All SLR users have the benefit of using a light metre.

Note that the photographers didn't have that privilege. A lot of old photographers never had that, so they learned how to metre the light with their eyes. A number of TLRs and medium format cameras would not have a light metre, so you had to buy one and attach it as an accessory.

My TLR doesn't have a light metre. I use the spot metre from my Yashica FX-3.

Anyway, cameras have three kinds of metring systems.

1. Spot.

2. Centrewieght.

3. Full matrix.

Spot metre:

As the name indicates, it metres only a SPOT of the image. The light metre is usually placed dead centre. On my Yashica FX-3 it is in the middle of the frame, where the split-image/micro-prism focus area is. This is the metre that is usually found in external light metres that are used in the studio.

2. Centrewieght:

This is similar to the spot metre, but it covers a bit more area. It also is in the middle of the frame. In dSLRs this is in the area that is around the centre indicator in view finder.

3. Full matrix:

The full matrix system is one that tells you how much light there is in the ENTIRE frame. The light metre measures how much light is coming in the entire shot and gives you a reading.

All light metres are calibrated to give you a reading that equals to zone IV, or roughly around 15% gray. That means if you shoot a white wall by following the light settings that the camera gave you, you would get a picture of a wall that is 15% gray.

Each of these metres have their advantages.

I personally prefer spot on my film camera. The reasoning behind that is because i have more control over what I am shooting since I shoot a lot indoors and in low light.

See, it is better to use full matrix when you are shooting in a place that has steady lighting, such as outdoor in the sun, since it is continious it will be more advantagous to use full matrix. That doesn't mean you can't use anything else. I still use spot metring. However, be careful when you are standing in the sun shooting shade or the other way round. A lot of light metres get fooled, especially full matrix, since the shade and the sunlit areas give different readings, it might give you a reading that might not give you what you want.

The reason I like spot is simple, because then I can use the Zone System easily. Using spot is simple, I point the centre of the frame onto what I want to expose, and I metre. This lets me use the Zone system ruling of having the shadow with detail area. I just metre and I step down by one stop and I shoot. Very helpful.

However when you are using a dSLR it is a bit different. Having the camera on P and shooting in sequence will give you problems if you aren't using full-matrix or exposure lock. I was shooting a bunch of bikers a while back and they were passing by my camera and I had it on continious. When I looked at the pictures, I realized that most of them didn't have the same exposure, or rather, most of them didn't have the same look in terms of lighting, even though everything was stationary and the only factor that was changing was the bikes passing by.

It turns out its from the metring system I had. On spot, the camera metred exactly the middle of the frame, and since that was changing the whole time, it gave me a variation of picture lighting. If I had full matrix on I wouldn't have gotten such a variation. It might still be there, but it wouldn't be that wide.

First take a look at what you want to shoot, and choose what you want. I choose full matrix for sun light, and spot for everything else.

Sometimes I stay spot if there isn't a lot of constrast in my scene.