Metering is one of those subjects that makes hobby photographers put their hands over their ears and sing "la la la, I can't hear you." That's because metering is a bit of a complicated subject. After all, who wants to talk about the way your camera estimates exposure? Shouldn't that stuff happen behind the scenes, so the photographer can focus on important stuff, like composition?
It's true that many point and shoot cameras - especially the inexpensive ones - have a fixed metering system that doesn't give you any control over how the meter analyzes light and chooses an exposure. But that doesn't mean that it's not important to understand how to use the different metering modes, if your camera offers them. With just a little bit of effort you'll discover just how useful switching between metering modes can actually be.
How does your camera estimate exposure?
Photographic light meters fall into two different categories: reflected light and incident light. An incident light meter measures the amount of light that falls on a scene, while a reflected light meter measures the amount of light reflecting off a scene. All internal camera meters are the latter variety, so I'm not going to spend much time talking about the former except to say that it is important to understand the difference, so that you'll know the limitations of your camera's internal metering system.
Incident light meters are always external, and they are far more precise than a reflected light meter because they can't be tricked by the amount of light that is reflected back from a scene. Have you ever tried to shoot a snowy landscape and been disappointed by your results? This is because snow is highly reflective - as that raccoon shaped sunburn suggests - and your reflected light meter is fooled into thinking the scene is brighter than it actually is. This results in underexposed photos. (Here's how to fix it)
But unless you're particularly interested in purchasing an external incident light meter, you are stuck with the meter that is built into your camera, so it's a good idea to understand how it works and which modes are most useful in what situations.
Reflected light meters such as the one in your camera are basically guessing at the amount of light in a scene, because all objects that exist in our world have a different capacity for reflecting or absorbing light. That snowy scene, for example, is a lot more reflective than, say, a meadow. The light meter attempts to compensate for this by assuming that most scenes average out to what photographers call "middle gray," which of course could also just be a color that is somewhere in the middle between a highlight and a shadow. In short, your meter is not actually smart enough to know the difference between black, white and gray, so you have to be.
Fortunately, most DSLR manufacturers (and some point-and-shoot manufacturers, too) have been kind enough to provide us with some options, which combined with a little know-how will help compensate for the shortcomings of the reflected light metering system. For most cameras, this translates to three different metering modes, each of which is useful in particular situations, and none of which is useful in all situations.
The three basic types of metering are matrix (also called evaluative, multi-zone, segment, honeycomb or electro-selective pattern metering, depending on who made your camera and how cool you want to sound when you talk about its metering system), center-weighted and spot metering (which also has a sibling known as partial metering). Here's a quick run-down on the differences:
Matrix or evaluative metering
No, you do not have to take the red pill to understand matrix metering. It's actually a very simple concept: The meter divides the scene up into zones, then analyzes each zone for highlight and shadow. It then takes an average for all the zones and determines the exposure based on that figure. Though the idea is simple on the surface, matrix metering systems actually use a complicated algorithm, and most manufacturers do it in their own special way, which is kept secret from the general public. Depending on the manufacturer, matrix metering systems may average just a handful of zones or more than a thousand of them. Other factors besides light are taken into consideration, such as the point within the scene where you have focused, the distance between the camera and the subject, and the colors within the scene. Nikon even has an on-board database of exposure information for more than 30,000 different photos that the metering system can refer to when determining the exposure for similar scenes.
Center-weighted metering gives the most importance--usually between 60 and 80 percent--to the light that is concentrated in a circular area in the center of the frame. The corners are given much less importance, though they are usually included to a small degree in the calculation. Some cameras will even allow you to adjust the size of the circle. This is usually considered to be the most consistent form of metering, since most photographic subjects are near the center of the frame and rarely fall into the outer four corners. For this reason, center-weighted metering is often the default metering system used by many point-and-shoot cameras of the variety that don't allow the user to have control over the metering system.
Spot or partial metering
Spot and partial metering operate under the same basic premise: the light in a much smaller part of the scene (usually the center) is measured and the exposure is set based on that reading. With spot metering, this is usually a space occupying 1 to 5 percent of the total scene. With partial metering the space can be up to 15 percent of the scene. Depending on the camera's manufacturer, you are either stuck with a metering from the center of the frame, or you can actually tell the camera which part of the frame you want it to take the reading from.
