Assignment 06 Depth of Field

Some of the important aspects of still photographic images that we can manipulate in the camera (the old fashioned film pre-photoshop way) are focus, shutter speed, ISO ("film" speed, remember ASA ratings?), and lens aperture or opening size. Let's take a look at the effect of changing the aperture while concurrently changing other settings to produce roughly the same exposure.

The "f" number setting describes the aperture of the lens. It's the ratio of the focal length of the lens to the size of the lens opening. Aperture effects depth of field. Depth of field is the range of distances between which things in the image appear to be in focus. A large aperture (small "f" number) will produce a shallow depth of field. A small aperture (large "f" number) will produce a deep depth of field. You can experience this effect without a camera by simply squinting to read the funny papers when you've left your reading glasses at home on the night stand. For reference, all images here were taken with a Nikon D80, an old manual focus 50mm f1.8 lens, available light (no flash), and a tripod to prevent shake during long shutter speeds. ISO for all shots was 800. The lens was left focused on the cat for all three shots. The only things changed were the aperture and shutter speed. Remember: large opening (small "f" number) + short shutter speed = small opening (big "f" number) + long shutter speed. The key is to keep the same amount of light coming to the sensor (or film).




This first shot was taken at f1.8. Note that the only thing in focus is the cat and imediately adjacent piano keys. Shutter speed was 1/20th of a second.





The second shot was taken at aroung f5.6 or f8 (I didn't write it down, and the camera doesn't record it from a fully manual lens). Shutter speed was 1/3rd of a second. Notice that a few more keys appear to be in focus.





This last shot was at f22 and 5 whole seconds. Notice how all the keys appear to be in pretty good focus. The camera is really squinting here, eh?


You can achieve some similar results even with point and shoot cameras, though you might have to fool the camera by fiddling with some of the auto settings. The main thing is to figure out how to adjust things so that you can control the depth of field. Now get out there and shoot some - digital film is cheap!