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Thread: Meter & f-stop discrepancy

  1. #1

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    Meter & f-stop discrepancy

    This is a question that has been bugging me for a while, and I'd like our prodigious panel of experts to weigh in on it, please.
    My light meter (Minolta Flash Meter III) reads in 1/10th stops.
    My vintage lenses have settings in partial f-stops...
    I understand that f6.3 would read as "f5.6.5" on my meter, but f4.5 on my Wollensak Velostigmat equates to what, exactly in tenths-of-a-stop on my meter?
    Thank you!

  2. #2

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    Re: Meter & f-stop discrepancy

    f/4.5 is one-third of a stop slower than f/4, so it would look like "f4 .3" on a meter with 1/10 stop display.

    To find the 1/3, 2/3, and full stop f-numbers, multiply the f-number by 1.12, 1.26, 1.41 respectively - so the third-stop increments are f/4, 4.5, 5.0, 5.6. But they would look like "4, 4 .3, 4 .7, 5.6" on a meter with 1/10 stops - I've always found this not very intuitive.

    To find the 1/2 and full stop f-numbers, multiply by 1.19, 1.41 respectively, so the half-stop increments are f/4, 4.8, 5.6.

  3. #3

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    Re: Meter & f-stop discrepancy

    The full f stop series are the integer powers of the square root of 2 or (√2)^X where X is an integer. Each integer represents a full stop.

    Thus:
    (√2)^1=1.414
    (√2)^2=2
    (√2)^3=2.828
    (√2)^4=4
    (√2)^5=5.657
    (√2)^6=8
    (√2)^7=11.314
    (√2)^8=16
    (√2)^9=22.627
    (√2)^10=32

    For tenth stops, you add tenths of stops to the integer exponents. (√2)^4=4 and (√2)^4.1=4.141, (√2)^4.2=4.287, (√2)^4.3=4.438, (√2)^4.4=4.595
    So your f4.5 lens is about .3 stops from f4

    Similarly (√2)^5=5.657 and (√2)^5.1=5.856, (√2)^5.2=6.063, (√2)^5.3=6.277, (√2)^5.4=6.498
    So your f6.3 lens is about .3 stops from f5.6

    You can also work backwards using logs. The exponent X = Log(f stop)/Log(√2)
    For the f6.3 lens exponent X = Log(6.3)/log(√2) = .79934/.15051 = 5.31 Which is .31 stops from F5.6

    Similarly, for the f4.5 lens exponent X = Log(4.5)/log(√2) = .6532/.15051 = 4.34 Which is .34 stops from F4

  4. #4

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    Re: Meter & f-stop discrepancy

    jeeesh..."half stops and a tweak" works fine for me - and is about all this old brain can manage!

  5. #5

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    Re: Meter & f-stop discrepancy

    Quote Originally Posted by John Layton View Post
    jeeesh..."half stops and a tweak" works fine for me - and is about all this old brain can manage!
    You're right, of course. For all practical purposes there's no need to go to tenths of a stop. Even half stops are more than enough for critical work, so the mental aerobics aren't necessary.

    One thing worth noting, however, is that both the Minolta and Sekonic digital meters suffer from "misleading" displays. They always show the wider f number followed by a tenth decimal graphic. This is problematic when the exposure value is more than half a stop higher. In that case, the closest full f stop is not the one that is being displayed. If you're not aware of this, then you could potentially be almost a stop out. For example, suppose the measured exposure value is 0.1 stops less than f8. The closest full f stop is f8, but the meter will display f5.6 with a graphic showing another 9 steps. If you set your lens to f5.6 you'll potentially be overexposing by about a stop. These meters usually have an alternate analogue bar graph to make it easier to see the closest full or half stop, but beginners may not be aware of this. In many respects, the old analogue meters with needle pointers provide more intuitive results.

  6. #6

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    Re: Meter & f-stop discrepancy

    On the Sekonic you can change the display if you find it “misleading”.

  7. #7

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    Re: Meter & f-stop discrepancy

    Generally correct today. The days of needing to achieve better than 1/3 f-stop for high quality color transparencies are done and gone. Given the photographic materials today, how they are used and expectations 1/2 f-stop is GOOD enough.


    Bernice


    Quote Originally Posted by sharktooth View Post
    You're right, of course. For all practical purposes there's no need to go to tenths of a stop. Even half stops are more than enough for critical work, so the mental aerobics aren't necessary.

  8. #8
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    Re: Meter & f-stop discrepancy

    Quote Originally Posted by Dugan View Post
    My light meter (Minolta Flash Meter III) reads in 1/10th stops...
    That's just an artifact of standard modern electronic components using decimals.
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  9. #9

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    Re: Meter & f-stop discrepancy

    Thanks so much for the excellent answers!
    I tend toward the "Kentucky windage" approach most of the time, but it's good to know the basis of the math.
    Cheers!

  10. #10

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    Re: Meter & f-stop discrepancy

    Quote Originally Posted by Bernice Loui View Post
    Generally correct today. The days of needing to achieve better than 1/3 f-stop for high quality color transparencies are done and gone. Given the photographic materials today, how they are used and expectations 1/2 f-stop is GOOD enough.


    Bernice
    Are you saying that modern transparency film has increased latitude or that we just aren't using transparencies any more? Because I'm planning to shoot transparencies for stereo viewing.

    In general, with ambient light and negative film I don't sweat it, maybe a little tweak of the aperture a half stop as has been mentioned.

    But in the studio with strobes, it's the other way around - I'm not setting the shutter and aperture to the light, I choose the shutter and aperture and adjust the strobe power until it's what I want. In this situation, I appreciate the 1/10 stop readout.

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