For those of you who are learning to use manual mode, you're going to have to get comfortable with the light meter, that little bar in your viewfinder that goes from -2 or -3 to 2 or 3. That alone is enough to start taking better pictures, but there's more to it.
The light meter in your DSLR measures how much light is coming to the sensor and calculates whether the picture will be underexposed, over exposed, or exposed ideally. It's represented in a number line from -3 to 3 or -2 to 2, depending on your particular camera.
Each number on the light meter is equivalent to 1 stop, or EV (Exposure Value). It's separated into 1/3 segments, which makes adjusting exposure easier.
Try This at Home!
Grab your camera and set it to manual mode. Compose a shot and press the shutter button halfway to start metering. If the meter reads negative, slow the shutter, open up the aperture, or increase the ISO. If the meter reads positive, speed up the shutter, stop down the aperture, or lower the ISO. When the meter gets to 0, take the shot.
The light meter is divided into 1/3 segments for a reason. Each of the exposure parameters, shutter, aperture, and ISO, have different impacts on exposure. A difference of 1 stop on the light meter is equivalent to:
1/2 or 2* Shutter Speed,
1/2 or 2* ISO, or
sqrt(2)* or 1/sqrt(2) Aperture f-number.
Luckily, as light doesn't just float around in full stop increments, you can adjust the settings in fractional increments as well.
Shutter and aperture, by default on most DSLRs, is adjusted in such a way that one "click" of the scroll wheel will modify the parameter by 1/3 of a stop. ISO may not be so easy; Some cameras increment ISO in full stops, while others increment in 1/3 stops.
Example: My Canon 6D can adjust ISO in 1/3 stops, just like my girlfriend's 5D mark II and my friends 60D, but my T2i can only adjust in full stops.
To get better at understanding what you can do to get the images you want, first determine if your ISO is adjusted in 1/3 or full stops. You can do this by setting the ISO to 100, then incrementing it one "click." If you're now at ISO 200, you can only adjust in full stops. If you're at ISO 130 or 133, you can adjust in 1/3 stops.
Let's make a hypothetical. You're shooting a sunset and you've got the proper exposure, but you want more depth of field. So you increase your aperture from f/2.8 to f/11. How should you adjust shutter speed to compensate?
f/2.8 to f/11 is a total of 4 stops slower, or 4*3=12 clicks on the scroll wheel. (2.8->4->5.6->8->11)
To compensate, slow your shutter by 12 clicks.
Scenarios that Knowing This Could Help You
Low Light Shooting. You've got a Canon 50mm f/1.4 attached to your camera, and as such, you should keep the shutter speed above 1/60s (full-frame) or 1/80s (crop-body) to prevent camera shake. You've got the aperture at a fairly wide f/1.6, but you're still getting -1 2/3 on the light meter at ISO 800.
Remember that -1 2/3 on the light meter is equal to five 1/3 stops.
(Full Frame; 1/3 Stop ISO)
Increment ISO to 2500 (+1 2/3 EV) - 5 Clicks
(Crop Body; 1/3 Stop ISO)
Increment ISO to 2000 (+1 1/3 EV) - 4 Clicks
Widen aperture to f/1.4 (+1/3 EV) - 1 Click
(Crop Body; Full Stop ISO)
Increment ISO to 1600 (+1 EV) - 1 Click
Widen Aperture to f/1.4 (+1/3 EV) - 1 Click
Slow Shutter to 1/60s (+1/3 EV) - 1 Click
Steady your hand
Increment ISO to 3200 (+2 EV) - 1 Click
Narrow Aperture to f/1.8 (-1/3 EV) - 1 Click
Increase Shutter to 1/100s (-1/3 EV) - 1 Click
You're shooting a race at the local track, and you want to get some panning shots. Your current settings are 1/60s - f/4 - ISO 400, and you're getting a +2 1/3 EV on the light meter.
