Exposure is the result of a combination of factors that control how light or dark the photo is. An underexposed photo is too dark, and an overexposed photo is too light. If you're the photographer, you get to be the judge of what's too light or too dark. The camera will have a go at it, and will usually get it about right, but don't let anyone tell you there's any correct answer other than what you want.
Correct Exposure
The 'correct' exposure for any photo is what you want it to be. If you want most of the image to be black, you might want what the camera would 'think' is underexposed.
Now we've got the pedantic bit out of the way, though, the correct exposure generally means making sure very little of the image is actually really black, and preferably none of it at all is actually completely white. If there are areas of very bright colour, it's also possible to 'blow-out' just one colour - a really bright red car might max out the red without having any white in the scene, which can spoil things too. We're back to being a bit pedantic there, though.
With digital cameras, dark areas can usually be rescued much better than light areas, so if conditions are tricky, it's often good to underexpose a little, and fix it after. You'll be able to do this to a greater expense if you're shooting in RAW, but the same still applies to JPEG images.
Automatic Exposure
Some people will tell you real photographers always shoot manual, and if you're using auto exposure, you're cheating. Fuck them. The camera can get it right almost all the time, so you probably might as well let it do its job. Or not, if you prefer. The right way is just what works for you. As long as the results are good, it doesn't matter what settings you used.
Almost all modern cameras use something called 'matrix metering' by default, and usually have options for 'spot' and 'centre-weighted average' metering. Spot can theoretically be useful if you want to expose very specifically for one small part of the image, and don't care if the rest is blown-out or completely black. Centre-weighted average metering is theoretically useful for situations when… no, I can't even make up a scenario where you'd want it. It's not really my area, as I almost always leave it on matrix. Maybe it does have its uses, but you're asking the wrong person.
My technique tends to be leaving it on auto exposure, with matrix metering, almost all the time. If there's reason to think it might not work, check how it looks, and adjust with the exposure compensation setting if you need to. With mirrorless cameras, you can see the exposure live before you take the shot. Most of the time, you'll know it's going wrong before you actually press the button.
There's one main scenario where I do use spot metering - high contrast street photos. If you have harsh light, and you want the sort of photo where the shadows are black, maybe with a person just stepping into some bright light, spot metering for the bright bit makes it easy. I have a button set up to do spot metering, and hold the result. I can put the centre spot on a bright area, hit that button, and it's locked until I hit the button again to unlock it. Then I just wait for the right victim. Er. Subject.
I'm including under the heading of 'automatic exposure' here all the main modes for this - fully auto, program, aperture priority and shutter priority, along with the various scene modes most cameras have these days. We'll talk more about the differences between how these modes work another time.
What Controls Exposure?
The exposure you get is a combination of three main settings.
Aperture
The aperture is a gap in the lens that lets the light in. When it's wide open, at the lens' widest aperture, it's completely out of the way, so the lens is letting in as much light as it can. You can close the aperture down to let less light in through the lens. It controls a lot more than just the exposure, so we'll talk more about aperture another time. Aperture is very important to many photographers.
- You see the aperture blades used as symbols of photography in many places - they're the set of blades forming a hole that's generally somewhere between circular and hexagonal.
- Aperture Laboratories in the Portal games has a lens' aperture blades as their symbol.
- Apple's pro-level app for photographers was called Aperture, but it's discontinued now.
- Lenses with a wider maximum aperture tend to sell for a high premium both new and used.
For me, I tend to think of what aperture I want first, then adjust other things to get it. If light is low, that might involve a slower shutter speed, or turning up the ISO. If it's a bright day, I might use a neutral density filter to cut the light entering the lens, so I can still open the aperture up all the way.
In terms of exposure, though, all you need to know is that a wider aperture lens in more light, so on a bright day, you may need a narrower aperture, and in low light, you need a wider aperture. If you're relatively new to this stuff, the most confusing part is the way apertures are numbered. It's backwards. The bigger the number, the smaller the hole is. So an aperture of f/2 is (quite a lot) wider open than an aperture of f/8. This is because of Reasons, but it's probably best to just accept it for now. Small number, big hole. Big number, small hole.
Shutter Speed
This is simply how long the shutter is open for. If the shutter is open for longer, more light gets in. The problem is that if the shutter is open for long, the chances are something will move during the time it's open. If things are moving about, you need a faster shutter speed. If things are moving fast, you need an even faster shutter speed. On the other hand, if it's dark, you need a long, slow shutter speed. If it's dark and things are moving about, you have a problem. Damn things. Tell them to keep still.
The slightly awkward twist to the numbering here is that in almost all normal circumstances, it's in fractions of a second. 1/60th of a second is a fairly long time for the shutter to be open, and 1/2000th of a second is a short time. Because you're thinking of it as a speed, though, the bigger numbers are faster, so it still feels like it makes sense. We'll talk a bit more about shutter speeds another time.
ISO
In the days of film, some films were more sensitive to light than others. They had a higher ISO number, and we called them 'fast' films. They'd have been great, except they made for rather grainy results - the quality wasn't so good. So you had to pick a balance - good image quality, or sensitive film that let you get pictures in less light than you'd otherwise need.
We still have to make pretty much the same choice, but at least we don't have to put different film in now. We can change the ISO setting, which changes the sensitivity of the image sensor, so it works with less light. The problem is pretty much the same as it was with film, though - we get a worse image. The sensor doesn't really get enough data at higher ISO settings, so the image we extract from it isn't as good. We get 'noise' in the image - a very tiny pattern of colouring. The camera can try to remove it automatically, but if it does, it will lose some of the detail that you really did want in the photo.
Some cameras are better than others at higher ISO, and the main factor is the size of the sensor. A big sensor will tend to give better images at high ISO settings than one with a smaller sensor. It's also something that's improved a lot with time. Newer cameras usually have much better high ISO performance than older ones.
Stops
All three are measured in completely different ways, but we tend to refer to the 'units' they change in as stops. We let in one more stop of light. We open the aperture by one stop. We make the shutter speed one stop faster. A stop is either a doubling or a halving of the light. When you make the aperture one stop wider, you're doubling the amount of light that comes in by doubling the size of the hole opened up in the lens. If you make the shutter speed one stop faster, you halve the time the shutter is open.
They work together
If you did both of those things - opened the aperture by one stop, and made the shutter speed one stop faster - the exposure would stay the same. That's important, because it means you can keep the same 'correct' exposure while changing the aperture or shutter speed, as long as you change the other one to match, or change the ISO to match. If you're using automatic exposure, as I almost always do, the useful point here is that you don't have to give up control completely. If you set the camera in aperture priority mode (usually A or Av on the dial), you can set whatever aperture you want, and the camera will do its best to make it work for you. It will pick a shutter speed to get what it thinks is the right exposure, and depending on settings ('Auto ISO’), it will increase the ISO setting if the light is too low. That’s important, because controlling aperture and shutter speed are important to getting the image you want.