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10-25-2007, 06:26 PM   #1
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Autofocus: Horizonal edges ≠ Vertical edges ... normal?

I was goofing off the other day with a new lens, and was clicking around the living room at anything I found in my viewfinder. At one point, I was using the sharp, high contrast edge of a black picture frame against a white wall to lock focus from several feet away. But then I noticed that when I took a subsequent shot on the horizontal edge instead of the vertical (or the other way around), the camera slightly readjusted the focus. No biggie, I thought...AF cameras will often re-find focus with two successive half shutters on the same spot, much less two different spots.

But my curiousity was piqued, and I proceeded to let the camera focus on different parts of that frame. I kept doing half-shutters while moving from vertical to horizontal and vice versa, and sometimes staying on the same spot. One thing I noticed was that when I had two successive focus attempts on the same edge (even two different areas of the same edge), the camera would generally not refocus. The AF indicator would appear instantly and it would be ready to shoot without any lens movement. Sometimes it refocused, but only occasionally. But any time I switched between horizontal and vertical, the camera would refocus....without exception.

I was curious to see how much of a difference the camera was perceiving between two focus spots. So I also snapped a few pictures during this focusing experiment, and let the shallow DoF of my FA35 reveal any focus errors. It seemed that many of the shots focused on a vertical edge were slightly out of focus (don't know if it was front or back focus), and the ones targeted on a horizontal edge were generally well-focused.

This test was performed using the center focus point, which is a "cross" sensor, so it seems to me that the sensitivity and response to contrast should be the same. After a few shots, I put it on a tripod to make sure I wasn't changing my distance from the target. The frame was far enough way, and I was lined up squarely enough to it, that there was no real change in distance from point to point. Even if there were differences, the camera still couldn't always adjust to the right distance on the vertical edges while it could for horizontals. I picked a new target, some tile on a wall with contrasting grout, and the behavior was the same.

So, is there any reason why my camera (a K100D) might focus better on a horizontal than vertical edges, when they're of equal contrast?

10-25-2007, 06:56 PM   #2
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Page 46 of the manual mentions focusing difficulty on certain subjects...

Blah blah, blah..."Vertical or horizontal line patterns..."

I don't think a single line constitutes a "pattern" however.
10-25-2007, 09:10 PM   #3
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It depends on the orientation of the AF sensor. If you google "d70 focus chart", the first link is a wealth of AF information.

Anyways, pertaining to your question, as extracted from the site:

"Each sensor is only able to assess areas of contrast that are not parallel to the sensor. "
10-26-2007, 08:08 AM   #4
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I think the "cross" pattern is supposed to handle the issue of horizontal and vertical, on the basis that there is no single line that is both at the same time.

this goes all the way back to the concept of diagonal split images in the "antique" era of manual focus film slr's where horizontal split images sometimes were a problem because the only focusing references were also horizontal.

I think you had mentioned that you were focusing on a picture frame. Are you sure you are square with the frame?

is the focusing distance changing i.e. lens to focusing point, between the side frame and the top or bottom frame when you change focus? unless the frame is square it will be, and you may be close enough that this causes a shift in focus.

Also is there detail in the frame, that may be causing the focus to search a little? i.e. a pattern.

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