Getting De-Programmed: Learning About Manual Exposure
Ah, I see you're still with me after last week. Good.
I'm going to presume you have already set your camera on manual exposure mode (not manual focus though) and spot metering. If you don't know how to, you'd better consult your camera's manual as there are far too many variations for me to address here. I'm also going to apologize up front for using Nikon terminology for some things which may or may not match up to your camera exactly if it is of another brand; the nice thing about manual exposure though is that all cameras work more or less the same. I'm also going to assume you have slide film loaded in your camera since this is really the only way to tell what you are doing; with print film the lab will mask any mistakes you make so you'll never really learn how all this works.
Slide film can record a subject brightness range of about five stops of light; anything darker than this range will end up featureless black, and anything brighter will result in featureless white. Between these two extremes then lies the territory we have to work with. Anything in the middle will show as reasonably average or "medium" toned. This might be medium gray as in an 18% gray card, or it could be medium green, red or another color. What is important is not the hue, but how bright or dark that color is.
When you look through the viewfinder of a modern Nikon camera, you will see a display somewhat similar to the one shown at the right. The display varies to a degree from model to model, but in essence they are all the same. This display consists of the axis of a graph with zero at its center. I have no idea why Nikon made it read right to left, but the scale goes from the zero point to +1 and +2 on the left, and -1 and -2 on the right. The distance between each of these points represents a stop based on what the meter is reading from. This same basic display also appears on the LCD panel on the top of the camera body. If you adjust either or both of the aperture and shutter speed such that the meter reads on the zero point, what you are metering off of will come out "medium." And you've probably already figured out that if the meter reads above zero, the subject will end up brighter than medium, and if it reads less than zero, what you are pointing the meter at will be rendered darker than medium on film. Beyond +2 and below -2 there are arrows for overexposure and underexposure respectively (assuming slide film with its range of five stops; print film has a somewhat wider latitude).
Modern Canon cameras have a display reasonably similar to this but theirs actually does read left to right. If your camera doesn't have a display like this, it may simply have a plus and minus with a center mark which works exactly the same way, but without giving you as much detail. Other older bodies may have a series of shutter speed numbers with one of them lit up. If your shutter speed is set to the one indicated, your subject will come out "medium." If you choose a shutter speed above or below the indicated one, your subject will come out correspondingly different than medium. There may be more variations I haven't come across before, so when in doubt, check with your camera's instruction manual. You may need to play around with your camera a bit to understand what it is trying to tell you, but the principle is the same regardless of how much it is presented to you on your camera's display.
Now then, how do you know what to meter on? Some people will tell you to place a gray card in the scene and meter on that, but this is generally not practical with landscape and wildlife shooting. Instead, you can meter on something that you want to render as "medium." There is no one "right" answer, but green grass and trees work well for instance, but not pine trees which tend to be about a stop darker than medium. If your camera provides an exposure scale, you can actually choose to meter on anything that interests you and place it on the scale where it should fall. For instance, if you are photographing a bright yellow flower, meter on it and adjust your exposure scale so it reads +1 (or maybe even more if it is really bright yellow). Once you get one tone placed correctly, whether it is "medium" or not, everything else in the same light should fall into place automatically. Once you have metered on your subject or whatever you feel most comfortable using, set your camera appropriately and then, without adjusting your focus or exposure settings, check a few more points including the highlights and shadows in your scene. Note where they fall on the exposure scale. If you like the exposure, fine; if not, you may need to make some compromises. Burned out highlights (above +2 on your meter) can ruin a picture, but consider letting the shadows fall into blackness (below -2) and become a silhouette. By playing with your meter and realizing what the results will look like, you will begin to be able to pre-visualize your results and make stronger images. Remember, it's not rocket science: there is no one "right" exposure. If you look at something blue for instance and need to decide how to meter it, just ask yourself if it looks "very dark blue," "dark blue," "medium blue," "light blue," or "very light blue." With more experience you can go half stops or thirds, but to begin with, just work with these five shades.
If you are in a high contrast situation with wide differences in lighting, you many find that not everything can be recorded the way you'd like to be able to, in which case you will need to resort to a graduated neutral density filter or other means of reigning in the brightness range. In bright noonday sunlight with deep shadows, you may find that it's just not possible to record a scene on film that looks great to your eye. Sometimes knowing when not to take a picture can be just as important as knowing when to trip the shutter.
It may take you a while to master manual exposure; it may also tend to slow you down when shooting, at least at first. You will probably make mistakes when learning also but don't be discouraged. It can take some time to learn to do this well. Have a bit of patience, and you will come to appreciate the ability to know what your results will come out as. This added control will make you a better photographer.