lengths

  1. #1

    Do you need addtional exposure for longer focal lengths

    I know that when you are focusing closer than infinity say with a 150mm lens you need to think about the possibility of increasing your exposure due to the bell ows extension causing some light falloff. I also realize this depends on how clo se you are focusing , how much the bellows is extended and that will determine the exposure adjustment to be made. What I am curious about is when you have a 2 10mmm on your camera and focused at infinity your bellows would be extended to 2 10mm between the lens and the focal point on the film plane. With this lens focu sed at infinity you would not need to make adjustments for exposure but would ex pect the exposure to be correct based upon the f-stop and shutter speed for expo sure as indicated by a light meter reading.

    Now say you put a 450mm lens on and the bellows needs to expand to 450mm between the lens and the focal point on the film plane. Do you need to make exposure ad justments due to the long extension of the bellows or I am just overly concerned by the visual impact of the bellows being expanded so much due to the 450mm len s?

    Regards,
    Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

  2. #2

    Do you need addtional exposure for longer focal lengths

    No exposure compensation is needed if the lens is focused at infinity. The f-stop is computed for the focal length of the lens.

    Good light,

  3. #3

    Do you need addtional exposure for longer focal lengths

    your lens stops are calibrated for infinity focus whatever extension, when it says f16 on your lens, this means f16 when focus at infinity !!!

  4. #4

    Do you need addtional exposure for longer focal lengths

    No difference in chrome density that I have ever noticed between using an 800mm T Nikkor (ca. 22" of extension for infinity) and far lesser focal lengths when measuring light with a (external) spotmeter set at manufacturer's recommended ISO. Thus NO compensation whatsoever is necessary for lenses at infinity distances (unless the f-stop scale is inaccurate, of course).

  5. #5
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    Do you need addtional exposure for longer focal lengths

    A rule of thumb for bellows correction is when you are focussing closer than 6-8 times the focal length of the lens, you will need to increase exposure for bellows correction (there are tables and calculators available). With long lenses you might need to pay particular attention to the tonal range seen by the lens, which may be significantly different than the average scene figures, depending on your metering techniques.

  6. #6

    Do you need addtional exposure for longer focal lengths

    the answer is very simple why you don't need increased exposure. An aperture (f)stop is defined as the ratio of the aperture diameter to the focal length, so you see, if you write it down you will see that if you increase the focal lenght the you have to increase the diameter to keep the same F stop, in other words if you increase the focal length you have to increase the "hole" through wich light passes to keep the same f stop.

  7. #7

    Do you need addtional exposure for longer focal lengths

    As everyone has pointed out, f32 with a 90mm lens does equal f32 with a 450mm lens if both are focused at infinity. But since infinity is so much further away with a 450 it is more likely that bellows correction will come into play than with a 90 because it is more likely one will focus somewhere short of infinity.
    John Hennessy

  8. #8
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    Do you need addtional exposure for longer focal lengths

    You've probably noticed that lenses get much larger as the focal length increases. It's this increase in size that lets enough additional light into the camera to adjust for the fact that when focused at infinity, the lens is further from the negative for higher focal length lenses.

  9. #9

    Do you need addtional exposure for longer focal lengths

    If i am not mistaken, the focus is not part of the equation, just the f stop (and you're shutter speed and film speed of course!!).

  10. #10

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    Do you need addtional exposure for longer focal lengths

    If we over-simplify for a minute let's think in terms of f10. 1/10th the focal length. For a 150mm lens at infinity it should be near 15mm. The aperture hole would be 1/10th the length. So a 450mm lens at infinity at f10 would have a 45mm hole for the light to come through. If you had the 150mm on and the bellows at 450mm because you're photographing something 3 inches away, The scale would say f10 when the hole is 15mm, but you no longer have a 150mm lens. In that case you have a 450mm lens with an innaccurate scale. Your 15mm hole at 450mm is 1/30 the focal length, not 1/10th so it becomes f30. You could do the long equation and get to the same place. This has been a useful shortcut for me because the jump to real numbers, f11/ f32 is easily made. I hope I didn't confuse you more. Sometimes what seems easy and logical to me completely befuddles someone else.
    He is no fool who gives what he cannot keep..to gain that which he cannot lose. Jim Elliot, 1949

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