Stop! In the name of light…

The topic I want to cover in this post is something rather basic – but I remember how it kept me puzzled for quite a while when I got more serious about photography. And when I finally understood what it’s all about I was like “duh!” – it helped me so much to handle my camera better, faster, and with more confidence. So I try to give an explanation in really really simple words – and hope it works. :-)

I can’t remember how many times I’ve read and heard about “fast” lenses, about “stopping down”, about “one stop” more (or less), “one more stop” of usable ISO, and so on, and so on. And I always wondered: what the hell IS a “stop” actually?

I guess the easiest way to put it is that a stop is a measure of light. It’s not absolute, it’s relative, and most often, it’s used to describe differences or changes in light. A change of one stop means half the light, or twice the light.

You can measure a stop in different ways. This has directly to do with the exposure triangle, of course.

  • By exposure time. It’s easiest to understand how the amount of light is half as much, or doubles: an exposure time of 1/500s is one stop faster than 1/250s, and half the amount of light will hit the sensor (or film) in 1/500s, compared to 1/250s. Or take an exposure time of 20 seconds: one stop more (light) would mean to double the exposure time to 40 seconds.
  • By aperture. This was the easiest to understand for me, because the aperture is (more or less) “in the way” of the light as it passes through the lens, onto the sensor or film. The series of full stops starting from f/2 would be f/2.8, f/4, f/5.6, f/8, f/11, f/16, and so on. Confusingly though, the larger the f-stop number, the smaller the opening of the aperture is – and vice versa.
    It’s easier to understand if you don’t think of the aperture as the opening itself, but as the mechanism that opens or closes (the blades). Very little of the aperture blades is in the way of the light when “using a lens is wide open” (what’s really meant with that saying is a wide open aperture) and more is in the way if we “stop down”.
  • By ISO. Somewhat similar to exposure time, this is pretty straightforward: when you halve or double the ISO, you double or halve the light that the sensor registers. ISO800 captures twice the amount of light than ISO400 (when exposure time and aperture are constant). As camera technology advanced, higher ISOs weren’t that noisy anymore, and thus people said that “this newer camera has one more stop of usable ISO.”

So if you “stop down” the lens from f/2.8 to f/4 (see above – this is one stop) you let only half the amount of light pass through the lens, and to get a proper exposure, you have to double the exposure time – let’s say from 1/100s to 1/50s. If you stop down twice (which, starting from f/2.8, would mean you end up with an aperture setting of f/5.6) you need an exposure time that is four times as long – let’s say 1/25s, instead of 1/100s.

How did this help me when I operate my camera? Well, most cameras allow adjustments to the aperture, exposure and ISO in steps of 1/3 of a stop (most Nikons can be configured to use 1/2 stop instead, but it’s not the factory default).

So, if I make a photo and notice that it is underexposed or overexposed (when I check the histogram on the camera’s display, or see the blinking highlight warning), I simply dial in “one stop” of exposure compensation – letting either more light in, or less.

The interesting thing here is that each “click” of the dial equals one step of exposure compensation, ie. 1/3 of a stop. If I cannot use a longer exposure time (because I don’t have a tripod to stabilize the camera, for example), I might need to raise the ISO sensitivity by one stop instead: I press the “ISO” qualifier button and again it’s 3 clicks of the dial, each 1/3 of a stop.

Or, if I can sacrifice the depth of field, I can open up the aperture, and again it’s 3 clicks for one full stop. I don’t have look at the actual number, I simply go click-click-click (in either direction, and for any of the three variables: exposure, aperture and ISO sensitivity) and I know I now have either halved or doubled the amount of light reaching the sensor or film.

Click-click-click. That’s one stop. It’s really that simple. I wish someone would have told me. That’s why I wrote this article. If it helps just one fellow photography fan out there, my mission is accomplished. Click-click-click. :-)

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