Shutter Speeds, f-stops (Depth of Field) and Camera Meters: A Quick Primer

In better cameras, you can adjust shutter speeds and f-stops. Adjustment of these two variables can result in wonderful creative effects. Knowledge of these settings empowers you as a photographer.

All photography is a compromise. You are balancing time (shutter speed) versus quantity (aperture/opening of your lens). Think of a faucet: You can turn it on full blast and fill a cup of water in five seconds. Think of "full blast" as a lens that is all the way open at a setting such as 2.8. If you turn the faucet half on (your lens is letting half as much light, at a setting such as 4.0), only half the water (light) is coming in, so to fill the glass you'll have to leave the faucet on twice as long. This length of time is the shutter speed.

Shutter Speeds

Typically, shutter speeds vary from a short 1/1000 of a second (which freezes action in sports photography) on down to slower speeds such as 1/500, 1/250 (still okay for action photography), 1/125, 1/60, 1/30 (this last is marginal for hand-holding), 1/15, 1/8, 1/4, 1/2, 1 second and "B" which stands for "bulb," a now-outmoded photographic technique, which in real life means you use a cable release and manually hold it down for a certain number of seconds or lock the cable release for extensive exposures such as astronomy photography. Use longer shutter speeds with a tripod for "impressionistic" photos or "silky" photos of waterfalls. The faster the shutter speed, the less light comes into the camera. Each shutter speed is twice or half of its adjacent shutter speed.

f-stops (Depth of Field)

Your other creative camera control is the aperture (opening) on your lens. We measure the size of the opening using f-stops. Each f-stop is twice as large or half as small as the one next to it. Thus, when you set your lens at f/8, and then adjust the lens to f/11, you're letting in half as much light. Conversely, if you set your lens at f/8 and then adjust it to f/5.6, you're letting in twice as much light. On a 35mm camera, you'll find f-stop settings like f/1 (wide open), f/1.4, f/2.8, f/3.5, f/4.0, f/5.6, f/8, f/11, f/16, f/22, f/32. To gain a bit of light, you'll also see non-standard apertures such as f/1.8. f/3.5 or f/4.5.

Why bother with all this? As you go up in f-stops, you increase the front-to-back distance (depth of field) in your photograph. Thus, if you take a picture at f/22 (a very small opening), you'll have most of your picture in focus. A way to remember this is as you go higher in f-numbers, more of the picture is in focus.
Another factor to keep in mind is that your depth of field decreases as your camera/subject distance decreases. Thus depth-of-field becomes a major challenge in close-up photography. Depth of field also decreases when you use medium or large format equipment, such as equipment that uses 120/220 or 4"x5" film. Depth of field decreases, too, when you use telephoto lenses.

Going back to our faucet analogy, the challenge with increased f-number (smaller aperture) is that less light comes in through the lens. So you must compensate with a slower shutter speed. And hence we have problems like camera shake. The choice of apertures and shutter speeds is yours.

Most of the time you will be safe in using mid-range camera settings. In sunny mid-day conditions, you can use an f-stop of f/8 and 1/250 of a second and do just fine with ISO 100 film. Automatic and inexpensive box cameras are preset to such aperture/shutter speed combinations.

Many newer cameras allow you to set them to either shutter-priority or aperture-priority modes. Typically, use shutter-priority for sports or action photography to ensure high shutter speeds with telephoto lenses. Use aperture-priority for landscapes.

Many cameras (especially older, manual models) have a feature called depth of field preview. Since nearly all lenses nowadays are automatic, meaning that you view your image at the wide-open setting until you take the photo, when the lens stops down to your desired aperture, you can't see how much of your image is in focus. When you press the depth-of-field preview button, the image will appear darker, depending on the f/stop you've chosen. But you will be able to see how much of your image is sharp. Older camera lenses also have depth of field markings on the lens barrels.

Film Speeds

Film "speeds" vary according to the sensitivity of the chemicals in the film. Films with bigger ISO numbers (such as 400) are more sensitive to light, so they're great when you work with low-light photographic situations or you have to do sports photography. However, such films aren't as high-quality as lower ISO films, such as films with an ISO of 64 or 100. So there' s a trade-off.

On some cameras you must manually set the ISO for whatever film you have in the camera, usually with a dial on the top of the camera (either right or left side). Newer automatic cameras and many point-and-shoots read a pattern off the film canister, called the DX coding.

Camera Meters

Camera meters are stupid, with one simple job to do. Using center-weighted needles or LED readouts, they work to make your picture fit a middle gray tone (we call this 18% gray, based on reflectance of a universally calibrated card). In the case of color, those middle tones may be beige, flesh, green, etc.

Most of the time this system works. In the case of print film, there's a lot of room (5 stops each way for Kodacolor) for error. The machine that processes film negatives at your local drugstore will adjust for exposure errors, trying (again) to make everything a middle gray tone, as most of the time snapshooters don't want their photos to be too light or dark. But there are a lot of special situations. If you're taking a picture of your kid skiing down a snowy slope, the camera meter will want to make the whole scene gray, and the snow will come out gray, with your child as a silhouette. In such situations, take a meter reading from a neutral source (or if you want to get fancy, use a spot meter, which reads a tiny fraction of your scene, such as that found in a one-degree or five-degree arc), and use that reading for your photograph, in spite of what your meter says. If your print photo is special and you don't like the darkness or lightness what you get back from the drugstore, a custom lab will print a better-exposed print for you, though keep in mind because of the handwork, this can be costly.

If you have to make an exposure in a difficult lighting situation, try these "ball-park" neutral sources: Meter off grass. Or, on a sunny day, try metering off the north sky at the horizon line. You can also meter off the back of your hand (light skin) and then "open up" one stop (e.g., your meter says f/8, so you go to f/5.6).

Backlighting (where the primary source of light comes from behind the subject) provides another challenge. Some so-called automatic cameras have special settings for backlighting. But in general use your brain: you can generally allow two more f-stops than what your camera meter says. Or meter off a similar front-lit subject and then use those settings. When in doubt, bracket: take a meter reading according to what your meter says, then take a photo that is a half-stop or stop over-exposed and another that is a half-stop or full stop under-exposed. Often, "correct" exposure is a matter of personal preference.

Camera Meter Broken (or you don't trust it or want to double-check it)

You can try the "Sunny 16" rule. This means that you should be able to get okay pictures if, on a sunny day, you set your aperture to f/16 and your shutter speed closest to whatever ISO you're using. For example, if you're using ISO 100 film, set your aperture to f/16 and your shutter speed to 1/125 (which is closest to ISO 100).

Meters can vary from one camera to the next and are often one of the first things to malfunction in a camera. Sometimes the meter is all right, but your shutter speeds are erratic (this is especially true in older manual cameras that have been stored for a long time; a reliable camera repair shop can recalibrate your shutter speeds for you.) Check your meter against the settings recommended in this section. If you find you're consistently getting incorrect exposures you can compensate by slightly adjusting your ISO setting.

You can use creative controls by varying your apertures/shutter speeds.The inside of your film box or a separate sheet packaged with the film will give you recommended settings for common photo situations. If you have an erratic camera meter, consistently use the same kind of film and begin memorizing some of the settings below so you can out-guess your meter. Here are some typical settings and their variations for ISO 100 film (note that higher shutter speeds apply only to certain brands of automatic cameras):