Monday, May 18, 2009
Maximize Sharpness - 5/18/09
A simple and effective test to determine lens sharpness
If you spend a lot of time online reading about photography, you find a lot of concentration on the technical aspects of equipment—like lens sharpness. As important as sharpness may be, it’s not the only thing that makes for good lenses or good photos. And if you spend all your time worrying about which new lens might be the sharpest, you’re missing the point. All modern lenses are incredibly sharp. The real key is to know how to use them to get the maximum sharpness out of each one.
Lenses are often said to be sharpest at two or three stops from wide open. That would mean that an f/2 maximum-aperture lens would be sharpest at f/4 or f/5.6. But in truth, that varies from lens to lens. The only way to find out exactly how sharp a lens is, and at what aperture it’s sharpest, is to test them.
Testing lens sharpness is simple, like photographic copy work. Set up a test chart, or even a newspaper, as if it were a piece of art to be copied. Tack the target to a wall, or hold it with magnets to a metal board. Use light sources that provide even illumination, and place them beyond 45 degrees from the camera axis—so the lights will be near the same plane as the resolution chart to avoid reflection into the lens.
Whether you’re using a plain old newspaper or a fancy resolution chart, the point is simple—photograph a detailed, flat surface across a range of apertures, and examine the pictures to compare the sharpness from one to the next. Set your lens to manual focus, and once you’ve established focus for the first shot, don’t adjust it again. Use a cable release or timer to ensure the camera is steady.
Strobes will work, but hot lights are simpler because of the constant exposure adjustments. For example, set up your target and lights and calculate the appropriate exposure at a low ISO for the lens set wide open—say, ½ a second at f/2. Shoot an exposure there, and then adjust the lens to f/2.8. Don’t forget to modify the shutter speed to maintain an accurate exposure—in this case, 1 second at f/2.8. Then continue on down the line until you get to the minimum aperture.
Opening the photos in your favorite browser, enlarge them to examine the same details of the scene in each picture. You will pretty quickly learn a lot about your lenses.
I tested a favorite 100mm f/2 lens and learned that its sharpest aperture is f/8. What surprised me almost as much, though, was how f/22 was not very sharp. It reiterated something I always knew, but hadn’t really seen in practice: small apertures put more in focus, but they don’t always make for a sharper point of focus.
My test revealed something else—how different apertures create differing amounts of vignetting. Stopped down to f/22, my 100mm lens showed no signs of darkening at the edges. But wide open at f/2, the corners of the frame were significantly vignetted. Scrolling quickly through the thumbnails animated the changes so I could watch the vignettes expand and contract quite clearly.
I could carry out additional tests on other lenses to find out each of their sharpest apertures, and sure I could then compare them to each other to determine which of my lenses is the sharpest. But really, what’s most valuable is to know just how sharp each aperture on a given lens may be. If I’m at f/16 and have no particular need for the added depth of field, why not adjust to an equivalent exposure that will allow me to shoot at f/8—the sharpest aperture on my lens?
Remember, though, every adjustment has consequences. If you change apertures for sharpness but don’t concern yourself with the dramatic changes to the depth of field, you could do more harm then good. Don’t treat sharpness as the end-all, be-all in choosing which aperture to use. But it is good to know at what apertures your lenses are the sharpest, and make use of that knowledge when you can.