Decrease ISO to 100 (-2 EV) - 6 Clicks
Increase Aperture to f/4.5 (-1/3 EV) - 1 Click
Decrease ISO to 100 (-2 EV) - 2 Clicks
Increase Aperture to f/4.5 (-1/3 EV) - 1 Click
You test out the new settings, but you're finding you want more of the cars in focus. Easy, just:
1/3 Stop ISO:
Increase aperture to f/7.1 (-1 1/3 EV) - 4 Clicks
Increase ISO to 250 (+1 1/3 EV) - 4 Clicks
Increase Aperture to f/7.1 (-1 1/3 EV) - 4 Clicks
Increase ISO to 400 (+2 EV) - 2 Clicks
Increase shutter to 1/100s (-2/3 EV) - 2 Clicks
Now you find you're getting just the right amount of blur on your 1/3-Stop ISO body, but not on your Full-Stop ISO body. To correct, slow the shutter back to 1/60s to create the same amount of motion blur as the other body. This adds +2/3 EV to your exposure. To correct for this, lower the ISO from 400 to 200, which is -1 EV, which means you've got a total exposure of -1/3 EV. To correct this, sacrifice a bit of depth of field by opening up the aperture from f/7.1 to f/6.3, a difference of +1/3 EV (or 1 click).
Understand a bit more now about how to compensate for tweaking settings for the shot you want?
If you're using a camera that uses 1/3-stops for ISO, you're in luck, because you have more flexibility when tweaking shot settings. Just remember that for every click of the wheel, you must also do a click in the opposite direction for another parameter, e.g. 1-click faster shutter = 1-click faster ISO or 1-click faster aperture.
If you've got a body that only increments in full-stops when adjusting ISO, you've got to be a bit more creative, and make a few more sacrifices. Since a stop difference of ISO may mean the difference between a noisy or non-noisy image (see ISO 1600-3200 gap on a crop body to know what I mean), you've got to remember that for every click of ISO change, you've got to change the other parameters a total of 3-clicks opposite to the direction of the ISO change.
Why's my picture so noisy? It's a question I've seen many times from people who have just started shooting in manual. They may have gotten the correct exposure, but their picture is very noisy. When looking at the EXIF data, I find that they've got an exposure of 1/1000s - f/11 - ISO 12,800, with a kit 18-55mm f/3.5-5.6 lens.
You've got to understand that ISO affects sensitivity, but also increases noise, aperture affects the depth of field, and shutter speed adjusts motion blur.
If they've got a proper exposure at ISO 12,800 - f/11 - 1/1000s, then the following settings would yield the same exposure, with less noise (assuming the lens is capable of this aperture at that focal length):
f/11 -> f/4 (+3 EV)
ISO 12,800 -> ISO 400 (-5 EV)
1/1000s -> 1/250s (+2 EV)
Net Exposure Change: +/- 0 EV
To make your own hypothetical shooting situations, throw together some numbers, then increment through using this easy to use chart:
Bold = Full Stop
Dark - Less Motion Blur
1/8000 - 1/4000 - 1/2000
1/1000 - 1/800 - 1/640
1/500 - 1/400 - 1/320
1/250 - 1/200 - 1/160
1/125 - 1/100 - 1/80
1/60 - 1/50 - 1/40
1/30 - 1/25 - 1/20
1/15 - 1/13 - 1/10
1/8 - 1/6 - 1/5
1/4 - 1/2 - 1" - 2" - 4"...
Light - More Motion Blur
Dark - Less Noise
100 - 125 - 160
200 - 250 - 320
400 - 500 - 640
800 - 1000 - 1250
1600 - 2000 - 2500
3200 - 4000 - 5000
6400 - 8000 - 10,000
12,800 - 16,000 - 20,000...
Light - More Noise
Dark - Sharp Background
32 - 28 - 25
22 - 20 - 18
16 - 14.3 - 12.7
11 - 10 - 9
8 - 7.1 - 6.3
5.6 - 5.0 - 4.5
4 - 3.6 - 3.2
2.8 - 2.5 - 2.2
2 - 1.8 - 1.6
1.4 - 1.3 - 1.1
1 - 0.7
Light - Blurry Background