Before I get to spot metering (don't worry, Melissa, I haven't forgotten!), a little bit on exposure. Not for any particular reason, even though spot metering has to do with exposure, this isn't necessary info, it's just that I haven't taken any spot metering pics yet, and have taken these. ;)
The other day, I was photographing a bunch of finished knitting projects for my knitblog and to update my Ravelry projects page. Normally, I keep my D40 set on +0 EV, which means that it doesn't over- or under-expose the images. It just does its best to properly expose them. Usually, this works really well. However, that day it was being cranky. Here's the +0 EV picture, taken in A mode, so I picked the aperture, and the camera picked a shutter speed to match:
Um...not fabulous. It looks a little dark, even though it's supposed to be properly exposed. The exposure histogram (from Picasa) confirms this:
Since the peaks of each curve are near the middle of the histogram, it's pretty well exposed, although there is a little bit of underexposure in the red channel, as evidenced by the small red peak on the left side. (Do I sound like I know what I'm talking about? Because I don't. But this is what I've read...so yeah, one day I'll actually understand these histograms. Just not today.)
Here we see the beauty of digital cameras - I could see immediately on the view screen that the picture looked dark (cranky camera!), so I bumped the exposure to +1/3 EV, which means that the camera automatically overexposes the picture by 1/3 of a step. (I don't know what this translates to technically...and the internet doesn't seem to be much help...so if you do know, please share in the comments!) Here's the slightly overexposed image:
Much better! Now, just for kicks, let's bump it up another 1/3 of a step and see what we get:
Yep, at +2/3 EV, it's definitely more overexposed-looking, as anticipated.
I ended up using the +1/3 EV image for the project page, because it looked the best to me.
Changing this setting, called exposure bias or exposure compensation, can be useful in other situations, too. Here's a good article from Digital Photography School: Using Exposure Bias to Improve Picture Detail. I've used it before in similar situations, when the camera was, e.g., exposing for the bright part of an image, but I really wanted to see details on the dark part. I'd set the camera to overexpose, so the dark part would be more visible.
Uh...so, actually, I was wrong, way up there in the beginning...this does tie in to spot metering. Now that I know how to spot meter, I would probably spot meter in a situation like that. Hmm...the question then becomes, when should you spot meter, and when should you use exposure bias...I think it would depend on the subject and composition of your photo...I'll have to investigate further!
In the meantime, while I think and research, go play with the exposure bias on your camera! Even most point-and-shoots have an option for this, in my experience. That's when it can really come in handy, because on an SLR, you can always just go to full manual mode and expose how you want by controlling the aperture and shutter speed. You rarely can do that on a compact camera, but you can change the exposure bias - giving you more control over the picture, which is a good thing. If you try this, let me know how it works out for you! And if you have any questions, ask away. =)