K200D autofocus modes - speed and accuracy

[grainbelt]

I'm coming from a p&s background, purchased a K200D a month ago. Now have the FA 50 1.4 as well as the kit 18-55.

I brought the 50 1.4 to a holiday party last night and thought I'd shoot some candids of my coworkers and their kids, since we were at a big arcade/indoor playground/lasertag facility. Lighting was fairly poor, I was getting shutter speeds of 1/100 at F2 and ISO 400, no flash.

My main issue was around autofocus. It is my understanding that the center AF will try to pick a focus point within a larger area than the other focus points. Trying to half-depress the shutter, get a focus point, compose, and shoot was painfully slow. Seemed to get a lot of hunting.

Perhaps my expectations are too high, but I was expecting very quick, nearly instant AF. Should I be manually selecting a different AF point (either above, below, or beside the center area) for peak performance?

Appreciate any insight.

[kjfaust]

you can use your flash as a AF assist lamp
then once your subject is in focus, just lower back the flash

[Marc Sabatella]

Originally Posted by grainbelt

My main issue was around autofocus. It is my understanding that the center AF will try to pick a focus point within a larger area than the other focus points. Trying to half-depress the shutter, get a focus point, compose, and shoot was painfully slow.

I'm confused. Were you using center point mode or not? That's usually the fastest way to to get any camera to focus - don't let it waste time trying to figure out which focus point to use. Or you could put the camera in user select mode and tell it to use the center point, which is usually the fastest / most accurate.

But in low light, yeah, AF can be slow even when the camera knows to use the center point. It helps to point the camera directly at a very clear target - something with a lot of contrast. The camera is going to have a hard time focusing on a plain white shirt, for example. Also, some lenses propvide better contrast and focus performance than other. The FA50 is not known for super fast AF performance compared to, say, the DA40.

[grainbelt]

Originally Posted by Marc Sabatella

I'm confused. Were you using center point mode or not?

I was using center point, and focusing mainly on faces. This was in fairly low light, indoors, and people were moving around, so it certainly wasn't the easiest situation.

For some reason I thought I read somewhere that the center point behaved differently than the others, but I may have misread it - been reading too much lately, and not shooting enough. -45F temps will do that.

[jeffkrol]

Originally Posted by grainbelt

I was using center point, and focusing mainly on faces. This was in fairly low light, indoors, and people were moving around, so it certainly wasn't the easiest situation.

For some reason I thought I read somewhere that the center point behaved differently than the others, but I may have misread it - been reading too much lately, and not shooting enough. -45F temps will do that.

you may have been reading about Canon. Not all AF sensors are equal. Fast glass actually requires a different type of sensor. As an example (but I believe not the only way to achieve it)
...... the 40D incorporates a diagonally mounted, high-precision cross-type sensor at the center AF point that is sensitive to both vertical and horizontal lines when fast lenses (f/2.8 or faster) are used, for for added accuracy and precision with these wide-aperture lenses. It's the world's first AF SLR where both............
Canon EOS 40D Digital SLR Kit with 28-135 IS USM Lens at Hunts Photo & Video - 100 Main Street - Melrose, MA 02176

A relevent thread can be found here:
Best lens for low light AF
And a source:
http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf
Aperture dependency
If the two “virtual AF apertures” are spaced further apart than the full diameter of
the exit pupil (as a result of our mounting a lens having a rather small maximum
aperture), there are no blue or red rays that can pass through to the detectors, and
focus detection will fail.
To avert this, the focus detectors are usually designed to have virtual AF apertures
near the edges of the exit pupil for a modest camera aperture—an aperture likely to
be available on most lenses to be used on the camera. Of course, this compromise
(a “reduced baseline”) results in a reduction in the precision of the focus detection
system.
In some cameras (such as the Canon EOS 20D), at one or more particularlyimportant
AF point locations across the frame (typically the one at the center), two
autofocus detectors are provided, one workable for modest aperture lenses and one
(with a “larger baseline”) workable only for larger aperture lenses. The appropriate
one is put into action based on the maximum aperture reported by the lens in place.
This arrangement exploits the greater precision in focus determination available
with a larger aperture lens in place.
#

[KjetilH]

The FA50 1.4 is, even though it is "fast" (aperture), the autofocus isn't that good. Probably a result of it being not very sharp wide open, where, obviously, the focusing happens.

[grainbelt]

Thanks much, that makes a bit more sense to me than the technical commentary above.

One thing I noticed was that the manual focus ring on the FA 50 1.4 has a nice long throw and a good tactile feel, so I may simply try using manual focus a bit more next time.

using the flash as a focus assist seems like it would not only distract the people I'm shooting, but also a be a bit inconvenient, ergonomically speaking. I've heard the speedlights have a focus-assist-only mode, and I'm planning to pick one up shortly, so perhaps that will help. My Canon S5 IS had a dedicated, red coloured lamp that served as a focus assist, and it had very responsive AF, for a p&s camera.

[aamir515]

Originally Posted by grainbelt

My Canon S5 IS had a dedicated, red coloured lamp that served as a focus assist, and it had very responsive AF, for a p&s camera.

well i had a canon s3 is and i was just frustrated by the low light focusing issue even with the af assist light and thats why i upgraded to pentax k100ds. i use it center focus point and the performance is excellent (compared to canon s3) even without assist light.

[KjetilH]

Remember, a P&S does not need anything like a super sensistive AF, due to the depth of field being so great. Think of it as treading a needle compared to throwing a ball into a swimming pool. The pool is much larger, and it doesn't matter much how lousy you throw, you'd still hit the pool.

[daleroy]

Originally Posted by grainbelt

Perhaps my expectations are too high, but I was expecting very quick, nearly instant AF. Should I be manually selecting a different AF point (either above, below, or beside the center area) for peak performance?

Appreciate any insight.

Unfortunately, Pentax DSLRs are not that quick at focusing in low light compared to Canon/Nikon. You'll probably get a few people tell you to use different batteries and the like (which may make it faster than it currently is for you), but will never make it achieve the same speeds as say a Canon 40D. Part of the reason for this is that the Pentax AF double checks focus before showing the shot as in focus. In normal outdoor light, it will be on par with the mid level Canon/Nikon's, but in low light, Pentax are really slow at focusing.