How it works - F/stop
Think of exposure of film in this way. Filling the bucket is the amount of light needed to properly expose your film. The two drawings illustrate two ways to achieve this filling of the bucket. Either a slow drip for a long period of time or a fast flow for a short period of time. Both will eventually achieve the same end, filling the bucket.
This works the same way with cameras:
1) The larger the opening [faucet turns], the more light [or water] can come in per unit time. [Aperature]
2) The longer the opening [faucet] is left open the more light [or water] comes in. [Shutter speed]
It is the combination of the two that achieves the exposure we want. There is however a third factor we need to consider if we want to make our system more universal.
3) Distance to the film affects the amount of light that reaches any particular spot. This is because light "spreads" as distance increases. We saw this when going from a wide angle to a telephoto. The image of the candle flame was small and concentrated in the wide angle camera and larger and more diffuse in the telephoto camera. BUT, for a given size hole, the same amount of light entered. Because this same amount of light had to "paint" a larger area in the telephoto camera [further away from the opening], the amount of light that reached any particular spot on the film was less.
Say we have a drop of red paint and we need to cover a white disc with this paint. The larger the disk we have to color, the more dilute will be the red pigment and the lighter the resulting shade of red:
|The size of the white disc is related to the distance from the drop. The further away it is the larger it is. In this case the drop represents the pinhole and the discs represent the film. A telephoto lens, which has a longer distance to the film, therefore receives less 'light' because the image is larger and more spread out.|
Putting this together we now know that for film to receive a given exposure of light we must know three things.
1) The size of the opening through which the light passes [Aperature]
2) The length of time this opening is available [Shutter speed]
3) The relative distance the light has to travel, thus affecting the area that the image spreads out onto the film [Focal length]
The "F/Stop" is defined as the ratio of the Focal length divided by the Aperature.
The total exposure that the film receives is then dependent on the F/Stop and the Shutter speed. Note that the size of the camera no longer matters. For a larger camera with the same F/Stop, the Aperature will also be larger to compensate. This makes it much easier for different photographers using different cameras to both be able to calculate the correct exposure for a given situation.
In practice the F/Stop has been agreed to be based on a system of common ratios. These are called the F numbers and follow the following progression:
1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64, 90, 128, 190, 256 and so on.
[These numbers may seem arbitrary, but they are in fact based on a formula involving the square root of two, which you don't need to know to use them. Note that every other number is a doubling or having, depending on direction of travel.]
Most lens cameras will stop at F16 or F22. Pinhole cameras typically starting at F128 and go on to F1024 or higher.
All cameras follow the same rules in regards to F/Stops or F numbers. Using the above scale of ratios:
Moving one scale unit to the right reduces the amount of light reaching the film by half. [two unit move equals 1/4, etc.]
Moving one scale unit to the left increases the amount of light reaching the film by two. [two unit move equals 4 times, etc.]
A larger number means less light, a smaller number means more light.