Almost since the day I began my renewed interest in photography, I’ve heard rumors of this thing called the “sweet spot” for a lens, which is vaguely described as around 2 stops above the minimum for most lenses. I’ve never actually taken the time to investigate what this means in real terms though, so I decided to test it out with my 85 mm prime. I took a photograph of the same scene at each aperture opening in the middle of my living room, so the light wasn’t necessarily great (even wide open the shutter was a 1/15). To test this theory of a sweet spot, I wanted elements that provided high black and white contrast and also bright primary colors, so I gathered some brass ornaments around the house, set them on an end table, put a chair behind it with a black slatted back and some brightly colored ornaments on that, and set up a white sheet of tagboard behind it all. Here’s the results of my experiment.
Traditionally I’ve been a guy who likes to shoot at as wide an aperture as possible for many shots, because I tend to like bokeh and softness in the background. Many times in the past, particularly with one of my lenses, I’ve been frustrated with chromatic aberration, and rather than fault my own meager photography skills I’ve been inclined to blame the lens as cheap. It is a cheap lens, but this experiment shows me that I need to be more mindful of what I’m shooting and what the appropriate aperture is. In the above test (done with a not-so-cheap lens) you can see chromatic aberration all over the place – at the edge of the chair slats, the front of the statue, and around the egg colors’ edges.
In the next example, taken at 5.5, the chromatic aberration is pretty much gone (some coloring you can see on a few of the edges is actually a reflection from the objects behind). There’s a nice softness to the foreground and background, but not so soft the objects look like smudges of color as in the first shot, and there is detail on the statue in this image that is lost in glare in the first shot.
The third shot, taken at f/22, actually seems to be a little out of focus, but the focus and distance didn’t change. I think the reason for this appearance is the flare we see off the highlight in the blue and on the brass in the front right. Wherever there is light, it has spread so much at an 8 second exposure that everything seems to be a little fuzzy or hazy; the detail is pretty much gone. Also, everything has the same focus and so there’s no real sense of depth. Plus, every flaw and speck of dust is immediately obvious, and would take much more post work to eliminate.
None of these settings is the perfect setting for every shot, of course. You wouldn’t need settings if there was only one “good” setting. But this experiment gave me much more clarity about lens behavior, and it’s knowledge I can use in the field to set up better shots and to understand what kinds of settings I can get away with (or will prefer) and what I can’t in any given situation.