i think i need to do some reading up
any body got any good recomendations??
Here's one helpful guide to get you started -
http://photography-on-the.net/forum/showthread.php?t=414088
It can be a very big topic, though, and it really depends how deep into it you want to go. One very important point to understand is that your camera's metering system is pretty dumb. It has no idea whether you are pointing the camera at a polar bear in a snow storm or a black cat in a coal cellar. What it will try to do, unless you tell it otherwise, is to guess that the scene (or the bit of the scene you are metering from ) should average out to a "middle tone", often called middle grey. Tricky lighting conditions can easily upset things, like a backlit subject in its own shade.
It is also worth noting that the camera's meter is relying on light
reflected back from your subject/scene. In contrast, a hand held light meter can be used to measure the
incident light that is illuminating the scene. The problem for the camera is that it doesn't know how bright the light is that is on your subject/scene. It only sees the reflected light. Dark things do not reflect much light so the camera is misled into thinking there is simply not very much lilght, which may not be the case at all. Conversely, a bright white car, or wedding dress, will fool the camera into thinking there is too much light, which also may not be true.
Different metering modes will give different results depending upon where in the scene you point the camera. Matrix/Evaluative metering will look at the whole scene, the distribution of light, and where the focus point is, and try to guess at what sort of scene you have and set an exposure accordingly. Centre weighted average will not really apply any intelligence to how the scene is made up (e.g. sky at the top) and will look at everything but give more weight to whatever is towards the centre of the picture. If there was a black horse as your subject the camera would probably try to brighten it up to make it nearer "middle grey" and thus overexpose the scene. If you had a white horse as your subject the camera would do the reverse. With experience you can make better judgements about the tonal content (brightness) of areas of the subject/scene and how the camera might interpret them. Then, using exposure compensation, you can bias the camera towards keeping things darker or making them brighter.
Partial metering and spot metering are more precise metering modes, allowing you to point the camera at specific areas of the scene and to decide how light or dark you want those parts of the scene to be. They give you more control, but also make you more responsible for the results. It's a bit like high risk and high reward. If you get it wrong it can go very wrong, but if you get it right it can be superb.
The beauty of manual mode, if the the lighting is pretty constant, is that once you have a correct exposure set you can freely adjust your composition and the metering/exposure will not keep changing each time you move the camera or zoom the lens. Once the exposure is set for the scene as a whole, and the light falling upon it, it does not matter whether you first photograph a black horse and then a white horse, or both together. The exposure will remain steady, and each will be exposed correctly. There would be no problem with grey horses, brown horses or any combination of colours and patches. Small horses will be as well exposed as big ones, as will people, dogs, flowers and pretty much anything else.
The autoexposure modes are great if the lighting keeps changing and you don't have the time or inclination to keep adjusting in order to keep up. However, if your subject and scene keeps changing, as you move about, you will need to be ready to adjust your exposure compensation to deal with the changing reflectivity of your subject/scene. In my experience, I tend to find that the tonal content of my subject/scene varies more often and more quickly than the lighting. Therefore I find manual exposure to give me more consistent and predictable results, most of the time.
Because the camera is seeking to average out the scene, or the metered area of the scene, to "middle grey", there is another useful technique you can use by metering from a standard "grey card". This is a special photographic card, calibrated to match the reflectivity that the camera expects. By metering from a grey card, with exposure compensation set to zero, or setting a manual exposure with the meter also at zero, you can get a pretty accurate exposure to suit the incident light hiting the scene. Your own palm makes a great substitute for a grey card, and can be easily used to set a pretty accurate exposure for the incident lighting. However, your palm is more refelctive (lighter/brighter) than a grey card, so instead of setting the meter at 0 you need to go a bit higher. If your palm is in the same light as your subject/scene then simply spot/partial meter from your palm and (if you have white skin) set an exposure about +1 1/3 stops on the meter. If you have dark skin then you may want a bit less. This approach removes the variability of changing refelctivity of a subjec/scene and replaces it with a known constant - your palm - and since palms don't tan they remain a steady reference all year round. If you meter for your palm then, usually, your scene will be well exposed too. Every rule has exceptions, so this doesn't apply in every situation, but it is a good starting point.
Another excellent rule/guideline is the "Sunny 16 Rule". When shooting in bright subshine the rule (guideline) suggests that at an aperture of f/16 you shutter speed should be the reciprocal of your ISO. e.g. at 100 ISO your shutter speed would be 1/100. At 200 ISO it should be 1/200 and so on. If you change your aperture to, say, f/5.6, which is 3 stops brighter than f/16, you would need to compensate with a -3 stop adjustment to shutter speed, ISO or a little of each.
Like I said, it is a big topic, and not easily condensed into a single post in a forum thread, but I hope that helps a bit. If you need to know more then feel free to ask more specific questions.