Spot metering is a very accurate form of metering in that it will give you a precise reading for a very small part of the scene, so it is most useful for shooting high-contrast scenes where your subject may otherwise fall into shadow or be washed out by very bright highlights.
When to use matrix metering
Matrix metering is good for scenes that are evenly lit, and for use during outings where you will need to capture shots quickly. Because DSLRs generally have very sophisticated matrix metering systems, this is the default setting for most photographers, the one to choose when it's not immediately obvious which of the other modes would be the better one. Matrix is that set-it-and-forget-it reading, and it's generally the better choice if you aren't completely comfortable with the idea of mucking with your metering system.
When to use center-weighted metering
Center-weighted metering is the go-to setting for portraits, since it will ,make sure the subject is correctly exposed ("expose for the subject") without giving much weight to the background. It is more predictable than matrix metering, which means you will get more consistent results. It requires a little extra thought than matrix metering does and is best used for scenes where you think you will need more control over where the camera measures the exposure. If you don't want the lighting in the background, for example, to affect your exposure, you should switch to center-weighted metering.
Good examples of scenes that benefit from center-weighted metering are high-contrast scenes such as those shot in full sun, especially outdoor portraits where getting the right exposure on your subject is more important than the exposure of the subject's surroundings.
When to use spot metering
Spot metering is one of those settings that is primarily used by professional photographers. Once you fully understand it, though, you can use it to good effect for backlit shots (metering for a backlit subject's face, for example, will stop your photo from becoming a silhouette). Spot metering is also good for shooting subjects at a distance or for macro photography, especially when the subject isn't filling the entire frame. You do need to take some care when using spot metering, because although you may get a well-exposed subject you may lose the rest of the shot.
Some other examples of situations where you might find spot metering helpful are scenes that are evenly lit but where your subject is significantly darker or lighter than its surroundings. For example, a white dog shot against a dark background or a person dressed in black standing in front of a white building. The moon at night is another good example of a subject that should be spot metered, since it is a very bright object on a very dark background. If you tried to use matrix metering to capture the moon, you would end up with a bright white circle without any detail.
Two Step Shutter
When venturing away from matrix/evaluative metering, you'll probably need to use the 'two step shutter' feature on your camera. That's a feature, which allows you to lock in your camera's meter reading for a period of time (while holding down the shutter). This is handy because center-weighted metering is just that - center-weighted - and you can't use it to meter an off-center subject (the same is true for many spot meters, too, depending on the manufacturer). Instead you will have to position your subject in the center of the frame, take the reading, then recompose and capture the image. See how here.
If you have a DSLR, another option for you is the AE Lock function.
Don't forget your exposure compensation
Exposure compensation (EV) can help improve your photographs if you're using a metering mode that consistently seems to be over or underexposing your photos. Remember that this is actually a common problem with all internal camera meters, since they can only measure the amount of light that is reflected off a subject - which means that they are prone to error. Certain types of scenes will always require some exposure compensation, regardless of which metering mode you choose. Snowscapes, for example, or beaches with very white sand will usually be under-exposed and will require an exposure compensation of at least 1 stop.
So which mode is better?
Which brings me back to the original question: which one of these metering modes is the best one? Well, like almost all photography questions, the answer is a resounding "it depends". Most of the time, you're probably going to want to choose either center-weighted or matrix metering, with the decision being made based on the type of lighting in the scene, and your own preference. Scenes with low contrast/even lighting, especially when you prefer not to have to selectively meter for your subject, are best tackled with matrix metering. Scenes with a higher contrast, especially those that would benefit from selective metering on your subject are best shot with center weighted metering. As for spot metering, reserve that for backlit scenes and for those shots where you have some extra time for experimentation.
Metering is a tricky subject to master, and like most other technical aspects of photography it is best conquered with lots of trial, error and wasted frames. And if you're like many hobbyists, you may just prefer to set it and forget it and stick with matrix metering, since that's the mode that will allow you to chase shots and capture them on the fly rather than have to think about metering every shot before you actually take it. After all, living subjects don't tend to stay in one place. Toddlers and pets are notorious for abandoning their cute antics while you are busy metering, and recomposing. So don't switch over from matrix metering just because you feel compelled to try something different. Instead, wait for those shots that just don't seem like they're going to come easy, or for that location that seems to be giving you consistently bad results. Know the different situations that each metering mode is adept at, and be ready to switch over when the situation arises. Experimentation is a good thing, but don't lose any pictures over it.
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