Light Meters
Types
There are two general types of meters: incident, which measures the light falling on the subject, and reflective, which measures the light reflected from the subject. Both measure light, they just do it differently. Both come as hand-held, separate tools (from a camera). Both work on the same principles.
A light meter, regardless of type, assumes that any scene being metered is average. This means it assumes that there is an even spread of tones from dark to light. In photographic terms, the meter reading will render the scene middle grey. (I know there's a lot of italics here, but these terms are pretty important in understanding this metering thing). Middle grey is just that— a medium grey tone. It is more precisely a neutral grey of 18% reflectance. Now, how most meters come about rendering the scene middle grey is by averaging. That is, they will sample different parts of a scene and average the samples together to get a reading. This averaging works pretty well, in general. As long as a scene contains a reasonable spread of tones, the reading you get from an averaging reflective meter will do fine. (It can be tuned, tweaked, and torqued however— see exposure methods).
In the Camera
Most 35mm SLR cameras have a built-in reflective light meter. These meters usually use a kind of amoeboid-shaped averaging pattern, based on some esoteric scientific high-falutin' calculation that IS rocket science. Most manuals even have a picture of it. There are many cameras with different meter methods, such as center-weighted and spot available, but let's stick with the basic averaging here.
So, you set your ASA for the proper film speed, telling the meter what speed film you are using. You turn the camera on, and point it at the subject (also known as composing the image). Now, you take a meter reading. This usually involves pressing down on the shutter release a bit. In the viewfinder, any number of things might happen. You may see a row of red, green, and yellow lights. There may be a number, with plus or minus sign(s). There may be a needle moving up and down, with some stationary circle or gap. Yeeesh! The idea is to get things centered. READ YOUR MANUAL! You simply need to kow how to read your meter.
What the Meter is Telling You
What an exposure (E) consists of is two variables: the amount of time the shutter is open (T), and the size of the lens opening or aperture (A).
E=A/T
This formula indicates that E, exposure, is the result of an inverse relationship between shutter speed T and aperture A. What IS E? What is it's actual value? That depends on the scene. Balancing out the meter by setting the camera's aperture and shutter speed values indicated by the meter will give you an average scene as far as exposure. That is, there will (ideally) be an even amount of darks and lights.
Now, often will be the case that one or the other of the camera settings controlling E– shutter speed or aperture – will not be what you want. Either the shutter speed is too slow or the aperture is too wide (see shutter speed and depth of field). So you need to change one. Aperture and shutter speed have an inverse relationship, so to maintain the same E value, or exposure, increasing one means you must decrease the other, or vice-versa. The same exposure value will have many different aperture/shutter speed combinations:
| A/S equivalents for ƒ11 @ 1/60 | ||||||
|---|---|---|---|---|---|---|
| ƒ2.8 @ 1/1000 | ƒ4 @ 1/500 | ƒ5.6 @ 1/250 | ƒ8 @ 1/125 | ƒ16 @ 1/30 | ƒ22 @ 1/15 | |
All of the combinations above will give you the same amount of exposure, but they will also give you a different image. Notice again that as the aperture size decreases (the ƒ number gets smaller), the shutter speed increases. The realtionship is inverse, and each setting represents a stop. So 1/500 of a second is one stop different from 1/250, and ƒ8 is two stops from ƒ4 (ƒ4, ƒ5.6, ƒ8).
How the Meter Tells You
This is often the most confusing thing about meters, mostly because in-camera meters pretty much all read differently. Much like the layout of dashboard instrumentation in automobiles, the meter display of cameras, even the same model made at different times, changes. Some are as basic as a needle that is centered in a slot (by adjusting the aperture and/or shutter speed). Some use a series of lights—red being way off, yellow close, and green on. There are all kinds of different ways, some very intuitive, and some that looked good on paper but don’t work very well in the real world. Sounds weak, but the best place to start is the camera manual.
Making the Adjustments
Since exposure is a combination of shutter speed and aperture, to adjust for a given reading you change one or both of these settings, depending on the image you want. Shutter speed controls the appearance of motion, and aperture controls depth of field. One will invariably be more important than the other. Just remember that you want the exposure value to be the same, so when you change one setting, you need to change the other: if you increase your shutter speed to a faster setting (less time), you need to open up the aperture, to let more light in. Likewise, if you close down the aperture (less light), you need to increase the time to a slower shutter speed.
