I encounter quite a few hobbyists who don’t know what a “stop” is. A stop, in photographic terms, is a relative value for exposure to light, or a change in exposure by a factor of two, or +/-1 EV.
Got it? No? Okay then, if I change my exposure by one stop, I’m either letting in twice as much light, or half as much (depending on which way I’m adjusting). There are three ways I can adjust this on a modern SLR and each way effects other aspects of the photo. Understanding these other effects is important to controlling your image.
While you read this there is one important point to keep in mind. I am a photographer, not a writer. I’m not good a document structure and tend to drop into a ‘stream of consciousness’ style. Or, in my case, a ‘puddle of consciousness.’
Shutter speed, f-stop and ISO setting combine to keep the background recognizable, yet keep the subject separated from the background.
1. Shutter Speed: Go to your kitchen sink and turn on the faucet for 3 seconds. How much water came out? 6 ounces? Good. Now run it for 6 seconds. 12 ounces? Imagine that. Run it twice as long for twice the water. Your shutter works the same. If I shoot at 1/80 sec and my image is overexposed by one stop (+1 EV), I can speed the shutter up to 1/160 for correct exposure. That’s three clicks on the adjuster dial for most modern cameras.
There are ways this will affect you image. First, is blur. A faster shutter will freeze motion while a slower setting may give you a bit of motion blur to enhance a feeling of movement. Too slow, however, and camera shake can become an issue, especially at longer focal lengths. The second way shutter speed affects our image is ambient exposure. With flash photography, the flash is faster than your shutter (it has to be. look up “sync speed”). This means that changing the shutter speed doesn’t really effect flash exposure but it will change how much the ambient light contributes to the image. A great way to understand this is to try this little exercise:
Go out in the evening and shoot a victim subject against a sunset (or other situation where they’ll be in silhouette against the sky). Set your shutter speed to your max sync speed (usually 1/200 or 1/250) and close down your aperture so the sky is dark (just KINDA dark, not black). Take a shot (camera and flash should be in manual mode). Now turn on your flash and take a shot. You can see your subject now, but the sky hasn’t changed. change the power setting on your flash to get a nice light on your subject and you’ll see that you’re not effecting the sky at all. Now slow your shutter down one stop (3 clicks, or half the speed). Shoot. You should have more sky now, but the subject didn’t change. Slow it down another stop and shoot. And again. Eventually you’ll get to a point where the exposure on your subject will start changing. This is because you have some little bit of ambient light coming off the subject and you’ve gotten slow enough that it starts adding to the exposure. Unless you have a tripod and a very steady subject, you’ll start to see your image getting a bit soft now because of the blur factors I mentioned before.
2. ISO: ISO stands for International Organization for Standardization (I know that should be IOS. Go ask them, not me.). With film, there were two standards for designating sensitivity (film speed). The arithmetic (ASA) scale where doubling the number meant twice as sensitive, and the logarithmic (DIN) scale where increasing the number by 3 meant twice as sensitive. On film boxes you’ll sometimes see “ISO 100/21″ which would be half as sensitive as “ISO 200/24.” It’s usually just shortened to “ISO 100.”
Digital cameras work the same way. Setting you camera to ISO 400 makes it twice as sensitive as ISO 200 and half as sensitive as ISO 800. So, adding this to our shutter speed discussion, If my image is well exposed at 1/80 ISO 200 but it has a bit of motion blur that I don’t want, I can speed my shutter up to 1/160 to eliminate the blur and turn my ISO to 400 to compensate.
Starting to see how they work together? If I double one setting, I can cut the other in half to get the same exposure with different effects on the image. Higher ISO setting result in noisier images, So you should generally try to shoot at the lowest ISO you can.
3. f-stop: There’s a reason I saved this one for last. It’s the hardest to get your head around. Partly because of math and physics and partly because usually people use the terms slightly incorrectly. First, let’s talk about “aperture.” An “aperture” is an opening. In optics, it’s the opening through which light travels. The aperture of a lens is it’s diameter (lens here meaning the hunk of glass, not an entire lens unit for a camera). The f-stop of a lens is the ratio of it’s aperture to it’s focal length and describes the relative amount of light it can put at it’s focal point. The longer the focal length, the larger diameter it must have to put the same amount of light at it’s focus.
Now let’s talk about the lenses you’re used to. A camera lens is really a unit containing multiple lenses (referred to as lens elements) along with various electronics, focus/zoom controls, and maybe even a focusing motor. The values given (50mm f/2.0) are the effective focal length and maximum f-stop. If you were to replace the lens with a single piece of glass, that’s the size it would be. As I zoom in with my 50-150mm the overall length of the lens doesn’t really change, although it often will with cheaper consumer-grade lenses. Aperture is changed on camera lenses by varying an opening inside the lens by using an iris. As the diameter of the opening in the iris changes, more or less of the lens is used effectively changing the lens diameter. The aperture size and the focal length combine to determine the f-stop. Much of the confusion around these concepts arises from the common practice of using the terms “f-stop” and “aperture” interchangeably. A 100mm lens with an aperture of 25mm has an f-stop of f/4.0, while a 200mm lens with a 25mm aperture would be f/2.0. Same aperture, different f-stop. The terms are NOT synonymous.
Because of the math involved, f-stops follow the pattern of 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, … with 2.8-22 being most common on camera lenses. Those numbers represent full stops. In other words, f/4 gives half the exposure than f/2.8 and f/5.6 halves it again.
Higher numbers mean less exposure, but greater Depth of Field. Most lenses are sharpest, however, a couple of stops from their maximum aperture. When adjusting aperture, like shutter speeds, three clicks on the wheel mean on full stop on most SLRs these days.
Hopefully, you now have a better understanding of the relationship between shutter speed, ISO, and f-stop. Determin proper exposure and adjust from there taking into account, image noise, motion stopping, and depth of field.