The sharpest aperture on any lens is generally about two or three stops from wide open. This rule of thumb has guided photographers to shoot somewhere in the neighborhood of ƒ/8 or ƒ/11 for generations, and this technique still works well. It’s bound to get you close to the sharpest aperture. But the only way to know for sure which aperture is your lens’ sharpest is to test it.
All you need to perform this test is a light, a tripod and a cable release—or even better, a tethered connection to a computer. This will allow you to not only check focus precisely but also ensure you’re not causing any camera shake since you’ll be tripping the shutter remotely. You’ll also want a flat subject with some really fine detail. It’s this detail that will test the resolving power of your lens. Something like a page of newsprint with very small text is an ideal candidate.
Tape the newspaper flat to the wall in a position where it’s easy enough to position the camera and tripod so that the camera is level and parallel to the wall. Light the newspaper as if you were copying a painting—with a light on each side, positioned beyond 45 degrees from the camera to ensure no reflections on the paper. I like to use a constant light source, like an LED or a tungsten hot light, and then make sure the lighting is nice and even and you’re ready to begin.
Set your camera to manual focus, and lock in on an area of fine newsprint. Then, set a manual exposure at the lowest native ISO (something like ISO 100 works just fine) and choose an aperture at either end of the spectrum—wide open (the smaller numbers, like ƒ/2) or stopped down (big numbers, like ƒ/32). With the aperture set, adjust the shutter speed until the exposure appears correct. Depending on the intensity of the light, this could make for very long exposures of several seconds when you’re shooting at ƒ/32.
Let’s say you’re starting with an exposure of 1 second at ƒ/32. Shoot a picture, then adjust to ƒ/22 and half a second before taking another picture. Then adjust to ƒ/16 at one-quarter of a second, and so on, proceeding all the way down to an exposure at the widest aperture—ƒ/2 at 1/250th.
After shooting an exposure at every aperture on your lens, import the images into your computer and take a close look at the details. You’ll notice right away that the wide open exposures and the fully stopped down shots will both be not quite as sharp as those in the middle of the aperture range. (You’re also likely to see fairly pronounced vignetting when shooting wide open.) Using a program such as Lightroom, it’s easy to switch back and forth between frames, or even compare them side by side, in order to determine which aperture produces the sharpest results. It’s bound to be somewhere around three stops from wide open—so on an ƒ/2 lens that would be ƒ/5.6 or even ƒ/8. You’re likely to see the differences in sharpness in the definition of the smallest examples of type in the frame. The distinctions may be subtle between a couple of middle apertures, but you’ll know at least which ones are visually sharper than others.
Once you’ve determined which apertures produce notably sharper images, you can be sure to start there every time you’re using that lens. Of course, you’ll sometimes need to shoot wide open for shallow depth of field or stopped down for deep depth of field, but at least you’ll know what apertures to use when you don’t have a specific reason to change it.
Try this test with all your lenses and make note of which apertures are sharpest—and just as notable, which apertures aren’t. It will come in very handy with every new photoshoot you do.