You might not know this, but your camera is a little neurotic. Trust me. Or don’t, and try this little experiment so I can prove it to you.
The premiss here is your camera has a bunch of light metering points. These points don’t cover your entire field of view when looking through the viewfinder (or back of the camera) and, unlike your focus points, you can’t see them. But they are there and they measure how much light is hitting each and every one of them, then your camera uses that information to figure out which Aperture, Shutter Speed and ISO to use.
One thing most people don’t realize is a little change in focus can cause a little change in metering. If you are shooting in Manual you can ignore this. This is for the rest of us who shoot in Program or Aperture or Shutter Priority.
Here’s the experiment you can run:
To prove my point and to help you understand the ramifications this feature has on your photos, let’s set up a simple scene. You’ll need a dark object, maybe your black camera bag, and a white object, like a piece of paper but with a little writing on it to help your camera focus.
1) Set the objects on a table, side by side.
2) Make sure the objects are well lit, either by an overhead light or the sun, which counts as an overhead light, I guess.
3) Change your camera’s focus mode. As all cameras are a little different, you will be looking for a mode which allows you to take manual control over which focus points are used. IMPORTANT! We are not looking for manual focus here, just manual selection of the focus points. Big difference. Consult your boring camera manual if need be.
4) Set your camera to Aperture Priority and set the aperture to the smallest number you can achieve.
5) Set your camera’s ISO to an appropriate level given the amount of light you have. 400 should work just fine.
6) Set your camera on the table or on a tripod so it will not move. This part is important because we don’t want our metering to change because of a framing change, only because of changing focus.
7) Frame your scene so the middle focus point is right in the middle, so a little of both the dark object and white object are covered. Make sure there is not much else other than your subjects in the field of view.
8) With that middle focus point selected, make note of your camera’s shutter speed.
9) Now select a focus point that is squarely surrounded by the dark object. Again, make note of the shutter speed. Did it change?
10) Lastly, select a focus point completely over the white object and also note the shutter speed.
What you should have seen is a slight change in the shutter speed. Or maybe your camera is more sensitive and the change was larger. Either way, my point is to be aware of how this will affect your photography.
If you shoot on anything except Manual or Auto (meaning Program, Aperture or Shutter Priority) your camera’s metering will come into play. It is important to understand that the metering mechanism is affected by where your camera is focusing. While this is certainly not as drastic as using Spot Metering, as demonstrated in my previous post about Metering Modes, it is still a real effect that needs to be understood.