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Thread: What aperture is 2.8.2?

  1. #1
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    What aperture is 2.8.2?

    I noticed that lenses are marked with the aperture number and light meters are marked in 1/10 of a stop between whole stops. Does someone make a light meter that gives you the equivalent f stop number of say 2.8 and 2/10? On a meter that would read as 2.8.2. How would I know the decimal equivalent of say, f3? f12.5? Is f4.5 the same as 4 and 5/10?
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  2. #2
    Eric Woodbury
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    Re: What aperture is 2.8.2?

    2.8 plus 20%. I commend you if your process is that accurate. Just give it f2.8 and 1/5 or a nudge. Then again, better over-exposed than under. If that other number is 1 second, you are in reciprocity territory anyway.

    [BTW, I have this meter and the little ON switch was always sliding on and battery dead. Then I taped the switch off and the battery still seems to die, so now I remove the battery and tape it to the bottom of the meter for when I need it. You experience any of that?]

    e

  3. #3

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    Re: What aperture is 2.8.2?

    Quote Originally Posted by rbp View Post
    I noticed that lenses are marked with the aperture number and light meters are marked in 1/10 of a stop between whole stops. Does someone make a light meter that gives you the equivalent f stop number of say 2.8 and 2/10? On a meter that would read as 2.8.2. How would I know the decimal equivalent of say, f3? f12.5? Is f4.5 the same as 2.8.8?
    The f-number changes based on the square root of 2 (1.41421...), so f/2.8 x 1.414 = ~4, which is why the stop are:

    1, 1.4, 2, 2.8, 4, 5.6, etc.

    Figuring out tenths between those two numbers would be math something like taking 1.414 and subtracting 1 (= .414) and then dividing .414 by 10 (=.0414). So .0414 is one tenth, so to calculate 2.8+.2 stops, you'd need to multiply 2.8 by 1.0828 (.0828 being .0414x2), which gives you f/3.03184...so basically f/3.
    Last edited by MAubrey; 16-Jun-2018 at 15:49. Reason: typo

  4. #4
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    Re: What aperture is 2.8.2?

    Right, thank you, that helps, be nice to have that button on a light meter, to switch between decimals and working aperture.

  5. #5
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    Re: What aperture is 2.8.2?

    Quote Originally Posted by Eric Woodbury View Post
    2.8 plus 20%. I commend you if your process is that accurate. Just give it f2.8 and 1/5 or a nudge. Then again, better over-exposed than under. If that other number is 1 second, you are in reciprocity territory anyway.

    [BTW, I have this meter and the little ON switch was always sliding on and battery dead. Then I taped the switch off and the battery still seems to die, so now I remove the battery and tape it to the bottom of the meter for when I need it. You experience any of that?]

    e
    I have a couple and not that problem. I like them because they have the feature of self timing off, the battery never wears out, just turn it on and its on, it turns itself off after a while.

  6. #6
    ic-racer's Avatar
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    Re: What aperture is 2.8.2?

    What you have is probably more useful. The indicator gives you a visual as to how far to move the aperture lever between the two numbers that are actually marked on the scale.
    Other Sekonic meters, like the 558 I have, can show really crazy numbers. For example, when set for "T" and 1/3 increments, the readout can look like this as you scroll through the shutter speeds:

    3.6(2), 4.5(0), 4.5(3), 5.0(2), 6.3(0), etc. (f-3.6 and 2/10, f-4.5 and 0/10, f-5.0 and 2/10, etc.)

  7. #7

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    Re: What aperture is 2.8.2?

    Given all the other variables, these are rounding errors and false precision.
    Last edited by faberryman; 16-Jun-2018 at 16:26.

  8. #8

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    Re: What aperture is 2.8.2?

    Quote Originally Posted by rbp View Post
    Right, thank you, that helps, be nice to have that button on a light meter, to switch between decimals and working aperture.
    I should add that other variations (f-stop vs. t-stop, tiny variations in film speed, shutter accuracy, etc.) are going to make 1/10th stop differences irrelevant for the most part. If you just round to the nearest third or half stop, you're probably going to be as accurate as you can get.
    Last edited by MAubrey; 16-Jun-2018 at 15:51. Reason: typo

  9. #9

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    Re: What aperture is 2.8.2?

    Quote Originally Posted by MAubrey View Post
    I should add that other variations (f-stop vs. t-stop, tiny variations in film speed, shutter accuracy, etc.) are going to make 1/10th stop differences irrelevant for the most part. If you just round to the nearest third or half stop, you're probably going to be as accurate as you can get.
    To be a little more precise, a shutter’s speeds are accurate if they are +\- 30% of the marked speeds.

  10. #10

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    Re: What aperture is 2.8.2?

    The decimal setting is useful when lighting a set... If there is a trend where light fall-off is starting, 1/10th of a stop will not yet be seen by the film, but is starting (and you can start reading it)... Gives you a chance to even out the lights esp with strobes...

    Steve K

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