What is a stop of light?
"Stop it down" & "increase your ISO by a few stops" were two phrases that really confused me when I started in photography. I understood what it meant, but not the meaning. What is a stop of light?
Light is measured in stops
A stop of light is not a fundamental unit, but a method of measuring the increase or decrease of light in a photographic exposure. 1 light stop is either double (+ 1-stop) or half (- 1-stop). So, irrespective of your initial exposure, increasing it by 1-stop means twice as much light, and decreasing it by 1-stop means half the amount of light. So far, so good right? Well, it gets a little more complex when you think of more than 1-stop. For example, if you increased the light by 2-stops, it is actually 4x the light (2x2), and 3-stops is 8x.
In terms of how this affects your images, here is the same photo processed to show up to -3 and +3 stops difference.
Note how the histogram changes when you go up or down in light stops.
In practice however, it is not so precise. Photographers often "ball-park" the stops, estimating and approximating it as dealing with exponentials in our heads is tough. Generally though we are only ever 1-stop out in our exposures, unless something went horribly wrong! :)
We control the stops of light using ISO, Shutter speed and Aperture, but each is slightly different when it comes to how to calculate light stops.
ISO and a stop of light
Stops of light in terms of ISO is the easiest to deal with. Double the ISO an you have double the amount of light sensitivity and therefore 1-stop of light. The same also applies in reverse. So if I was using ISO 800 and I wanted 1-Stop more light, I would increase it to ISO 1600, or if I wanted 1-stop less light, I would use ISO 400. This is basic maths.
Shutter Speed and a stops of light
The shutter speed determines how long the sensor is exposed to light and is measured in seconds, or fractions of seconds. Doubling the shutter speed means the sensor has half the about of time exposed to light and therefore 1-stop LESS light. Inversely, halving the shutter speed means the exposure is twice as long, and you have GAINED 1-stop of light. So if I started with a shutter speed of 1/500, and I wanted 1-stop more light, I would reduce the shutter speed to 1/250. Likewise if I wanted 1-stop less light, I would increase the shutter speed to 1/1000. It is still basic maths, just a little more advanced. :)
Aperture and the f/stop
Up until now, it has been fairly straightforward, but alas, here we are with aperture. The aperture of a lens is denoted by an f/number, which corresponds to how much light the lens lets in. Unlike shutter speed and ISO where you can double or half to increase or decrease by 1-stop of light, for aperture it is in multiples of 1.4!!! This is because lenses are circular, so if you want to double to area of a circle you increase the diameter (and radius alike) by 1.4 (flashback to school maths anyone?). So, increasing the diameter of lens opening by 1.4 will let in 1-stop more light, and reducing it by 1.4 will let in 1-stop less! Just to make a little bit harder, the f/number is inversely proportional to the area, meaning the higher the f/number the less light it lets in. A change of 1-stop of light using aperture is called an f/stop.
So if you were using at f/5.6 and you wanted 1-stop more light, you would divide 5.6 by 1.4 to get about 4, so you would use f/4. If you wanted to let in 1-stop less light, you would multiply 5.6 by 1.4 to get 8, use f/8. This sound like a drag to calculate each time, and it is. Most people just learn the f/numbers off-by-heart without really understanding their meaning.
The bottom line
- A stop of light = double or half the amount of light in respect to a photographic exposure (image)
- Doubling the ISO will INCREASE the exposure by 1-stop
- Doubling the shutter speed (making it faster) will DECREASE the exposure by 1-stop
- Increasing the aperture by 1.4x (an f/stop) will DECREASE the exposure by 1-stop
- Forget about the f/stop calculations and just learnt them. 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16. :)