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08-01-2009, 05:35 AM   #16
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my former k10 could never focus properly on say a bunch of trees in the distance.
don't know if i'm asking too much of the AF system, but it seems to not know what to focus on or since i'm using a wide angle lens, it just picks a spot that it thinks is right.

oh, well. that's what MF is for

08-01-2009, 05:48 PM   #17
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QuoteOriginally posted by pbo Quote
I was also wondering what "cross type" focus point means? I was kinda thinking (just intuition and such) that it'd mean the single point where "cross" crosses would be in fact the point camera focuses on and then relied on the red square as a guide to that point's position... But then yesterday (or was it day before yesterday?) I found out that was wrong.
Hi pbo,

Cross type focus sensors can more easily detect objects that have contrast edges that are either vertical or horizontal. In the SAFOX VIII and VIII+ systems, the nine central focus sensors are of this type, with the two outside sensors more sensitive to vertical edges.

wlachan's post shows how the sensors are placed on his cameras, but this is a variable between bodies since the AF sensor assy is independent of the camera's image sensor, and only has to be aligned within the established mfg's tolerances.

The greater sensitivity to horizontal and vertical contrast edges brings up another factor in how the AF sensor "chooses". Contrast edges in the AF sensor's area that more closely align to either vertical or horizontal relative to the image sensor's orientation will usually be preferred by the system. If the AF system, for example, had to choose between a horizontal or vertical line, and one that was oriented at 45 degrees, it would probably choose one of the first two, all other things being equal.

I shoot a lot of birds in dense foliage with long teles, relying very much on AF. I will on occasion, when shooting handheld, tilt the camera to match an angled contrast edge on the bird to help the AF system lock on what I want it to.

I'd suggest that you try testing the location and area of the focus points that you tend to use. It always helps to know your equipment. A tripod is pretty much a necessity to do this as you need the image to be stable in the VF to be precise.

Scott
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