By Carl Weese
Quite a few TOP readers have been asking about the low light AF performance of the Pentax K20D. It's no secret that the AF function in Pentax dSLR cameras through the K10D was not at the level of the industry leaders. It happens that I've been busy shooting all sorts of things in the six weeks or so I've had the camera, but low light situations didn't happen to be part of it. Then, this past Sunday, I went to a nearby Historical Society for the opening of an exhibit entitled "The Fabric of Marriage: Wedding Dresses." The title was irresistible.
The little town of Washington has a long history both as a retreat for the world-weary well-to-do, and as something of an artist colony. The combination continues to this day and makes for interesting exhibits at the Historical Society. This one included material reaching more than two centuries into the past, along with more recent material, displayed with a whimsy that approached the surreal.
Of course I started taking pictures the minute I walked in, then I realized I was testing the K20D in pretty difficult low-light conditions. The light was dim enough to need exposures in the range of 40th to 60th of a second at ƒ/4 or ƒ/3.5 using ISO 800. It was also really harsh with huge jumps from highlight to shadow. The photographer for the local paper took one look and got out a flash gun:
So how did the K20D do? I was once again impressed by the "unannounced" improvement in AF performance of the K20 over the K10. Given the situation, I chose to set the camera to S(ingle)AF, and Selective focus point. With this combination, you can use the control wheel to punch your way around the 13 AF points available in the finder. Focus is actuated when you press the shutter release half-way. If none of the focus points let you grab focus on the right part of the picture with exactly the framing you want, you can shift the framing before shooting. If you want to shoot another frame, you can grab focus again with much less disturbance to your framing than using center-only. In C(ontinuous)AF you are stuck with framings allowed by the need to shoot with an active focus point in the right place.
For, it turned out, 99 frames, the K20D with, for a while, the 35mm ƒ/2.8 Macro, and then the 21mm DA Limited, did at least as well as I could have managed with a first-rate manual film camera system. By which I mean a rangefinder Leica, or something like an old Nikon F3 with fast (ƒ/2 or ƒ/1.4) lenses. It flat-out did better than I could possibly expect to do by focusing this equipment manually. Between the relatively slow (dark finder) lenses and the inherent reduced perceptual size of the view in an APS-C sensor camera's finder, not to mention the hair-trigger abruptness of the focus helicoids in both these lenses, I wouldn't even try to focus visually.
At this light level, focus speed was not instantaneous. It wasn't "so fast I didn't notice." But I'd describe it as "fast." More important, it was positive. By that I mean that when I pressed the button halfway, the mechanism almost always made one direct move to focus, and stayed there. It only "hunted," a couple of times, and when it did hunt, I could immediately see that I'd selected a target that just didn't have enough detail or contrast for the system to grab. Even more important than that, when I examined the files on screen, it turned out that the AF had been consistently accurate.* You need all three of these, in some reasonable compromise combination. It doesn't matter how fast and positive your AF system is when it grabs focus point if that point is a few inches behind or in front of the actual target. It doesn't matter how accurate the system is if the pictures all go away before you can release the shutter. You need a workable combination.
Next couple posts in the series will continue to look at issues revolving around AF, while looking at specifics of some lenses the Elves at Golden, Colorado, have sent me to review.
*AF performance and the K20 AF adjustment feature are proving to be an eye-opening experience that I will be reporting on further. Here's the teaser: tests on official focus targets at very close range, and flat targets at more realistic shooting distances, indicated one level of adjustment for my 21mm (the potential utility of the lens was the reason I bought into the system) but real-world results began to show front focus with this adjustment instead of the rear focus it had displayed before. Next time....