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Old Jul 9, 2003, 5:51 AM   #1
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Default AUTOfocus problems?

something i never knew before but read the other day; is that apparently autofocus works best when the centre of the focus area is pointed at an area with some definable contrast, i.e. an area of the frame which has some light and dark patches next to each other; the more contrast the better.
I never experimented with this before, but as soon as I did i noticed an improvement in auto focus, including in low light some of the time.
If there isnt any distinct difference between any areas of lighting and there arent any clearly defined lines within the LCD frame or the viewfinder, then you can look for somewhere at about the same distance which does > and lock the focus then re-compose and shoot.

Im only a 1 year old in photo years, so maybe everyone else already knows this. Is defining lines of contrast - how autofocus works?
Anyway Im interested, does this tip work for others as well as it does for me?
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Old Jul 9, 2003, 6:37 AM   #2
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You are correct. I first learned this years ago when Minolta introduced their Maxxum auto-focus series of SLRs. Often you need to focus on another part of the picture area and hold the shutter release halfway, recompose and shoot.
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Old Jul 9, 2003, 7:51 AM   #3
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Some AF points only pick up on lines (i.e. contrast) in either horizontal or vertical directions. My Canon 10D has 7 AF points, but only the center one picks up on contrast in both directions. I believe the rest are sensitive to only vertical lines.

AF systems also need enough light to work. This is kinda obvious, but the tricky thing to remember is that your eyes are much better at perceiving light than a camera is. So you might be able to see fairly well, but it might be too dark for the camera's AF system.

There are much fancier AF systems on the really high end digital cameras ($2,000USD and up) which have multiple more sensitive AF points which also can work better in lower light. Its all a cost game of what features ot put in, but still hit their price point.

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Old Jul 9, 2003, 9:45 AM   #4
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Autofocus systems use image processing of the through lens image in realtime (rather like sharpening/unsharpening utilities) to detect edges and see how diffuse (in brightnes terms) adjacent pixels are.

So, you need good contrast for the software to distingish differences in levels of a few pixels on edges, and of course well defined edges.

But if you don't have them in the scene (say a plain coloured wall), just pan the cam to something defined, at the same distance, half press the shutter to capture the distance and return the cam to the shot frame. Now you can see why low light is a problem - no contrast on the edges. VOX
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Old Jul 9, 2003, 11:22 AM   #5
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Here's the details courtesy of How Stuff Works:

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