Reading the Light Meter in the Viewfinder
Okay. All three settings are in your hands now. Shutter, aperture, ISO, you've turned all three dials yourself at this point. So this lesson is about the thing that tells you if you turned them the right amount.
Look through your viewfinder right now. Down at the bottom, or off to the side depending on your camera, there's a little scale. Numbers running from negative something to positive something, usually -2 to +2, with a mark in the middle at zero. That's your light meter. It's the camera telling you, in real time, whether it thinks your current combination of shutter speed, aperture, and ISO is going to give you a properly exposed picture, or whether you're headed for something too dark or too bright.
There's usually a little indicator, a dash or an arrow, that slides along that scale as you change your settings. Center it on zero and the camera's meter thinks you're exposed correctly. Slide it toward the minus side and you're underexposing, the picture's going to come out dark. Slide it toward the plus side and you're overexposing, too bright.
Here's the thing though. Zero on that meter is not the same as "correct." It's the camera's best guess based on the light hitting the whole frame, averaged out. The meter doesn't know you're photographing a bride in a white dress or a black dog in a dark room. It just sees light and tries to average it to a medium gray. So a lot of the time you'll want your indicator sitting somewhere other than dead center, and that's fine, that's you making a decision the camera can't make for itself.
How to actually use it, step by step
Put your camera in manual mode, since that's where we've been living. Point it at something in your house, doesn't matter what, and look through the viewfinder.
Find the meter scale. Get familiar with where it is, because on some cameras it's easy to miss if you're not looking for it.
Change your shutter speed slower, one click at a time, and watch the indicator slide toward plus. Change it faster and watch it slide toward minus. Now leave shutter alone and do the same thing with aperture, open it up and watch the indicator move, close it down and watch it move back.
Get the indicator to sit right on zero. Take the picture. Look at it. Nine times out of ten for a typical daylight scene, zero on the meter gives you something reasonable.
Now point at something unusual. A bright window with backlight, or a dark corner of a room. Get the meter to zero and look at your result. You'll probably find the meter got fooled, the window blows out white or the dark corner goes muddy gray instead of staying dark. That's the moment you understand the meter is a suggestion, not a rule.
The dial you have to check every single time
I'll be honest with you, this is the part of the lesson I care most about, because it's the one that bit me.
There's a separate dial on most DSLRs called exposure compensation, and it lets you tell the camera "actually, bias everything two stops brighter" or darker, even while you're in manual or in one of the priority modes. It's a small dial, usually right under your thumb where it's grabbing the camera.
I spent an entire morning up American Fork Canyon one fall photographing a stand of scrub oak that had turned this really good orange and red, the kind of color that only lasts about a week and a half before it drops. Beautiful morning. I shot for two hours, moved around, changed compositions, felt like I was getting good stuff.
Got home, pulled the memory card, and every single frame was blown out. Overexposed bad. Took me a while to figure out why, because I hadn't touched my shutter or aperture in any way that should've caused that. Turned out I'd bumped that exposure comp dial with my thumb early on, probably reaching for the shutter button, and it had been sitting at plus two the whole time without me noticing. Two hours of shooting, gone. That bothered me more than it probably should have, but it also cured me. Now I check that dial literally every single time I turn the camera on. Before I check my shutter speed, before I check my aperture. It's the first thing I look at.
So that's your homework built right into the lesson. Find that dial on your camera. Know what it looks like, know where the zero mark is, and get in the habit of glancing at it before you start shooting anything that matters.
When to trust the meter and when to ignore it
My opinion on this, and it's not a complicated one: get the meter close, then trust your own eyes on the LCD more than you trust the number. The meter gets you in the neighborhood fast, which is valuable, but it doesn't know what you're actually trying to say with the photo. If you want that scrub oak to look rich and a little moody instead of bright and cheerful, you might deliberately sit your meter at minus one and like the result better than zero. That's a choice you get to make once you understand what the meter is actually measuring.
Before next time
Practice getting the indicator to zero on five totally different subjects around your house or yard, one bright, one dark, some in between, and notice which ones the meter actually gets right. And check that exposure compensation dial before you even pick up the camera bag next time. Just make it a habit now while it's cheap to build.