For over two years, I used my first digital SLR exclusively on the "automatic" settings. While I did get some nice images (including my best sellers) I regretted not learning how to use the manual settings. I regretted it until March 23, 2007. Yes, I remember the date because it changed the way I take pictures. Brian Peterson's book "Understanding Exposure" walked me through the steps methodically and clearly and I cannot thank him enough. His book is one of my frequent recommendations when people ask me what to read. I also quote often from my notes taken at a weekend seminar presented by the Rocky Mountain School of Photography.
So, relying upon what I have learned from the previous sources and what I have learned in practice by trial-and-error, here is a summary on how I use the manual settings on my digital SLR. (Warning: This is NOT a definitive detailed essay of this subject. Just a quick overview to get the novice started.)
Spin the dial to the "M" setting for manual.
The three settings are: ISO, aperture or "F-stop", and shutter speed.
ISO: This determines how sensitive the digital processor is to light. Lower light shooting environments require a higher setting and vice versa. If you want a candid shot of the kids by the fireplace, or if you are shooting at night without a flash, set the ISO to 1600 or 3200. Full sun outdoors, you could use an ISO of 100, 200, or 400. The trade-off on higher settings is a more grainy image.
F-stop: Depth of field. You know how some photos have the people or object in front of or behind the main subject totally out of focus? Your eye goes right to the kid in the middle with the bright eyes and big smile. It's because that kid is the only part of the image that is tack sharp and it was done that way on purpose because the photographer used a low F-stop, probably F-2.8. You may have seen landscape shots in which the flowers in front and the mountains in the back were all in focus. It was a higher F-stop, most likely F-32. The aperture setting is my favorite artistic tool in photography. It allows the photographer to choose which aspects of the image will be in focus. Note: Higher F-stops require slower shutter speeds.
Shutter speed: Do you want to stop the action? Do you want some blur? Pull out your camera's operations manual and find the dial to spin that controls the shutter speed. In addition to the action/blur control, shutter speed also has a major influence on how light or dark the image will be. The faster the shutter speed, the darker the image and the better the action will be stopped.
When you look through the viewfinder there will be lighted graphs, charts, etc that let you know what settings you have chosen. There will also be an indicator to show if the shutter speed and F-stop are adjusted for what your camera thinks is the "ideal" setting. Canon calls this the "standard exposure index" and it appears across the bottom of the viewfinder under the focusing screen. When you change the shutter speed, you will notice the indicator move. Of course as you experiement you will learn where on this line you want the settings for the effects you desire.
1) The kids by the fireplace shot needs some light. Maybe open the curtains before they come into the room. (Candid shots require some preparation.) F-2.8 or 5.6, ISO of 800, and experiment with shutter speeds to get a clear, sharp focus with no blur.
2) Humming birds at the feeder in bright sunlight require very different settings. F-5.6 is still good, but I go with an ISO of 400 and shutter speeds of up to 1/2500 and faster.
3) A fall landscape scene offers many options and this is typical for me. I mount the camera on a tripod and use a cable release expecting a slow shutter speed. F-32, ISO 100. Depending on the available light, clouds or overcast, this set up could require a shutter speed of several seconds. A slower shutter speed here will also help you get that silky smooth texture to moving water in your image.
Finally, be sure to keep your camera's manual in your equipment bag for easy reference. My last tip is to take a lot of shots. And I do mean a lot! One afternoon I went out and shot fall color and moving water for a couple of hours and filled two memory cards with almost five hundred images. After you have tried the suggestions I have made in this article and experimented with several thousand shots, you will be more confident with the manual settings and never go back to "auto" again. Happy shooting!