After trudging through all those scary numbers, we now know that ISO 200 is twice as fast as ISO 100, that 1/60 second is half the time of 1/30 second, and that f/4 is twice as fast as f/5.6. If I did my job well, we even learned that without anyone’s brain dripping out of their ears and onto their keyboards.
Note: the word “fast” in photography generally means “captures more light” when it’s talking about ISO or aperture. When talking about shutter speed, though, “faster” means just the opposite– 1/60 is faster than 1/30, but 1/30 lets in twice as much light. It’s confusing.
What might or might not be obvious now is that we can change two different things and keep the exact same exposure. For example, each of these settings will produce the exact same exposure:
|ISO 100||f/4||1/60 sec||baseline|
|ISO 100||f/2.8||1/120 sec||faster (wider, more light) aperture, faster (less light) shutter speed|
|ISO 200||f/4||1/120 sec||faster ISO, faster (less light) shutter speed|
|ISO 200||f/5.6||1/60 sec||faster ISO, faster (wider, more light) aperture|
|ISO 400||f/5.6||1/120 sec||ISO four times as fast, shutter speed twice as fast (less light), aperture half as fast (narrower, less light)|
The first line is our baseline measurement. On the second one, we opened up the aperture wider to let more light in. In order to keep the exposure constant, we need to capture less of the light, so we keep the shutter open for half as long.
On the third and fourth lines, we increase the ISO, which records more light To balance it out, we either keep the shutter open for less time, or we make the aperture smaller– either of those will let less light in.
The fifth line is tricky! We’ve quadrupled the ISO, which means we’ve increased it by two stops. In order to compensate for that, we made the aperture smaller by one stop and we also made the shutter speed faster by one stop.
The fancy technical term for this is reciprocity– it really just means that if you change one setting, you can balance it out by changing another one, and get the same result.
So how would you use this? Here’s an example. Let’s say you’re taking a picture of a group of people, some of them standing behind the others. You put the camera in aperture priority mode and pick f/4 and ISO 100. When you try to take the shot, you find out that the best shutter speed is 1/30, and that’s too slow for you to hold the camera steady– you need at least 1/50 sec. What do you do?
Well, you can try using f/2.8 and 1/60. That will get you the same exposure, but at a faster shutter speed. But wait… now you’re at f/2.8, and when you focus on the front row of people the back row is out of focus, or vice versa. You really need f/4 to get everyone in focus.
Finally, you set the ISO to 200, which lets you use an aperture of f/4 with a shutter speed of 1/60. Voila! You get your shot.
The way exposure settings play together is a little bit like having a squishy ball in your hand. When you squeeze one part to make it smaller, another part gets bigger. Squish two parts, and the third one gets a lot bigger. Squish squish squish.