Aperture, speed and sensitivity are the tree parameters that we have to decide when it come to get a correct exposition of your photographs.
Aperture refers to the quantity of light that your lens will get trough. The maximum aperture = length of the lens at infinite focus / width of the frontal glass. So a 50mm with a frontal glass of 25mm has a max aperture of 2. Inside the lens, there is diaphragm that you can close to change this value. Each value of the scale is the previous value times the square root of 2 and it divides by 2 the quantity of light.
So for a 100mm F2.8, the next step is 2.8 * sqrt(2) = 4. Set at f4, half the light go through.
The speed refers to the time that the shutter is open to expose the film (or the sensor). It’s written in fraction of a second. 8 means 1 second divided by 8. As you can guess, higher is the number, faster is the shutter (1/1000 is faster than 1/8) and each step half the quantity of light to impress the film.
The sensitivity defines how much light the film needs to be properly impressed (corrected exposed). We talk of ISO (or ASA). For each step, we need to twice the previous number by 2 and it needs half of the light for correct exposure. ISO 100 need twice more light than ISO 200. ISO 32000 need twice less light than ISO 16000.
Let’s take a few examples:
1/125 at F2.8 on ISO 100 is the same than 1/512 at F2 on ISO 200
Let’s detail the calculation:
1/125 -> 1/512 = 4 times less light
F2.8 -> F2 = twice more light
ISO 100 -> ISO 200 = The film requires half the quantity of light
Why do you want to set those parameter your self ?
There are several reasons for that.
– You need depth of field. The DOF is the range of distance in focus. The smaller is the aperture, the wider is the DOF. So a lens closed at F8 will have more things in focus than the same lens opened at F2. This is important if you have 2 subject you want in focus at different distance. The DOF goes 2/3 behind the point of focus and 1/3 before.
– You need less depth of field. For a portrait, you want to isolate the subject from the background, so you need to open your lens to reduce the DOF.
– You want to give an impression of speed to a moving subject. You can do that by decreasing the speed and move the camera to follow your subject why you make the photograph. It will blur the background and give a feeling of speed.
– You want to catch a moving target and increase the speed of the shutter (that’s why lenses used by photographers of sport are very big, to catch as much light as possible and use the fastest possible speed).
– You want to have a feeling of grain on your B&W photographs. Of course there are filters to do that, but real photographers use high sensitivity films (or increase the sensitivity of their sensor) to get the same result.
– You want to make photographs in the night, so you need films or sensor with high sensitivity.
Exemples goes on and goes on. Fact is, once a photographer understands those basic principles and is able to calculate fast the correspondant values between an exposure and another (keep the same exposure while changing those parameters), he gets a whole world of possibilities and opportunities to make shots differently.