With automatic exposure in digital cameras so good, it's tempting to just leave everything on automatic. It feels like a daring move just to flip the dial one more notch to "P"! (Of course, P is just automatic with some options to make things a bit brighter or darker than the camera would like.) Here are some tips for the beginner or intermediate photographer to encourage you to get beyond P into the exciting world of Tv, Av, and even M on the camera's mode dial.
The simplest way to keep track of all the settings is to realize that (for a typical picture) we want some numbers to be as big as possible, and some numbers to be as small as possible. But when the numbers go ridiculously far in the good direction, that causes other undesirable things to happen, so the ideal picture tends to have numbers that "meet in the middle".
What are these numbers? Shutter speed. Aperture. And ISO. Those are the big 3. Normally, the camera trades off between shutter speed and aperture. The third number, ISO, only comes into play when there's no combination of the shutter speed and aperture that works. (We'll cover ISO last.)
With Tv and Av, you control one number (either shutter speed or aperture) and the camera judges the other one.
Tv means that you control the shutter speed. It's the stupidest mnemonic ever devised. Just remember that A = Aperture, so Tv must mean the other one.
For most of the shutter speed range, the camera shows numbers representing fractions of a second, so a bigger number represents a briefer time when the shutter is open. "30" on the display means 1/30th of a second. "200" means 1/200th of a second. For most pictures, we want this number to be as big as possible. A picture taken at 1/200th of a second has relatively little chance of blurring due to the subject moving or camera shake. A picture taken at 1/400th of a second has even less, and so on. When the number starts to get low (60 for some people, 30 for others, even 15 or 10 if your hands are really steady), it's difficult to get a sharp shot while holding the camera in your hands.
This concept is relatively easy to understand, so Tv is a good first step for someone just venturing beyond P on their camera dial. Pick a number between 100-500, dial it in, and see if the camera will shoot without flashing any warning lights. If warning lights blink at you (meaning not enough light is getting in, and the camera can't find a good aperture value to "meet in the middle"), dial the number down a little bit and try again. Don't let the number get lower than your personal minimum, which you can only find through experimentation.
Av means that you control the aperture width. Easy to remember, but aperture is a tricky concept to understand. It's the width of the lens opening, which you never really see when you're behind the camera. So you have to take it on faith that the camera is doing something different as you dial different numbers.
Again, for the typical picture, we want this number to be as big as possible. Big numbers mean a larger area can be in focus (in terms of how far away things are). If the number is too small, Uncle George is in focus but the mountain behind him is blurry. Or the tree 2 feet behind Uncle George is in focus but he's blurry.
Here there's less chance of things going wrong, and a fairly reasonable set of values that work for most situations. Shoot with aperture values of 5.6, 6.2, or 7.1 and you can get good results for most situations. If there's plenty of light and you have a good SLR, go up to 16, 22, or higher and pretty much everything is guaranteed to be in focus, no matter how many close and distant objects are in one picture. That high chance of getting an in-focus picture makes this the mode that most photographers eventually end up with, once they advance to a certain level.
M is for manual, another easy one to remember. Now you have to pick the numbers for shutter speed and aperture. That would be intimidating to do if the numbers came out of thin air. But with a digital camera, just look at the numbers the camera gives you in one of the other modes, then plug in those numbers or ones close to them. You can either review a picture you already took and get the numbers from it, or half-press the shutter button and watch the numbers that appear on the display. (The camera tells you what numbers it would use, if you took the picture right at that moment.)
Manual mode is not needed in normal situations, only when something odd happens with the light that tricks the camera into taking shots that are too bright or too dark. If you are taking a picture that's half in shade and half in bright sunlight, you might have to start with the camera's suggested numbers, then dial a higher number for shutter speed to make things darker, or a lower number for shutter speed and/or aperture to make things brighter.
Remember I said that things go wacko if you push the Tv or Av numbers too far in the good direction? Basically, increasing those numbers lets in less light, which reduces the amount of motion blur (for Tv) and blur due to physics-related properties of light (for Av). Go too far though, and your pictures will be too dark, or the camera won't be able to come up with a small enough value for the other number. (As the Tv number goes up, the Av number goes down, and vice versa.)
So, what if you get a picture that's too dark (or flashing warning lights) when you bump the shutter speed to what seem like minimal values, like 60 or 100 for shutter speed, or 4.5 or 5.6 for aperture? That means there's not enough light overall to find a happy medium. That's where the ISO value comes in. Unlike the other 2, we want to keep this number as small as possible, and only increase it until the warning lights stop blinking, or the histogram stops being scrunched up on the left. With a point and shoot camera, ideally you'd never let it go above 100; going to 200 or 400 means you're in a dim situation and have no other way to get a bright enough shot. With a DSLR (particularly Canon), you might go to 200, 400, or 800 just as insurance in case lighting conditions darken unexpectedly; and if you're really in a dark spot, you might shoot with 1600 or even 3200. The negative factor with a high ISO value is a different sort of blurriness, random speckles throughout the picture, referred to as noise.