You’ve taken the day off of work to head out to the local state park for your favorite activity, landscape photography. You’re up well before dawn to catch the perfect sunrise in the ideal location; maybe you even scouted the spot before in order to ensure everything’s just right. You’ve got your camera settings dialed in and you’re all ready. The last step is to ensure everything is perfectly sharp, so you’re taking the added step of switching from autofocus to manual in an effort to really dial it in precisely. You’ve really gone to great lengths to get this shot as evidenced above, but are you about to make a big mistake right before you click the shutter? Are you undermining all your efforts to ensure your shots are sharp?
The big mistake is zooming your lens to recompose after you’ve focused on the subject.
This zoom focus technique isn’t just for landscape shooters, of course. It’s true for anyone composing a scene they want to be perfectly sharp. And isn’t that usually all of us?
Some photographers zoom in as close as possible on their subjects in order to focus precisely, but then they undermine that sharpness when they zoom out to recompose. This is because most zoom lenses are varifocal, meaning that their optical construction is such that focus changes as the focal length (or zoom) is adjusted. If you focus first and then zoom the lens to recompose, your pictures aren’t going to be tack sharp.
But wait, you might say, I do this all the time and I’ve never noticed an issue. Consider yourself lucky. This could be because you’re using wide-angle zooms so the depth of field covers you or you’re shooting with a smaller aperture for the same result. Even focusing at infinity for subjects a great distance from the camera can mask errant focus, but rest assured that the focus does change when zooming, so it’s best practice to break this risky habit.
The kind of lens that doesn’t require re-focusing after zooming is called a parfocal lens. These are difficult to build and consequently very expensive, but they’re more common in cinema and video applications where zooming during recording is a standard technique.
You can use your camera’s magnification controls to check focus precisely immediately after capture or even shoot tethered to check your sharpness before and after every shot. But if you do nothing else, it’s the best standard practice to get in the habit of focusing only after you’ve zoomed the lens to ensure your shots are as sharp as possible.