Higher (faster) shutter speeds (250, 500, 1000) tend to "freeze" moving subjects and contribute to a sharper image.  The shutter speed was probably set at 1/500 sec or faster for the image below.

Lower (slower) shutter speeds (15, 30, 60) are necessary for proper exposure in low light situations if a flash is not used.  This night shot of downtown Kansas City required a tripod and a shutter speed of one second.

If a flash is used, the shutter speed must be synchronized with the flash so that the shutter is open when the flash goes off. This will either be the 1/60 (60) setting or the "X" setting, depending upon the camera. That is why the "60" on many shutter speed dials is colored red. A shutter speed faster than 1/60 will produced a "chopped off" frame if used with a flash, because the shutter is in the process of closing when the flash goes off. 

Many conventional camera lenses have a diaphragm that is similar to the iris in the eye. It can be adjusted to vary the size of the aperture (pupil) and control exposure. It is controlled by an aperture ring on the lens that is calibrated geometrically in f-numbers, or f-stops.

Each successive f-number lets in either twice as much light, or half as much light, as the previous number, depending upon which direction you are going. For example, an f-stop of 2.8 will let in twice as much light as an f-stop of 4. An f-stop of 11 lets in half as much light as an f-stop of 8. You may recall that the shutter speed settings are also set up a geometric sequence.

To better understand f-stops, try this exercise with an adjustable 35mm camera, as illustrated with this animation.  Set the shutter speed dial to "B", meaning the shutter will stay open as long as you are pressing the shutter release button, and it will close when you release the button.  Now open the camera back (with no film in the camera), press the shutter release, and look at the open shutter.  Aim the camera at a light background and change the f-stops on the lens.  You will see how the size of the lens opening changes as you change the f-stops, regulating the amount of light that enters the camera. 

Depth of field

Varying the size of the aperture (f-stop) not only affects exposure, it also affects depth of field. Depth of field is the distance between the closest object in a photograph that is in focus and the farthest object that is in focus. A smaller aperture, such as f11, f16, or f22, increases depth of field. It is the same principle as the pinhole effect.

The following photo demonstrates excellent depth-of-field.  The flowers in the foreground are in sharp focus and the building in the background is in sharp focus.  An f-stop of f22 was used for this photo.

In medical photography it is important to maximize sharpness in all areas of the photograph in order to record a useful image. The increased magnification used in ophthalmic photography decreases the depth of field and thus the sharpness of the image. Camera systems designed for close-up photography of the eye make use of high f-stops (f22, f32) to maximize the depth of field.  The macro photo below was taken at f32.  The use of electronic flash makes it possible to use a high f-number.  Notice the "reflex" image of the flash reflected in the cornea. 

As we learned in Module 21, each type of film must have a certain amount of light exposure for a properly exposed photograph, otherwise the photo may be washed out or be too dark. An exposure meter may tell you that the proper settings for good exposure in a given situation are an f8 aperture and a shutter speed of 125.

What if we want to increase the depth of field by using an f16 aperture? With this change we are cutting down the amount of light getting to the film. We must compensate by changing the shutter speed to let in more light... but by how much?

Remember that each "stop" on the shutter speed dial and on the aperture ring represents either doubling or halving the amount of light. If we close down the aperture two stops from f8 to f16, then we must slow down the shutter speed by two "stops" from 125 to 30.

Close-up camera systems used in clinical ophthalmic photography and in surgical photography get around this problem by using electronic flash to control exposure. The f-stop is set at f22, or f32 (depending upon the lens), the shutter speed is synchronized with the flash at the 60 or X setting, and the flash output is changed depending upon the level of magnification being used.

The exposure meter

In the "old days", many cameras did not have an exposure meter.  Kodak included cookbook directions with each role of film.  If you were taking photos on a sunny day and the film had an ISO of 100, the instructions told you to use an f-stop of f11 and a shutter speed of 250.  If it was a cloudy day, you would use f8 and 100, and so forth.

Today, most cameras that costs over $100 have an exposure meter.  The meter measures the amount of light coming through the lens, and it tells you (manual mode) or the camera (auto mode) what would be an appropriate combination of f-stop and shutter speed for the given level of light.

With older 35mm film cameras, it may be a matter of adjusting the f-stop ring and the shutter speed dial until a pointer matches up with the needle of the exposure meter.

Newer cameras, including digital cameras, have several automatic modes.  The completely automatic mode chooses everything for you.  You just "point and shoot".  The semi-automatic modes include a shutter priority mode and an aperture priority mode.

The shutter priority mode allows you to set the shutter speed, and the camera chooses the f-stop based upon the current exposure meter readings.  You might make use of this if you wanted to freeze the action at a sporting event, by choosing a shutter speed of "1000".  At this setting, the camera would probably have to use a very wide open f-stop, perhaps f-2.5.  A smaller f-stop might be possible if you were taking photos in bright sunlight, and/or using a high ISO film.

The aperture priority mode allows you to set the aperture, and the camera chooses the shutter speed based upon the current exposure meter readings.  You might choose an f-stop of f-16 in order to have good depth-of-field in your photo.  At this setting, the camera may have to choose a relatively slow shutter speed (60?) unless there is plenty of available light (sunshine) or a high speed film is being used.

Hmmm, you might be thinking to yourself, why don't I just use a high speed film all the time, then I will have more choice over which f-stops and shutter speeds to use!  Nice try, but surely you know by now that almost every good thing has a trade-off.  Remember from Module 21 that higher speed films produce a "grainer" image, reducing the quality of the image.  The same holds true for digital sensors.  Although there is a range of exposure levels for a digital sensor, "pushing" the exposure produces "noise", reducing the image quality.

However, there is a way that we can artificially produce more available light - with electronic flash, of course.  More on that subject, and more, in the next module.