by David Tong
After writing my previous articles discussing exposure (here and here), I received a couple of emails asking me “what is metering and how do I use it?”, and it made me realize that while the concept is simple, it’s not as easy to understand with all the jargon that goes with explaining what metering is.
Virtually all 35mm cameras that were sold after 1980′s have built-in meters. The built-in meter helps the user and/or the camera’s sensor to determine what exposure settings (shutter speed and aperture) the camera should use to obtain a “proper” exposure. As mentioned in my previous article, the camera’s meter can be pretty accurate for “average” scenes where the bright, middle, and dark shades of the scene are pretty well distributed, like the image above.
The image has bright areas (clouds, sky, and the light building), a lot of midtone (the front building, the road, the trees), and ample dark areas (the areas under the elevated highway). That is a pretty average scene, and most of our photos do fall under such lighting conditions, which makes sense to rely on automatic metering.
However, there are ways to “fool” the camera’s meter.
As I mentioned previously, the camera meter determines the midtone in the image, then averages everything out, and depending on the metering mode used, where you point your camera can yield dramatically different exposures on the same scene. Let’s use visuals to explain further. (The images below were shot indoors, just in case you see any glass reflections)
This is a gamma chart that shows the gaps between dark and light areas of an image. The whole area represents what your camera can see between black and white points, and the middle swatch is midtone.
Image 1: Original Image – Auto Exposure via “Evaluative” Metering
The red dot represents the midtone value of the scene, that’s the halfway point of our histogram represented as the dot in our graph. If we remove all the colors, this is what you’ll see.
The RGB values in the Info palette indicate R127,G127,B127 for that same spot (if 0 is black, and 255 is white, midtone would be 128, so we’re pretty close).
What would happen if we meter somewhere else? Let’s say we want to get as much details under the highway (the shadow areas) as possible. I pointed the center of my viewfinder towards the dark area under the bridge (red dot again), half-pressed the shutter button to lock the exposure, and then recompose to take the same shot.
You just told the camera that red dot is midtone, as a result, you exposed for that area “properly” while the rest of the scene is averaged. The result is more details in the shadows, but no details in the areas that are much brighter than the area you metered. Again, if you desaturated the image, you’ll see that the metered area is “midtone”, similar to Figure 3.
Metering for shadows is also known as “preserving shadow detail” or “sacrificing highlights”. Note that the histogram is leaning heavily to the right.
If you use the gamma chart as a basis, you shifted the middle grey swatch 3-4 slots to the right, and you lost 3-4 slots of bright image details as it falls outside your camera’s exposure range already.
Conversely, you can do the opposite, preserve the highlight details while sacrificing shadows. You’re now shifting the middle swatch to the left (blacks).
Note how the histogram is now leaning towards the left of the image. We metered the bright cloud (indicated in red) and by doing so, you told the camera’s meter that “that part should be midtone grey”. After desaturating, you’ll see that the cloud area now has a similar midtone RGB values as the previous two “metered” areas in Figures 3 and 5.
We can clearly see more details in the clouds, but the dark areas are now void of details due to the shift in exposure scale.
I decided to shift the original image’s exposure slightly higher to bring out more shadow detail and sacrifice just a little cloud detail for my final shot.
You, as the photographer and camera operator have the final say of how much detail you want to retain and sacrifice in the scene. You can instruct the camera where the “midtone” is in your image, not the other way around.