One of the big advantages to using a SLR over a point-and-shoot is being able to shoot in manual mode, opening the doors to more creative photography. Manually setting the sensor sensitivity (ISO), aperture, and shutter speed for the correct exposure can be quite satisfying. I especially enjoy it because it involves doing some simple arithmetic in my head. It boils down to figuring out the appropriate sensor sensitivity and the amount of light you allow to hit the sensor. The amount of light allowed to hit the sensor is determined by how wide the shutter is opened (the aperture) and for how long (the shutter speed). That’s all there is to it.
Shooting digital means that you get instant feedback and taking pictures is free. If you have time to set up for a photo, you can fiddle with the settings until you get the exact right exposure. Another great way to learn is to use one of the other camera modes (like aperture or shutter priority) to suggest starting values and then tweaking those. These modes also very useful when you’re under some time pressure but would still like to have some control over your camera.
So here’s what’s going through my head when I shoot manual:
Pick an ISO: A lot of digital cameras, when left to automatically set their own ISOs, seem to pick particularly high values for them (indoor scenes can be quite dark). A high ISO can introduce a lot of noise into the photo. For black and white conversion this is not a huge deal because it looks like film grain and can give your photo an artsy feel. I haven’t gotten around to purchasing it, but Noise Ninja is supposed to be quite good. Another solution is to invest in the new Canon 1D Mark III, which produces images with a lot less noise at high ISOs. I typically try to stay at ISO 400 or below.
Figure out your effective focal length: The actual focal length that I’m shooting with is 1.6x what it says on the lens (since I have a Rebel XT, which does not have a full-frame sensor). This is important for the next point.
Figure out your shutter speed: When handholding, the shutter speed should be at most the reciprocal of the actual focal length because camera shake is accentuated by long focal lengths. So for my 50mm prime, which is actually 80mm, the shutter speed should be at most 1/80 sec. Image stabilization technologies go part of the way towards allowing photographers to relax this requirement, as do monopods. A good tripod gives you even more flexibility here.
Start with the “Rule of 16″: To get the proper exposure in bright sunlight, at f/16, the shutter speed should be the same as the ISO (1/100 sec for ISO 100). Recalling the descriptions on films with different ISOs (100 for sunlight, 200 for cloudy days, indoors with flash, 400 for indoors, assuming other settings equal) gives me a very rough estimate of how much light to expect in non-bright-sunlight settings. This gives you a baseline to work with; since you’ve figured out your shutter speed you can now calculate the appropriate aperture after reading through the next point.
Change stops: The Rule of 16 can only take you so far; indoor lighting situations can vary a great deal so you’ll probably need to change the amount of light that hits the sensor. If you start reading about photography techniques, you’ll run into the word “stop.” The difference in the amount of light between two stops is a factor of 2. To double the amount of light, double the ISO, divide the f-number by 1.4 (sqrt(2) to be accurate), or double the amount of time that the shutter is open. If you don’t need to change the amount of light but would like to change something else, you will have to change the other variables to compensate. If you reduce the aperture number by a factor of 1.4, which widens it to allow twice as much light in (and also narrows the depth of field of your image), you should either halve the ISO or the shutter speed to preserve the exposure. The f-number is inversely proportional to the radius of the opening, and the amount of light is determined by the area of the opening. Photography might be too easy had the aperture measure been determined to be proportional to the area of the opening.
Check the histogram: The image histogram is an incredibly important tool, something I check after just about every single shot I take. It’s not good enough that you are not clipping shadows or highlights. For best results, the histogram should be located to the right as far as possible, even if that means your photos look overexposed. You’ll thank yourself later when you go to post-process because there is a lot more information content on the right side of the histogram. This has to do with the way that digital sensors are different from traditional film (Thanks to CRV for pointing this out to me).
Keep your equipment’s limitations in mind: Photos tend to be sharpest when you don’t shoot at your maximum (smallest f-number) aperture and you’re not at the extreme ends of your zoom lens. Sometimes this is hard to avoid doing, especially when you use the long end of a zoom lens. When handholding my 70-200mm f/4, I can see the image through the viewfinder move around, which makes me quite paranoid. Around dusk I’ll typically set the ISO to 400, keep the shutter speed to 1/640 sec or less, and aperture to f/5.6 at its widest. If the photo is overexposed, I reduce the shutter speed first before fiddling with the others.
So usually when I’m taking a photo I’ll set the ISO to something appropriate, Figure out my minimum shutter speed based on the focal length, figure out what aperture would be appropriate for that shutter speed (by consulting the Rule of 16 if I’m outdoors), and go from there. Having a lot of light available outside gives you a lot of flexibility; once you get indoors everything is much, much dimmer. A decent flash is pretty much a necessity. If you can’t use a flash (maybe because you don’t want to kill the ambient lighting) you may find yourself pining for new equipment and features, like lenses with extremely wide apertures, image stabilization, and camera support (monopods and tripods). You also begin to really appreciate bright, natural light.
After writing most of this I searched around for similar resources and found an excellent guide to exposure calculation. Definitely worth a read; important points are in bold.