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Taking a picture is, at the simplest level, a question of manipulating three variables; composition (what you see in the viewfinder and thus capture), focus, and exposure. The first is largely an art (one I'm still working to perfect). The second has special techniques associated with it, like selective focus/depth of field, but the third is where all the hocus-pocus of technical photography comes into the picture (literally). All modern digital and 35mm cameras have their own metering, which means they'll read light levels and either signal you how to set the camera, or they'll set it for you. The exposure setting can be viewed as a combination of two settings, metering mode and exposure mode.
Metering mode determines where/how the camera's meter reads the light in a given scene. There are three major metering modes, and all three are likely to be supported on higher-end cameras:
- Matrix mode, where the camera reads the light in a number of "zones" of the image and compares these zone readings to a database of "good" exposures to determine how to set the exposure. Matrix metering is good for from-the-hip shots where you don't have much time to fiddle with things, and it will give reasonable exposure most of the time. You can use matrix mode with any of the exposure modes, but it may not make much sense in manual (which we'll get to in a minute).
- Center-weighted mode, where the exposure is taken from a weighted average of the center-of-the-view subject and a graduated area radiating out from that point. This mode is helpful when you're taking a picture of something specific and want that something to be the primary determinant in the exposure, but where surrounding stuff will also have to be considered at a slightly lower priority. A lot of people swear by this mode, and it's worth giving it a try, especially where the lighting in a scene is complex.
- Spot metering mode where the exposure is calculated from a single very small point at the center of the viewfinder area, usually a couple of degrees. This mode is essential when you are trying to be sure that a very specific thing is exposed correctly no matter what happens in the rest of the scene. It's also the basis for metering for "semi-manual" exposure setting, a topic we're going to get to in a minute.
Exposure is set by a combination of lens opening (aperture) and shutter speed, so there are a large number of combinations that will produce the same exposure, differing in how well they'll stop motion (faster shutter speeds do that better) or how much will be in focus (depth of field is greater with small lens openings, which means longer shutter speeds). The exposure modes determine how the camera will balance these two variables:
- Programmed exposure mode means that the camera will set both shutter speed and lens opening for you based on a table that will be found in the manual. The idea is to balance the need to keep shutter speed high enough to avoid shake and the need to keep it low enough so that there's reasonable depth of field. This mode is great for those same from-the-hip shots because you don't have to set much of anything, but you should avoid using it where light levels are either very low or very high.
- Shutter priority mode lets you set a shutter speed and the camera will calculate the lens opening based on the meter reading. This mode is great if there is a minimum shutter speed you need to insure gets set, either to stop action or to eliminate shake effects with long telephoto lenses. In fact, you should probably use this mode with any lens longer than about 200mm unless you have a tripod or monopod.
- Aperture priority mode lets you set a lens opening and the camera will pick the shutter speed. This is good if you're looking for specific depth of field and can let the shutter speed wander around a bit. Usually it's a good mode to set when you're using a tripod, but you have to be careful if you have any telephoto lens on.
My own recommendation is to use Matrix/Programmed when you're unsure about variable conditions and won't have time to set up a shot, and to use the other modes when you have more time.
But What's the Right Exposure?
That sounds like it ought to be easy, but it's not. To understand why, you have to understand what the meter on the camera is really measuring. When you meter something, the camera's exposure system assumes that the "subject" is of average tone-not white snow or a black jersey. It attempts to render that subject as "average" exposure. Of course, all those "averages" don't let you quantify much about this topic, but fortunately there's a technique from the old days of film that will help. It's called the "Zone System".
Let's imagine a scale of ten zones. The extreme bottom of the scale is pure featureless black (Zone 0) and the top zone is featureless white (Zone 10) (the system uses roman numerals for the zones, but I'll use the more familiar numbers here).. The zones in the scale are all separated by one "stop" of exposure, the difference between f5.6 and f8 at constant shutter speed, between 1/125th and 1/250th second with constant aperture. On this scale, Zone 5 is the "average" we were talking about. Think of this as what grass might be properly exposed at if it were shot on a hazy day with no distinct shadows. Technically, it's the exposure you'd give an 18% gray card to get it to look 18% gray in the output print.
