Metering Patterns & Exposure Control
by George Schaub of Shutterbug
When we think of exposure our first instinct might be to rely on the metering system in the camera to always provide the correct exposure, regardless of the direction of the light or the conditions under which we expose. And for the majority of shots we would be fine, thanks to the incredible sophistication of the metering systems in cameras today. Whether the automated system is called evaluative, matrix or pattern, the scheme works by way of the system breaking the image up into numerous zones. These zones are then analyzed for not only the light in the zone itself but in the relationship of the brightness of the light in each zone to other zones. All this is then referred all to a complex look-up-table that feeds back an exposure solution. This now works with lighting situations previously considered impossible for an automatic metering system, including backlight, strong sidelight and even bright dominant scenes, such as snowy fields on a bright sunny day.
Two issue comes to mind here: one is the fact that you might not enjoy photography as much by always relying on such automation. There is a bit of a challenge involved in making your own readings and figuring out how best to interpret the light in the scene. True, with RAW these days you can be a bit off and recover nicely in post exposure processing, but those who shoot film, and those who shoot digital and want to get the exposure correct for their interpretation right out “of the box” then full matrix automation is less fun, and interesting.
The other is that there are indeed times when these patterned exposure setups fail. Now there is no way to predict exactly when this will happen, but having worked with a few different cameras I can tell you that it is indeed different for every system. In some it seems like the setup is still made for color negative film and the matrix system delivers consistent overexposure in high contrast scenes. In others evaluative seems to go off a bit when there is a dark dominant setup (lots of dark areas with side lighting), again overexposing.
That’s when taking control over how your system reads light comes into play, and the chief tool you have for that is the way you set up the metering pattern, aided and abetted by exposure compensation controls and auto-exposure lock. There are two main alternates to patterned metering, and that’s center-weighted averaging (CWA) and spot readings. Think of CWA as an oval that sits within the finder, with the majority of light information taken from that general oval and less from the edges of the viewfinder frame. Spot takes readings from a very specific area within the frame, defined by a circle within the finder. Some cameras allow you to expand that circle if you think it would be better for the way you work.
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While a brave effort, the patterned meter “wants” to open up the dark area in this image and while it does so overexposes the brighter part of the frame. Even with some shadow/highlight fixes there’s little hope for recovering the overexposed highlight here. This is a tough shot for any automatic meter, be it evaluative, matrix or patterned.
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The easy solution for this shot is to look at the light, consider your options and take control. Here a center-weighted-averaging pattern was used. The finder was pointed toward the right side, the exposure locked and then the shot was recomposed. This solved the exposure quickly and easily and balanced the light and dark side of the image “as seen.”
With these patterns you revert, if you will, to the “old” way of exposure reading, which means that wherever you take the reading from will yield an exposure that averages the readings to a middle gray or, in the case of the spot, yields a middle gray brightness value on the brightness area you have read from. This middle gray works wonders when you have a range of brightness values in the scene—it in essence sorts the values for you, thus records the brighter areas brighter and the darker areas darker on whatever medium you are recording. So, if you have three general brightness values, such as f/4, f/8 and f/16, the averaged exposure will be f/8 (two stops from each side, if you will) and that means that all the brightness areas will be fine. This is a very generalized description but it’s pretty much how it works.
With spot reading you actually “place” the small area in the finder on the brightness value you wish to read and then make your decisions from there. Want to “move” a brighter value to middle gray (a strategy for saturating a bright color, for example)? Then just place the spot over that value and use that reading. Want to insure bright highlights and not deepen shadow values to excess? Then read the highlight and use +1 exposure compensation, or slightly more in very bright conditions.
While matrix, evaluative or whatever might “solve” all these exposure situations I always think it’s more fun and challenging to tackle it yourself, and in the bargain gain lots more exposure control. Learn how to work with spot and CWA patterns and you’ll rarely have to reshoot to get the exposure right.
