What is "metering"

I'm sure someone more technical will elaborate, but metering is how the camera works out what exposure to offer you. The camera is basically seeking a mid-tone gray (- 128 between 0 (black) and 255 (white)). The camera usually offers evaluative (i.e. averages out from the whole scene, of center-weighted, takes more note of the centre part of the image, or spot (just one area of the image). Depending on what you are shooting, you can vary these settings on your camera. The camera doesn't always get it right, so you have AEB (automatic exposure bracketing) which takes 3 shots, one under, one exposed and one over, based on how many stops you select. If there's a lot of bright in the picture (snow scene) the camera gets confused and often gives you gray snow. If its very dark (inside a church, for instance) the camera gets confused and may try to brighten the image. There are a lot of variables. Sticking the camera on green box auto just lets the camera do it for you, but since the camera doesn't know what you are trying to photo, and is just measuring light, it often gets it wrong in tricky lighting.
Hope that helps.
 
When metering, you are using the camera, or a light meter, to determine the correct exposure based on the available light. Hope this helps.

Edit: Beaten to it by Photodiva. I thought that was quite a good technical explanation!
 
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Ah ok, so its measuring the light to get the correct exposure. So is this why when i focus the camera at the sky the landscape is much darker than when I focus on the land?
So is this done at the same time as when you focus on the subject by pressing the shutter half way?
 
Cris_L said:
Ah ok, so its measuring the light to get the correct exposure. So is this why when i focus the camera at the sky the landscape is much darker than when I focus on the land?
So is this done at the same time as when you focus on the subject by pressing the shutter half way?
Click to expand...

As already said, exposure metering is the business of measuring the brightness of the light, so you can deliver just the right amount for correct exposure. Exposure setting is the other half of the deal, which is converting that meter reading into a combination of shutter speeds and apertures etc. Of course, the two operations are done simultaneously by the camera when you press the shutter release half way, as you say.

The main problem with metering is that while want you want to do is measure the brightness of the light (known as an incident reading) what you actually do is measure the brightness of the subject, which is a reflected light reading. The difficulty is that a predominently dark subject will give a different reading to a light toned one, even though the exposure for both should be the same.

There are various ways around that, which is what the different metering modes are designed to help with. The most sophisticated metering method by far the the evaluative/matrix mode which takes dozens of different readings from all over the frame. In this way, it not only knows the average brightness, but also the difference between the brightest and darkest areas, and how much of the frame they occupy.

Furthermore, the camera then compares this data to a programme derived from thousands of test images and makes further adjustments based on that. For example, if it detects a large bright area at the top of the frame, it will tend to downgrade that value on the assumption that it is sky and what you really want is a meter reading from the darker foreground. It's actually much smarter than that, but you get the idea.

It's obviously desirable to get the exposure right first time, and IMHO evaluative/matrix is by far the best way of doing that in most situations. But for ultimate exposure control, you should refer to the LCD, and especially the histogram and a very clever function known as blinkies - the highlight over exposure warning that flashes black and white on the LCD. Enable blinkies in the menu.

The big difference between what the meter tells you and what you see on the LCD, is that the meter reading (any meter reading) is only a guess at what the exposure should be, whereas the LCD shows you exactly what you have actually got.
 
Richard has nailed this question in one post........again..........:D
 
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