F-stop memorization and math

edited September 2014 Posted in » Nikon D3100 Forum

I am struggling with remembering fstops and hot to utilize math in conjunction with aperture. I understand that the largest aperture is 1.0 and everything below is a fraction but cannot understand/memorize the math that is involved when switching fstops. Is there a guide/graph out there that explains fstops and a simple way to remember how to calculate between stops?



  • edited September 2014
    Once upon a time when cameras did not have so many bells and whistles, this was a bit easier. Shutter speeds progressed by approximate doubling and halving, and F stops were a standard halving progression as well: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22. Each of these stops changed the exposure by doubling or halving. The actual numbers represent a complex calculation of the ratio between actual aperture size and focal length, but the stops were largely standardized though many old lenses included a half stop or so added to the maximum aperture (so you'd have a 3.5, a 4.5. a 6.3, and so forth). The rest of the stops would be standardized so that you could always rely on the basic action - click the lens a stop wider, and click the shutter a stop faster to keep your exposure the same. Most older lenses only had one stop detents making it easy to adjust exposure even in the dark.

    I don't think there's an easier way of remembering the stops than just to memorize the common full stop numbers. Shutter speeds are direct - half or double the speed is a one stop change. ISO numbers are also direct - half or double the ISO number is a one stop change as well.

    On the D3200 and I presume the 3100 as well, the electronic selector proceeds in 1/3 stop increments. Three clicks moves you a full stop. So if you have a desired exposure value that says "this speed at this F stop", you can change the speed by one click faster, and restore the exposure value by changing the aperture one click wider. If you are shooting manually, and start with the meter's recommendation, each click represents a 1/3 stop deviation.

    Here's a chart of equivalents if you want it on paper. Many charts of varying complexity exist and can be found if you hunt.

  • edited September 2014
    Thank you! It's making more sense now! Luckily I just need to work in whole stops for now, but I'm still confused how in an upcoming quiz I am going to know the difference between stops and exposure and how to compensate. In time it will all be clear, thanks for the reply!
  • edited September 2014
    Aperture stops are the most difficult to remember as they don’t follow any apparent pattern. Then again it’s only 10 numbers (from f/1 to f/22), so it’s not beyond rote memorization.
    Here’s my own strategy for remembering them:

    7 of the 10 stops are whole numbers. That is: 1, 2, 4, 8, 11, 16, 22
    So if you’re taking a written test, write them out first. Start with 1. Then multiply it by 2 and write down the number. Repeat this 3 more times and you’ll come up with 1, 2, 4, 8, 16.
    Most lenses have a minimum aperture of f/22, so remember that and write it down. Half that number to get 11.
    Put the numbers into sequential order and you now have 7 of the 10 aperture stops.

    The other 3 stops are 1.4, 2.8, and 5.6. You’ll need to just remember them somehow. For me, I find it easy to relate them to something concrete.
    For example:
    f/1.4 is the max aperture of most high-end primes.
    f/2.8 is the max aperture of most fast zooms.
    f/5.6 is the max aperture of the kit lens at the long end.

    (Or you could just remember 1.4 and then its next two multiples of 2)

    Again, put the numbers into sequential order and you’ll end up with the 10 full stops between f/1 and f/22.

    Once you have a firm grasp of what the full stops of aperture, shutter, and ISO are, it becomes simple to adjust the aperture and then compensate.
    Let’s say you start off with your exposure at f/2.8, 1/30, ISO 400.
    You now want to shoot at f/5.6 which means you reduced the amount of light by 2 stops.
    So if you want to maintain the same exposure, you must compensate those 2 stops of light with shutter and ISO.
    The options are:
    f/5.6, 1/8, ISO 400 (slowed shutter by 2 stops)
    f/5.6, 1/30, ISO 1600 (increased ISO sensitivity by 2 stops)
    f/5.6, 1/15, ISO 800 (slowed shutter by 1 stop and increased ISO sensitivity by 1 stop)
  • edited September 2014
    I never realized this until just now. But it appears that the aperture stops do follow some sort of pattern that makes memorizing them a piece of cake. See the following:

    1 x 2^0 = 1
    1 x 2^1 = 2
    1 x 2^2 = 4
    1 x 2^3 = 8
    1 x 2^4 = 16

    1.4 x 2^0 = 1.4
    1.4 x 2^1 = 2.8
    1.4 x 2^2 = 5.6

    11 x 2^0 = 11
    11 x 2^1 = 22

    So you just need to remember 3 numbers (i.e., 1, 1.4, and 11) and then take their powers of 2. Then put them all in sequential order.
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