Independent of composition, photographers have three scene variables to play with when setting up a shot: light, depth, and motion. And not so coincidentally, we have three exposure parameters with which to manage those variables: shutter speed, f-stop, and ISO. The exposure parameters have a reciprocal relationship—an increase/decrease in one can be compensated by a corresponding decrease/increase in another—but merely getting the “correct” exposure with a random combination of exposure values can yield wildly different results. This is why I never want my camera making decisions for me, and always photograph in manual mode. It’s also why I tell my workshop students that they should be able to defend (explain their reason for) every exposure value.
My approach to metering
As a landscape photographer who is always on a tripod, I’ve removed camera shake from the exposure equation, so the only motion that concerns me is motion in the scene. Unless I’m trying for a motion effect (for example, blurring moving water or freezing wind-blown leaves) I use the f-stop and ISO that yields the absolute sharpest, cleanest image—no quality compromise (such as a large f-stop or high ISO) to subdue camera shake. This allows me to approach every scene at ISO 100 (most camera’s best ISO) and f/8 – f/11 (the maximum depth of field possible with minimal diffraction, and also the range where most lenses tend to be sharpest), and only deviate when my scene variables dictate.
Once I determine my composition, I refine the f-stop to ensure to depth of field that achieves my desired effect, then dial my shutter speed to whatever value delivers the proper exposure. My ISO only moves from 100 when motion in the scene requires it. For example, photographing at night requires a shutter speed fast enough to minimize star motion and I’m force to increase my ISO to 800, 1600, or higher, depending on the amount of light and the focal length at which I’m shooting.
About this image
In this image from last Monday’s snow day in Yosemite, I wanted to smooth the rapidly flowing Merced River to smooth chop that I thought might distract from rest of this spectacular scene, but first I had to get everything else in place. I on a fairly tight vertical composition that eliminated everything on the left and right of El Capitan. I used the snow-capped boulders in the foreground to create a little depth, and included just a little sky for some extra color (if the clouds swirling about El Capitan had been more interesting, I might not have done this).
Aligning the foreground boulders was both tricky and awkward. First, I had to wade through a couple of shallow pools to reach a one-rock island (not much bigger than a basketball), then thrust my tripod into the water as far to the left as I could and still operate my camera. A focal length too narrow would have cut off important elements, and too would have introduced distractions just outside this frame. Where the camera was positioned, I couldn’t get my eye to the viewfinder without going for a swim, and there was too much light to see my LCD clearly. So I pulled my camera from the tripod and framed the composition to get the right focal length. With my composition established, I decided f/10 was the largest aperture that would ensure front-to-back sharpness, and focused on a ripple a little behind the boulders.
Finally ready to shoot, I returned the camera to the tripod and used what I could see of the LCD to guess the final composition. After each click I removed the camera and reviewed the image in my electronic viewfinder (another reason I love the EVF), adjusting two or three times until the composition was right.
With my composition framed exactly as I wanted, my exposure dialed in, and my focus point set, I was ready to play with the motion. I started by dropping my ISO to 50 (almost as good as ISO 100, but with slightly less dynamic range), but still couldn’t get the shutter speed long enough without shrinking my f-stop to a less than ideal, diffraction inducing number.
The solution was my Singh-Ray Vari-ND filter. Because my Vari-ND is 77mm, I had to stretch to hold it in front of my 67mm Sony/Zeiss 24-70 f4. Dialing the filter ever so slightly darker between exposures and adjusting my shutter speed to maintain the correct exposure, I ended up with a variety of motion effects. I found that this one, at around 3 seconds, smoothed the water enough while retaining just enough texture for visual interest without being distracting.
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