Here are the zones in the system:
- Zone 0 is featureless, pure black.
- Zone 1 is below the normal underexposure limit; will likely print black on most paper.
- Zone 2 is the darkest zone that will show any detail-the deep shadow zone of your images.
- Zone 3 will show distinct detail and texture.
- Zone 4 is the "faces in shadows" zone.
- Zone 5 is normal exposure of a neutral (18% gray) object
- Zone 6 is the "brighter" zone, faces in brighter light or snow/sand in shadow go here.
- Zone 7 is the bright zone, the lightest zone that will reproduce detail/texture.
- Zone 8 is the highlight zone, where some gross detail may be visible but most is lost.
- Zone 9 shows only minor variations in exposure, not enough to hold detail or texture; prints white.
- Zone 10 is pure white.
When you use Matrix Metering, the camera is reading "zones" in all of the areas of the picture and trying to establish an exposure that makes sense for the pattern of highlight and shadow areas present. For example, the matrix database might say (in effect) "If I meter highlight on the top zones and roughly equal exposure across the bottom zones, this guy is shooting a landscape with sky showing, and it's essential to expose the landscape part reasonably even if we blow out the sky a little".
When you use center-weighted metering, the system looks at the central point and asks what exposure would put it in Zone 5, then adjusts that slightly up or down depending on the reading of the surrounding area, to insure that area isn't opaque black or featureless white.
When the zone system is really good is when you use spot metering. In fact, with spot metering and the zone system, you can deal with most difficult lighting situations (except ones of very high contrast, which we talk about in our tip on dynamic range). How do you do it? Here's three approaches.
Zoning on the Average
If you have a "subject" in the picture, and if that subject is of normal tone (grass, our 18% gray card), you spot meter on that subject. The camera will expose for Zone 5 when you spot meter. If you have time, you can also check the dynamic range of the scene. To do that:
- Remember your "base" exposure setting from the subject.
- Move your spot meter to the brightest area of the image where you expect to hold any detail at all. This should give you an exposure of about two stops less (if you were at 1/500th at f8, you might now be 1/2000th at f8). If there is more of a difference than that, you're going to blow out your highlights.
- Move your spot meter to the darkest area of the image where you expect shadow detail. This should give you an exposure of about three stops more (1/60th at f8 in our example). If it shows a larger difference, you're going to have opaque shadows.
If you see that you're going to blow out something on the highlight or shadow end, you can try to shift your "average" setting in the direction of the problem (darker if your shadows are shot, and vice versa) by a maximum of one zone, or you can try to deal with it via our dynamic range tip.
Zone to the Max
Another approach to using the zone system is to pick the lightest area you want to hold detail on as your target. Here's the process.
- Select the lightest area where you expect to hold detail at all (snow, if you want the texture, for example). Meter on this in spot mode. Remember, the camera will put this in Zone 5, so you want to move it up to Zone 7, meaning you'll now increase your exposure by two stops (either by dialing in compensation or using manual exposure and dialing in an overexposure of two stops-latter is easier).
- If you have time, check the average/dark object areas to make sure they're around where you want them (Zones 5 and 2, respectively).
This technique is good for shots in snow, on sand/water, etc.
Zone as Low as You Go
The final technique is to pick the darkest area where you want to hold shadow detail and measure on it. Remember, the meter will put it in Zone 5, so you will now want to underexpose three stops to push it down to Zone 2.
This technique is good for sunrise/sunset/mode pictures, because the natural behavior of camera metering would be to expose all pictures as though there was full daylight (matrix metering tricks this to a degree, which is why you shouldn't use zone exposure techniques except with spot metering). Pick an area you want to be dark (deep shadow, pure black), meter on it, slip it down to the right zone (1 or 2) by underexposing the indicated number of stops, and you're done!
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