First lesson : collecting light

Water Balloon Model

Imagine that you have balloon filled with water. You pierce a hole in it and collect water in a glass. Now three things decide how much water you collect,

1. The size of the hole you pierce
2. The time for which you collect
3. The speed with which water flows, which depends on amount of water in balloon (pressure).

In photography too, the first step is to learn to collect enough light, or say photons. In daylight photography, (3) is given to you. Depending on the time of day, clouds, Sun etc. the intensity of light is given. You are allowed to vary (1) and (2).

In photography terms used are,

Exposure: Amount of light collected
Aperture: Size of hole you pierce
Shutter speed: Time of collection

Now, accept some of weirdness in naming, units of selection. Shutter speed is measured not in terms of speed but time units. If your camera shows a shutter speed 25, that means 1/25s (one-twenty fifth of a second) is the time for which shutter is open. But if it says 3"5, it means 3.5 seconds- wierd:-), just look for presence or abscence of ".
Aperture should ideally be measured in terms of radius of opening, but it is inverse - weird :-), stay on to find more funny things. So apertures are measured in terms of F-number. A F-number of 5.6 is more open compared to 22. So F-number is 1/radius.

And these things are measured in stops. So u can give stops in apertures and take apertures and get the same exposure. So down two stops in aperture (make the hole smaller) and go up two stops in shutter speed (make the time larger) and you get same exposure.

Exposure doesn't have units, its measured in stops. So you say exposure is normal, or two stops under or two stops over.

And it is not enough to know these, it should go into permanent memory as numbers in maths have gone into. If you cannot follow these then you cannot follow any photography discussion.

Revise this lesson with better visuals here.