An Introduction to F-stop and Shutter Speed

by Mike Schumacher

Two terms you will hear are f-stop and shutter speed. They have to do with the amount of light the lens will supply to the film or digital chip. Shutter speed is the amount of time the shutter in the lens will stay open. F-stop is the amount of light the lens will let in while the shutter is open.

Shutter speed is measured in fractions of a second and for longer exposures in seconds or even minutes. Most common shutter speeds are 1/15th of a second to 1/500th of a second.

Common f-stops range from f1.8 to f22. The smaller the number the larger the opening for light to enter. An exposure of f1.8 at 1/250th would let more light in than f16 at 1/250th.

Depth of field is the area from near to far that is in focus. Smaller f-stop numbers (larger opening) will have a smaller depth of field. If you are making a portrait of a person and want just the person and not the background in focus (shallow depth of field) you would use a smaller f-stop number probably f2.8 - f5.6. If you were doing a landscape and wanted everything in focus from near to far (large depth of field) you would use a large f-stop number probably f11 - f16.

Shutter speed will control blur. Fast moving subjects require a fast shutter speed. A portrait would not need a fast shutter speed. You might want that shallow depth of field. But remember.....small f-stop = more light. So.....

The amount of light available will now dictate what shutter speed can be used for proper exposure.

As you can see from the above example the amount of light available will dictate the shutter speed / f-stop combinations that can be used. Which combination you use will depend on what you are trying to take a picture of. A fast moving car, a landscape, a flower? Is it high noon in summer or late evening in winter? Shallow depth of field or large depth of field?

As you can see f-stop and shutter speed are very inter-related. Both f-stops and shutter speeds can be measured in stops.

The stop is a basic unit of light in photography. Stops can be controlled by shutter speed, f-stop and iso setting.

Shutter speeds double or half the amount of light.

Standard shutter speeds are: 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500 1/1000 second

A step faster halves the amount of light (minus one stop), and a step slower doubles the amount of light (plus one stop).

Going from 1/60 to 1/30 second doubles the amount of light. Going from 1/30 to 1/60 second halves the amount of light.

Iso is the sensitivity of film or the digital sensor to light. The higher the number the more sensitive the film or digital sensor is to light.

Standard iso settings are: 50 100 200 400 800 1600

Increasing the iso one step doubles the amount of light (one stop). Decreasing iso one step halves the amount of light (one stop). Caution should be used when adjusting iso to a high value. Higher iso settings can introduce noise, especially in the shadows. Aperture settings are described in f-stops. F-stops specify the light gathering ability of the lens. A specific f-stop on one lens, say f8, will be the same on another lens.

Standard f-stops are: 1 1.4 2 2.8 4 5.6 8 11 16 22

If we use f4 as an example, going to f5.6 halves the amount of light (one stop) and going to f2.8 doubles the amount of light (one stop). In our f4 example, going to f5.6 would be referred to as closing down the aperture and going to f2.8 would be referred to as opening up the aperture. Aperture also controls depth of field. F16 will have more depth of field than f5.6. Depth of field can be used very creatively to direct the viewer's eye.

One scene can have many different f-stop and shutter speed combinations.

f/4 at 1/250 is the same as f/5.6 at 1/125 or f/8 at 1/60 or f/11 at 1/30.

Which one you use will be dependent on what you are trying to achieve.

Freeze action or blur? Large or small depth of field?

If you've gotten this far you might be wondering, why do I care about all this? It is very useful to understand these inter-relationships. Controlling aperture for depth of field and shutter speed for freezing or blurring action are basic tenants of creative photography.

More importantly, we've all had fantastic photo ops and tried to capture what we see. The human eye can see between 12 to 14 stops of light. Your digital or film camera can see about 5, maybe 6 or 7 if you play a bit in the darkroom. So when we are in the forest with the sun shining through the treetops, we have at least 5 stops of light or more between the shadows of the forest and the sunlight at the top of the trees.

Matrix metering combines the highlights and the shadows to create an average. So if you have bright sunlight and deep shadow, it creates a compromise that has either plugged up shadows or blown highlights. This is where knowledge of stops in photography comes in handy.

We can shoot multiple frames and use exposure compensation. We could shoot 3 frames….one frame at -1/3 (stop), one frame at the correct exposure (according to the meter in your camera) and one at + 1/3. Some cameras will only do ½ steps, consult your cameras manual to use exposure compensation.