6.4 Metering Modes

One common point of confusion in first-time DSLR users is the difference between exposure modes and metering modes.  Put quite simply, exposure is what happens to the imaging sensor when a certain amount of light reaches it, while metering involves measuring how much light is coming in through the lens (in the case of TTL, or through-the-lens, metering).  Thus, the former deals with how much light we want, while the latter deals with measuring how much light is available.  Once we know how much light is available, we can set the exposure parameters so as to ensure that the right amount of light is collected by the imaging sensor, resulting in an image with the right amount of brightness.
    In manual exposure mode, when using a DSLR, we can effectively ignore the camera’s meter, because we can take a few test shots and adjust the exposure parameters (aperture, shutter speed, and ISO) until we like the image seen on the camera’s LCD (or until we like the resulting histogram).  In the autoexposure modes, however, the camera needs to utilize its internal TTL meter to determine how to set the exposure parameters.  Because different amounts of light fall on different parts of the imaging sensor, it matters which part of the image is used for metering.  That’s where the metering modes come in.  Different metering modes simply provide the photographer with different ways of telling the camera how to perform metering—that is, how to read the light levels.  More specifically, they allow the photographer to specify which parts of the image should be used for measuring the light level.  For bird photography, that’ll typically be whichever part of the image contains the bird.  Unfortunately, on most DSLR’s there is no “Bird Mode” that would instruct the camera to find the bird in the frame and take the meter reading from that part of the scene.  As such, we have to make do with the standard metering modes, which are described below.

6.4.1 Evaluative/Matrix Metering

One of the most popular metering strategies—known to Nikonians as matrix metering and to Canonites as evaluative metering—is to take luminance readings from locations (called “zones”) across the entire frame, and then to adjust the exposure parameters according to either a sophisticated computer algorithm or by matching the observed readings to a large database of predefined scene patterns, or “profiles”.  Because these algorithms and profile databases have been so extensively tweaked by the engineers at the camera manufacturers’ labs, they tend to work very well for a very large number of photographic scenarios.



    One problem with evaluative/matrix metering modes for bird photography is that in challenging lighting situations you often have to overexpose or underexpose the rest of the scene in order to properly expose the bird.  If, for example, the bird is perched in a shady foreground surmounted by a large, bright background (or if the bird is back-lit, meaning that the bird is between the camera and the sun), then under some evaluative/matrix metering modes, autoexposure will result in a bird that is too dark (unless you’ve dialed in a large exposure compensation value). 



Note, however, that some evaluative/matrix metering modes can work well for back-lit subjects, as long as the subject is large enough in the frame to be detected by the metering mode’s algorithm.  Because the implementation of these modes differs between camera manufacturers (and even between different models produced by a single manufacturer), some experimentation will be required to find out how well this mode works for you.  Note also that on some cameras the evaluative/matrix metering mode will assign greater importance to the metering zone containing the active autofocus point (see section 6.7 for a discussion of AF points); if your camera happens to be one of these, then evaluative/matrix metering may indeed work well in those cases where you’ve got the bird positioned right at the active AF point—that is, as long as you’re not using the focus-and-recompose technique (discussed below).
    Although there are some very successful bird photographers who strongly prefer evaluative/matrix metering, the more popular option among wildlife photographers seems to be spot metering, which is described next.

6.4.2 Center-weighted and Spot Metering

For photographic applications in which there’s an explicit subject in the scene (such as a bird), or simply for those photographers who like to have greater control over metering, many SLR cameras allow meter readings to be taken from a small area of the scene—typically from the center, though sometimes also from other locations.  This is generally known as spot metering.  The spot in question (where metering takes place) is typically on the order of 2% to 5% of the size of the visible frame, though it can range up to around 15% on some models.  On low-end cameras the spot is often limited to a single, central location, and may be indicated by a faint circle depicted in the viewfinder (though the metering sensor may in fact be larger or smaller than this circle suggests).  On many higher-end models, the spot used for metering is co-located with the autofocus sensors, and on some of these models you can tell the camera whether you do indeed want the metering to be linked to the active AF point, or to always default to the center spot.  As noted above, some models provide a mode that behaves like evaluative/matrix metering but attaches greater importance to the zone containing the active AF point, resulting in a sort of hybrid matrix/spot metering mode.


    The problem with spot metering, for bird photography, is that the metering sensor typically doesn’t have the precise size and shape of the bird you’re trying to properly expose.  Depending on how large the bird appears in frame, the metering sensor may be larger or smaller than the bird.  If larger, then the meter reading will be influenced to some degree by non-bird elements of the scene, which may or may not be ideal for your purposes.  If the spot is smaller than the bird (and in general you may not know whether it is or not, since the glyphs in the viewfinder may not indicate the actual sizes of the sensors) then the meter reading will reflect the luminance of only part of the bird, and for birds with both white and black areas this can result in large variation in meter readings.

