Hi, I'm Joe. I teach photography at the School of Visual Arts in New York. I want to talk a little bit about how you control exposure.
You control exposure in your camera three ways, f-stop, shutter speed, and ISO.
F-stop is how big is my hole. Shutter speed is how long is my hole open for. And ISO is how sensitive is my hole.
I know exactly what it sounds like. I'm saying it this way for a reason. You won't forget.
F-stop is the size of the hole in your lens. It's also called the aperture. And it has an impact on the sharpness in your pictures as a function of focus through something called depth of field. The smaller the hole, the less light it lets in, but the more depth of field you get.
Depth of field is the amount of distance in front of and behind your point of focus that's just as sharp as your point of focus. And the smaller your aperture with the higher your f-stop number, the greater your depth of field is going to be. The lower the number, the bigger the hole, the less your depth of field is going to be.
Shutter speed controls sharpness as a function of motion. If you've got a very fast moving subject, you need a higher shutter speed to eliminate the appearance of motion blur in your picture.
And there's really two kinds of motion that you concern yourself with in the entire world. There's the motion of your subject, but there's also your motion.
You also have to pick a shutter speed that's going to eliminate the motion of your own hands and your breathing and your heartbeat from your pictures, and usually the longer the focal length of your lens, the more telephoto it is, the higher the shutter speed you need to use to eliminate your own hand motion from your own picture.
ISO controlled sharpness is a function of something called resolving power through signal noise or graininess. The higher your ISO, the more little random flecks of color are generated in your image that really shouldn't be there, that don't have anything in common with the tones around it and that has a tendency to make your image appear grainier and grainier and grainier, or fuzzier and fuzzier and fuzzier. And that eliminates the overall ability of your camera to capture and hold fine detail.
The way you choose which one to favor kind of depends on what you're doing. If you're photographing a car race or kids playing in the park, your number one issue is going to be motion and you're going to pick a higher shutter speed to arrest the motion, and then you're going to choose f-stop and ISO that allow you to use that shutter speed.
If you're photographing a still life, like an object on the table, you could put your camera on a tripod and in that case, your issue is probably going to be depth of field. You want enough depth of field for your object and the things around it to all be sharp all the way through and to make that happen, you're going to have to choose a smaller aperture, a smaller f-stop to increase your depth of field. And you're probably going to wind up using a longer shutter speed so that you can still let in enough light with that smaller hole.
If you're photographing like a work of art or a painting or something like that, your painting is probably not moving that fast. It's flat, so you don't need a lot of depth of field. In that case, your issue is going to be resolving power, graininess. You don't want to add a lot of camera grain to somebody's original work of art on the wall. You're going to want to choose a low ISO so that you're not adding anything to the painting that wasn't there in the first place. And in that case, while you're using a low ISO, the object's flat, you can probably use a large aperture. The object's not moving, so you can use a lower shutter speed.
And those are a few tips for choosing your f-stop and your shutter speed and your ISO when controlling exposure.