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Relationship between apertures, depth of field (DOF) and shutter speeds
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Most of the topics discussed here apply more to single-lens reflex (SLR) cameras that offer a wide range of creative settings. Shutter speed and aperture settings can be adjusted on some point-and-shoot film and digital cameras also. The concepts apply to all cameras, but it may not be possible to make adjustments as they are managed "internally" by the camera's computer chips and sensors. The new-age digital SLR cameras also have full control over shutter speeds and apertures—just like their film camera predecessors. The format here may seem a little strange. I've decided to start with shutter speeds, realizing that there is no perfect place to start because it is difficult to discuss one (shutter speeds) without the other (apertures). So the sequence is:
This is a superficial review only covering some of the main aspects of apertures, shutters speeds and depth of field. There are more things in play than presented here. The images and text here are owned by Clive Schaupmeyer. You are free to view them for your personal learning. Images are not to be copied elsewhere. Email if you have questions. Some of this stuff is available in this handout. It may not open for you. Just save it and then open it. Cheers! Clive |
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Shutter Speeds A camera's shutter performs the same function as a shutter in front of a window−it keeps the light out. When the shutter is opened, light enters and exposes the film. The shutter speed is the length of time the shutter is open. The longer the shutter is open the more light enters. On some cameras, shutter speeds range from Fast shutter speeds are used to "stop" motion and can be used to reduce the amount of "blur" resulting from camera movement (shake). Sometimes slow shutter speeds are desirable to create a "special" effect. Slow shutter speeds may be required in low light conditions. (At slow shutter speeds a tripod−or other stabilizing technique−is needed to prevent unwanted blur.) 1000…500…250…125…60…30…15…8…4…2...1 |
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Some examples of apertures, DOF and shutter speeds Before getting into apertures and depth of field (see farther down), it may help to review these images showing the relationship between apertures, shutter speeds and depths of field. The relationship between apertures, shutter speeds and depth of field is obvious in these images below. Wide apertures (small f number like f 2.8) result in faster shutter speeds which is good for stopping action, as shown in the top set of images. (For the record, those are potatoes on a conveyor belt moving uphill from left to right.) If your scene or subject requires a deep depth of field (lots in focus from front to back), then you will need a small aperture (large number). However, the shutter speed will be reduced and that may be a problem. However, if you want to show blurred action, then this is good. The selection is sometimes a compromise. Pay special attention to the images in the right column. Note that as the shutter speeds get longer, the potatoes blur more and more because they travel uphill while the shutter is open. The images in the middle column show how depth of field increases as the aperture gets smaller. |
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| Shutter speed ranges and their common applications or limitations |
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| Aperture or f-stop number The aperture is a "hole" in a mechanical structure in the camera lens or camera body that regulates the amount of light which passes to the film−at any given shutter speed. A large hole (low f-stop number) lets in more light than a small hole (high f-stop number). See images above. Technically, the ƒ number (f-stop) is the ratio between the diameter of the aperture and the focal length of the lens. All this really means is that for any given f-stop, the actual diameter of the aperture (in millimeters) is larger for a telephoto lens than for a wide angle or "normal" lens. (This presents some engineering issues, and "fast" [wide aperture] telephoto lenses get incredibly expensive.) This already is "too much information," and we won't worry about the "lens physics" details here. The standard f-stop numbers are: 1.4…2…2.8…4…5.6…8…11…16…22. |
Some expensive lenses will show more numbers at one or both
ends of the scale. The largest theoretical aperture is f1, although I am not
sure if such a lens is physically possible to build. Some large-format cameras
have pinhole apertures at f45.
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Exposure reciprocity This IS a simple concept that means: for any given light condition, a wide range of shutter speeds and aperture openings (f-stops) will result in the correct exposure. If your camera light meter indicates that f5.6 and 1/60th second is the correct exposure, then all of the combinations below would give the same exposure if the light was always the same: 
So what? First, go back and look at the images above. Why should a photographer care about changing shutter speeds
and messing with apertures? Simply, because some photo opportunities may
require fast or slow shutter speeds. And some may require wide or narrow
apertures to affect the depth of field. Briefly, shutter speeds must be at minimum levels to avoid camera
shake or "stop" action. Wide apertures reduce depth of field and
small apertures widen depth of field. This interchange of shutter speeds and aperture
sizes is useful for improving picture quality and impact on cameras that will
allow these changes. |
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Depth of Field Depth of field is the range in front and behind (of where the lens is focused) that is also in focus.The range of depth of field (DOF) varies with focal length, aperture size and distance from the camera to the focus point. The effects of aperture size on DOF are
illustrated in the images above. Sometimes it is desirable to limit the DOF and a wide aperture (low f-stop number) is used. (And shutter speed is therefore increased.) However, some subjects require a deep DOF and the aperture must be narrowed (to say, f16). The shutter speed may then become too slow and a tripod must be used. (Wind can affect depth of field. If photographing a field of wild flowers it may be desirable to select a smaller aperture such as f16. However the resulting shutter speed may be 1/60th second or slower. The DOF is therefore compromised−or you can select a faster film. If the flowers are waving in the wind they will not be sharp when photographed at this speed. It is more complicated than
this, but this is just about enough for now. Clive Schaupmeyer
Coaldale, Alberta |