    One alternative to spot metering is center-weighted metering, in which a large, central region of the frame is used to collect a majority of the metering information, while the rest of the frame still contributes some information.  Some people find this a reasonable compromise between spot metering and evaluative/matrix metering, though how well it will work for you in practice will depend both on the types of birds you tend to shoot (i.e., whether large or small, and how close you typically get to them) and on how your particular camera implements the center weighting—i.e., how large the metering sensor is, and how the relative importance is distributed between the central sensor and the other zones spanning the frame.
    No matter which metering mode you choose, if that mode gives higher weight to any individual spots, and if you’re using auto-exposure (as opposed to manual exposure), then you need to be aware of the auto-exposure lock function, or AEL.  This is especially important for the method of focus-and-recompose, which will be described in detail in section 6.7.  Briefly, recomposing involves taking a focus reading from the bird and then shifting the camera’s angle so as to place the bird elsewhere in the frame, for aesthetic purposes (since you probably don’t want the bird to be in the exact center for all your photos).  The trick is to make sure that the meter reading is locked in place at the same time that the focus is locked; otherwise, after repositioning the bird in frame, the final meter reading may not reflect the brightness of the bird.  On many cameras you can lock the meter reading by pressing the AEL button (labeled “*” on some bodies).  Your camera’s intruction manual should tell you whether you need to hold down the AEL button or if pressing it just once is enough, and also what happens if you press the button multiple times; some high-end cameras allow multiple readings to be taken from different parts of the scene, and averaged together.





6.4.3 Metering in Manual Mode

Although the camera’s meter has no direct effect on the exposure parameters in manual mode, its reading is still available to the photographer if s/he wants to consult it.  Inside the viewfinder you’ll see a bar with regular intervals marked along its length.  This is the meter bar.  The zero position of the bar (halfway along its length) represents a “neutral” light level—the so-called 18% gray level, which historically was a good reading for film photography.  If the depicted reading is above this midpoint, then the camera believes the scene will be overexposed, and vice-versa for readings below the midpoint.  To the extent that you have faith in your camera’s meter, you may wish to adjust the exposure parameters so as to get the reading as close to neural zero as possible. 




    Personally, I ignore the meter altogether, except when trying to get an initial starting point for my exposure parameters at the start of a shoot.  To quote an experienced bird photographer I met once in Maryland, “The meter is the stupidest part of your camera; just ignore it.”  Given the much more information-rich histogram and highlight-alert features offered by modern DSLR’s, I don’t think that statement is terribly off the mark.  I make the vast majority my exposure decisions based on the highlight alerts that show during image playback on my camera’s LCD; this is in keeping with the ETTR philosophy outlined earlier in section 6.2.  More generally, it’s possible to make exposure decisions based solely on the histogram (nearly all DSLR’s can be set up to diplay an image histogram after each shot, rather than displaying the image itself).  Just keep in mind that the histogram reflects the overall light level throughout the scene, whereas a spot meter can measure light at just one point (which is useful if that point is co-located with the bird, and is a convenient size).  On the other hand, the highlight alert function is, in effect, a dense array of thousands of spot meters calibrated to the upper end of representable light levels, rather than to neutral gray.  In this way, we can view the highlight alert function as the natural evolution of spot metering for digital cameras (at least in manual mode).  It’s conceivable that some combination of histogramming and highlight alerts might be developed in the future, perhaps via Live View (see section 6.11), making the traditional fixed-zone metering system that much less useful for the advanced bird photographer.
    At this point it’s useful to note once again that all of the auto-exposure modes described in the previous section allow a manual override of sorts via exposure compensation (EC).  Though I now shoot exclusively in manual mode, I previously used Av (aperture priority) and found that the meter reading of my camera (a Canon 30D at the time) was consistently off by about 2/3 of a stop.  That is, I found that by dialing in an EC value of -2/3 I was able to get better exposures in Av mode on that particular camera model.  Different camera models have different biases in this regard.  During metering (regardless of metering mode or autoexposure mode), if you watch the meter reading in the viewfinder while turning the EC dial, you should see the reading go up and down as you vary the EC.  What you’re doing is telling the autoexposure system to aim higher or lower in its notion of what is an “ideal” exposure. 
    While shooting birds in Av I found that I was constantly adjusting my EC value up and down by a few clicks (each click on my camera is a third of a stop; on other cameras it may be a half or even a full stop per click—remember that a stop of light corresponds to a doubling/halving of the shutter speed or ISO, or multiplying/dividing the f-number by 1.4).  Once you find yourself doing this you should consider switching to full manual mode.  In manual mode you can choose which of the three exposure parameters (aperture, shutter speed, ISO) to modify, whereas in the autoexposure modes your EC settings affect just one of the parameters.  Also, in the autoexposure modes, the “baseline” reading that the EC value adjusts can vary as you follow a bird between sunny and shady locations; although this can be useful when relying entirely on ambient light, for flash photography it can be more of a hindrance than an aid.  Flash photography is addressed in the next chapter.