Okay, just got home from the pub and will try and explain some of the technical side of photography (without slurring my words, hic!)
Here goes....
A camera is a light sensitive piece of equipment and you need x amount of light to for a photo to be correctly exposed.
There are 3 things that govern the amount of light that makes your photo:
1. Shutter speed
2. Aperture
3. Film speed ( ISO, ASA or DIN)
All of these are DIRECTLY linked.
1. Shutter speed. Faster shutter speed (eg 1/500th sec) lets in less light (like an eye blinking) but therefore captures moving objects without blur. Slower shutter speed (eg 1/30th sec) lets in more light but subjects can be blurry, due to subject movement or camera shake.
2. Aperture. Small aperture (eg f22) i.e. small hole in lens, lets in less light but creates greater depth of field (more stuff in focus). Large aperture (eg f2) i.e. big hole in lens, lets in more light, but creates less depth of field, which can be nice as it focuses the eye on the subject.
3. Film Speed. Faster film speed (eg 1600ASA) is more sensitive to light (needs less light) but can be very grainy (or pixel noise). Slower film speed (eg 100ASA) is less sensitive, so needs more light, but is far better quality.
phew!!
Light is measured in stops. 1 stop is half or double the amount of light.
So with shutter speed the stops are for example 1/30th sec, 1/60th sec, 1/120 sec, 1/250 sec, 1/500 sec, etc. Because you are halving or doubling the time between stops, the light level will be half or double between stops.
With aperture it's a little more complicated. Imagine a hole (in the lens) being open at f1. If you want to half the amount of light passing through that hole you need to half it's area. With pi, to half the area of a circle you times it by 1.4. Therefore to keep halving the area and creating 'stops' you end up with f1, f1.4 (1 x 1.4), f2 (1.4x1.4), f2.8 (2x1.4), f4, f5.6, f8, f11, f16, f22 etc. These are your aperture stops.
Film speed works in stops too, by halving and doubling it, so 400ASA is twice as light sensitive as 200ASA but 800ASA is twice as light sensitive as 400ASA. You end up with stops of 50ASA, 100ASa, 200ASA, 400ASA, 800ASA, 1600ASA etc.
So.....
If I was to take a photograph and the camera in auto mode told me that the correct exposure was 1/250sec at f2.8 at 400ASA. Lets say I wanted less depth of field and i knew my lens worked down to f1.4 (some cheaper lenses only work down to f5.6), so i wanted to shoot at f1.4, it is 2 stops between f2.8, which my camera told me was right and what I want, being f1.4. Because i am now letting 2 stops more light in my photo will be too bright so I need to darken it down somehow. I can either adjust the shutter speed 2 stops (two stops darker would be changing from 1/250th to 1/1000th sec) or make the film speed 2 stops less sensitive (from 400ASA to 100ASA).
Now this probably makes no sense, but hopefully it'll come naturally one day.
A good book to buy is Ansel Adams Basic Guide To Photography. He was a photographic technical genius.
Some great photographers don't have a clue about the technical side but rely on their creative eye, and produce amazing results.
I think it is like art. To become Jackson Pollock, you must learn fine art first.
Good Luck
