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The subject line up there is a bit of a generalization, yet true for most cameras, be they DSLR or point and shoot. And not a lot of people realize this fact. Many of my students just assume the camera makes one determination about the lighting in the scene and that is that. If only it were that simple (and it is when you use Manual mode and don’t let the camera decide, but that’s another topic).
The truth is, most of the time your camera will alter the selection it has made for shutter speed, aperture and ISO (depending on which variable you have allowed it to control based on mode settings) because you focused in one spot or another. This happens often in the matrix or evaluative metering modes, when the camera is allowed to use all of its metering point as it sees fit. It will not happen when using spot metering, but can, occasionally, when using center-weighted. Concentrating on the full metering mode then, be aware of where you are focusing as that may not be where you want to meter.
For instance, take this unedited scene from a coffee shop concert I shot this weekend for an ukulele band named The Castaways:
The exposure looks about right. The settings are Canon 7D, ISO 6400, 35mm, f/5.0, 1/60. For reference sake, I was using evaluative mode (using all the metering points) and my camera has three vertical areas to focus on in the focus mode I was using (the following effect is existent in most any of the focusing modes, be it single point or in a group). Those areas look like this:
In the shot up top, I am focused on the red location. Now, let’s move to the green location, right in the center.
The settings are Canon 7D, ISO 6400, 35mm, f/5.0, 1/160. That’s 1 and 1/3 stops faster. Can you guess why?
It’s because the area of focus, when metered, has more of the light banner (which is also being lit by direct outdoor light) to contend with. It sees more lightness, as compared to the first focus point, which had more black and dark areas, and compensates. What’s going to happen when I focus on the banner, or the blue location?
Well now, things are looking bleak. Canon 7D, ISO 6400, 35mm, f/5.0, 1/250. 2/3rds of a stop darker than before and two full stops darker than the image we started with, which was a decent exposure.
If you are not shooting on Manual, and there is no hard-fast rule that says you need to, then be aware that your camera may change its decision on metering when you change focus. Give it a try yourself to see if your camera changes its mind with your focus.
Set up a target with black, gray and white sections that will align with your focus points. Keep light constant. Move the focus points to the black, then gray, then white and see if your metering changes. Try this in Aperture Priority mode to easily see the shutter speed change. Make sure you tap the shutter half way to start meeting anew after moving the focus points. Don’t know how to move your focus points? It’s time to bust out your manual.
This effect is neither good nor bad. It’s simply something to be aware of.