I dont really understand what metering is and when to use the various otptions such as center, partial, or spot metering. Also, I dont understand what exposure compensation is and when to use it. Finally, what are "stops" such as saying "stabilization" only works between 1 and 2 "stops"?
Thank you all and Merry Christmas!
As others have said, your camera's built-in meter tries to set the exposure values for an 18% gray exposure (due to historical paper contrast values.) Center-weighted metering just uses the light values from the center of the image, spot uses them from a specific point. In general, you can "trust" that your camera's metering database has seen enough "similar" scenes that it'll pick the right value in matrix or full-scene metering mode, or you can select what to meter based upon the tonal range in the scene. For example, if you have a rock in the scene that's around 18% gray and in the same light as your subject, you can spot meter the rock, and use that to set your exposure value for the entire scene. If there's something with the same tonal value as 18% gray, but in color you can do the same. If you know that say your fresh snow is 2 stops higher than 18% gray, then you can meter off the snow, dial in two stops of exposure compensation (or change either the exposure manually) and shoot away with the snow where you want it. The same goes for a black subject- the camera is going to say "Hey, that's 18% gray!" so you need to dial in negative exposure compensation to compensate for the fact that the black object is darker than the gray. You can purchase 18% gray cards to carry around, then you can spot meter off the card and get the lighting in the scene matched to the camera's meter. Obviously, you have to put the card in the same light as your subject- such as under a tree if you have someone sitting in the shade.
With digital, it gets a little easier. You can simply shoot something white, and examine your camera's histogram to see where the white part of the exposure lies. You want it as far to the right as possible without going over if you want any detail at all, or right on/over the line if you want to get a completely white, no detail exposure. If your white object is in the same light as your subject, then you can set the exposure that way by spot metering (some advocate using a fuzzy white towel,) but if your subject is in darker light, you'll have to open up the exposure to get a good shot- the easy way is by dialing in an appropriate amount of exposure compensation. Learning to use your histogram is good- if your camera doesn't do separate red, green and blue channel histograms, you may occasionally have an image where you blow out one of the channels inadvertently. Learning what conditions that happens in is a good thing.
Once you meter a scene and figure out what your exposure is (because the camera will do the 18% gray thing, you may want a scene to be darker and moodier, or lighter so there's no one 'correct' exposure, you get to decide what's important about a scene. You have three exposure values that you can modify ISO, shutter speed and aperture. ISO is the sensitivity of the recording medium, in the old days it was "film speed." On digital, it's the sensitivity of the sensor to light. All sensors have a "base ISO" that is their natural sensitivity (usually 100 or 200.) above or below that, the camera's electronics modify the signal to gain more sensitivity. This modification comes at the price of sensor noise, which make the pictures grainy. In film days it was the size of the actual grains- so we keep the term "grainy." The closer you are to the base ISO of your camera, the less noise the picture will have, but the more light you'll need. Shutter speed is how long it takes the shutter to expose your medium (film or sensor.) and aperture is how much light is let in by your lens. The aperture number is a ratio, so the smaller the number, the bigger the opening, or the more light that is let in.
High shutter speeds allow you to freeze subject movement and make it so that camera movement is less apparent in the image. Large apertures have narrower depth of field, or parts of the image that are in apparent focus from the plane of focus, which is parallel to your lens and sensor plane on most cameras (I won't distract you with the exceptions.)
Each full value of ISO, shutter speed or aperture is what's called a "stop" of light. It halves or doubles the amount of light necessary for an equivalent exposure. So, if you have 1/60th of a second at f/8 at ISO 400, and you change any single value one stop, one of the other values must be changed in the opposite direction to get an equivalent exposure- say 1/30th at f/11 at ISO 400 or 1/125 at f/8 at ISO 200.
There are a some "rules of thumb" that apply here in terms of getting a sharp image- the first is that it's best to hand-hold a lens at a shutter speed that's 1/focal length or faster. So a 500mm lens needs 1/500th of a second to hand hold for most people (some can go down to 1/250th, some need 1/1000th.) Obviously, this is only in terms of camera/photographer movement- you need fast shutter speeds to freeze a subject's movement. Image stabilization gives you one to three stops of photographer/camera movement above this rule of thumb. So, if you need 1/500th you can shoot at say 1/250th or 1/125th and still not see much photographer-induced vibration. In real tests, you'll often find that using a tripod provides a visibly sharper image than IS/VR does at slower shutter speeds, and as you go down the scale, the effects are less noticeable, as they are on shorter rather than longer lenses. The amount of impact depends highly on the implementation, in body or in lens. Currently, in-lens implementations are better, but obviously have to be purchased for each applicable lens.
A new rule of thumb with digital is that you should try to keep your aperture at f/11 or wider (lower number.) That's due to the diffraction limits of the physics of light combined with the sensor's photosite size. The smaller the sensor, with equivalent resolution, the more diffraction comes to play, as well as the larger the number of photosites. That's one of the reasons that a 6MP camera is "better" than a 12MP camera with the same sensor size in terms of diffraction- which is not what the megapixel fans will have you believe. With people and animal shots, a shallow depth of field isolates the subject and makes for nice images. With things like landscape shots, generally you want a larger depth of field, and with a larger depth of field you don't have to be as exact with your focusing. With a 6MP 1.5x crop factor (APS-C sized sensor) diffraction starts to impact sharpness at about f/12.9. With 12.4MP, it comes in at f/8.9. So, you get about a one stop difference between a Nikon D40 and a Nikon D2x, and the "pro" camera is more limited by diffraction than the "consumer" one.
I think that about covers it...
