In the good old days, cameras had only an M mode and a meter. Often the meter was center weighted, but you got what you got. I was reading about the zone system and too poor to buy a spot meter.
When I bought my 20D, I was strongly tempted to spend several hundred dollars more and get the 30D -- because it had a spot meter -- but I didn't.
The 1D Mark III has a 63 zone metering system (a 7 by 9 matrix) and allows the choice of 4 metering modes:
Some people have suggested the following: green grass is close to an 18 percent reflectance, making it a handy (unless you live in the desert) not so neutral grey card. Some say instead that grass is darker than that.
You cycle through these choices by pressing the metering button on the top left of the camera and rotating the dial. Rotating the wheel will change flash exposure compensation. The symbol looks sort of like an eye within a rectangle.
The 5D Mark II has the same 4 choices, but 35 zones (a 5 by 7 matrix). Partial metering is 8 percent, and spot metering is 3.5 percent.
The 20D has 35 zones, but only 3 choices (no spot metering). Partial is 9 percent.
One has to wonder about the lack of spot metering in the 20D, this should be something that could be provided by a firmware upgrade.
Some people have noticed that metering follows (and is weighted around) the selected focus points. This is the way the evaluative system is designed - the focus points indicate the subject and the rest of the image is the background.
This is the main theme of the Malaysia article -- metering follows the focus points. When manual focus lenses are used, metering priority is given to the center focus point.I always figured that that center weighted metering was a throwback to the days of old film cameras which had simple center weighted metering systems.
There is a button on the back of the camera, high and right and labelled with a "*" star or asterisk symbol. It is right where your thumb can quickly find it. This is the AE lock button. To use it, you first press the shutter half way to measure the exposure, then press the AE lock button to lock or latch it. You can press it again at any time to lock the current exposure value. You will see a "*" in the viewfinder to indicate that AE lock is active.
To clear out a locked exposure value, you have to wait 4-6 seconds until the display goes blank. Then when you wake the camera up, the AE lock is gone.
This button can be swapped with the AF-ON button using a special function. This can also get tangled up with special functions that set up back button focus, so be warned.
The point of all this is that you can lock an exposure, then move the camera to compose the photo differently. I often will do this when a bright sky will cause the camera to overexpose for example. Alternately you could just shoot in manual and pay attention to the meter.
This is a multi-spot feature unique to the 1D Mark III cameras. The following feature is available when the meter is set in spot metering mode. You can take up to 8 spot readings and the camera will average them for you. It also displays the last 3 readings alongside the meter so you can see the range of brightness in the scene. In some ways I think this display is perhaps the handiest aspect of all this. Multi-spot metering only works in Av, Tv or P modes; in M mode you just get a plain old spot meter, which I guess makes sense.
On a 1D-III body, you can use Control Function IV-12 to set the "length for timer". This gives you 3 different times you can adjust.
The feature is enabled using control function I-7. When enabled and a single autofocus point is selected, spot metering takes place at that point. When automatic autofocus point selection is active, spot metering always takes place at the central point.
Tom's Digital Photography Info / tom@mmto.org