What is an F-stop?

Written by: Brandon Yuong

apertureThis part is going to be a little technical and confusing, but please bear with me. What is an f-stop? An f-stop is a relative value that is used for measuring the opening of the aperture of a lens. It is also known as relative aperture, f-ratio, and focal ratio. The larger the aperture opening, the more light is let in, and vice versa. The measurement is written as f4 or f/4, but should be expressed as the fraction since that would be most accurate in portraying the actual physical dimensions of aperture (the larger number means smaller opening). For example, f4 is larger than f16, which doesn’t seem to make sense unless you think of them as fractions: f/4 (like 1/4) is larger than f/16 (or, 1/16).

Relating the Exposure Values

At its core, a stop is a measure of light. F-stop is a relative number that is directly related to a measurement of light. A stop of light is equivalent to doubling or halving the amount of light in a photograph. For example, increasing by 1-stop is doubling the light, by 2-stops is quadrupling, etc. Decreasing by 1-stop is halving the light, decreasing by 2-stops is for 1/4th the light, etc. Modern cameras can be adjusted to increase or decrease by 1/3 or 1/2 stops. For instance, my cameras shift by 1/3 stops, so I know that 3 clicks of a change is going to shift the exposure by 1 stop (when shooting in manual).

A stop of light is related to shutter speed and ISO by a factor of 2. So to double or halve the light with either, you will multiply or divide by 2, respectively. If I am at 1/500 second, and I want to double the light (1 stop), I would increase the shutter speed to 1/250 second (1/500 times 2). If I instead want to halve it, I would slow down to 1/1000 second (1/500 divided by 2). Likewise, if I am at 200 ISO and I want to double the light (1 stop), I increase it to 400 (200 times 2). For quadruple the light (2 stops), I increase it to 800 ISO (200 times 2^2). For 3 stops, I need to go to 1600 ISO (200 times 2^3). Basically, multiply/divide by 2 at each step until you’re where you want to be.

Finally, f-stops (aperture) is related to 1-stop differently than shutter speed and ISO: it is related by a factor of about 1.414 (approximately the square root of 2). To increase aperture by 1-stop (double the light), the aperture value must be multiplied by sqrt(2). The progression of aperture by 1-stop would be the following, if each iteration was multiplied by sqrt(2): f/22, f/16, f/11, f/8, f/5.6, f/4, f/2.8, f/2.0, f/1.4, f/1.0, etc. No matter how mathematical you are, it is easier to just remember these numbers for aperture. You’ll be quicker in the field that way, trust me. Alternatively, if your camera shifts by 1/3 stop, know that 1 click of the dial is 1/3 stop, 2 clicks is 2/3 stop, and 3 clicks is 1 stop.

Here is a chart for an example of the relationship between each exposure setting and f-stops:

F-stops Relative Light Aperture Shutter Speed ISO
Starting point 1x f/22 1/4000 s 100
1 2x f/16 1/2000 s 200
2 4x f/11 1/1000 seconds 400
3 8x f/8 1/500 seconds 800
4 16x f/5.6 1/250 seconds 1600
5 32x f/4 1/125 seconds 3200
6 64x f/2.8 1/60 seconds 6400
7 128x f/2 1/30 seconds 12800
8 256x f/1.4 1/15 seconds 25600

+1 f-stop example

+1 f-stop

Original image

Original image

-1 f-stop example

-1 f-stop