Digital Photography Technology

The term f stop is one of the mysteries to new and some older photographers, although it is one of the most important aspects of photography. All camera lenses have a method of controlling or changing the f stop, although on low cost cameras it is fully automatic, but on higher end cameras, it is can be controlled manually or fully automatic, depending on the photographer.

The f stop controls the amount of light to the sensor/film. A good photograph requires a specific amount of light through the lens, and that is controlled by the combination of the f stop setting and the shutter speed.

All lenses are designed for a specific lens opening and focal length. An example would be a 50mm f 2 lens. This equates to a focal length of 50mm and a maximum lens opening of f 2.  There is a definite relationship between the focal length and the f stop of a lens.  Using the formula ,focal length 50mm / diameter 25 mm, = f 2.  The diameter is the distance across the lens opening. This particular lens would  start at f 2 and probably go to f 22. There is no need to remember this formula, but it just gives some insight about some basic lens design.

When looking at the f stop markings on this lens, f 2, 2.8, 4, 5.6, 8, 11,16, 22, they probably do not make any sense, but remember that the smaller the number, the more light passes through the lens to the sensor/film. The difference between each of the numbers is a factor of 1/2 or 2 depending on what direction you are going. To go from f 2 to f 2.8 it decreases the light to the sensor/film by ½. To go from f 16 to f 11 doubles the light, and from f 16 to f 8 is four times. Once you understand this, many of your problems will be solved. The standard f ratings are 1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, 22, 32.  Some lens manufactures will have different f markings, but these are the standard.

Shutters are designated  somewhat the same way, but are easier to understand.  For an example, starting at 16 sec, 8sec, 4sec, 2sec, 1sec, 1/2sec, 1/4sec, 1/8sec, 1/15sec,  1/30sec, 1/60sec, 1/125sec, 1/250sec, 1/500sec, 1/1000sec, 1/2000sec, 1/4000sec. The same as the f stops, a factor of 2 times or ½.

ISO settings are the same. Speeds of 25, 50, 100, 200, 400 were common. Each one was either double or one half. This  corresponded to f stops and shutter speeds.

Then digital cameras came along and the design engineers
went wild. Now on any of the higher end digital cameras, there are an infinite number of f stops and shutter speeds. What has stayed the same is the ratio between the two. If  the shutter speed goes up, the f stop number decreases.  Another way of saying more light, faster shutter and visa versa.

Another  term  that is being used is a full stop. This means from f 5.6 to f8 is a full stop, again 2 or 1/2 times. Now all camera manufacturers have designed the cameras to be able to adjust for 1/3 and ½ stops. This makes for a finer adjustment, where a full stop might be too much or  too little exposure.

This should help you understand f stops and shutter speeds, and hopefully help you with any exposure problems that you had.