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03-29-2011, 11:57 AM   #1
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Auto Focus Adjustment

I'm playing around with my K-r since in lower light (not even necessarily low light) it does seem to have a focusing issue and I see the AF Adjustment option but I'm not sure where to start with this. For example, what does +1 vs -1 get you? What does this adjustment do?

Thanks, guys!

03-29-2011, 05:11 PM   #2
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What you want to do is set your camera up on a tripod or at least a table. Focus on something that has both a foreground and a background. Then look at your image on your computer. See whether it focuses in front or behind the subject. That's what the + and - adjustments are for. Ideally, you would focus on a chart designed for this purpose. Search for a focusing chart. Here's one:
http://focustestchart.com/focus20.pdf
03-29-2011, 11:49 PM   #3
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I'm gonna jump in here with a bit of heresy.
I think focus charts are generally misused and overall misleading.
Most lenses have some curvature of field, and so a focus chart is pretty much guaranteed to give a false reading. Combine this with the fact that it is practically impossible to align things to the perfection that is required to ensure that they would give a correct reading even with a flat field lens and to my mind, they are well nigh useless.
I also found when I tried using one that when I got things adjusted correctly at the rather close distances that these things are used at, that my normal focus distance was out significantly.
Here's how I adjust my lenses.
I set my camera on a tripod and focus on an object that is a more normal distance away for the lens in use.
I take an adjustment bracket series at -5, 0 and +5, and see which is better focused.
I then bracket adjust around the best focus in smaller and smaller increments until I am satisfied.
This gives me a real world AF adjust that works for making pictures of what I take pictures of, rather than nice crisp pictures of meaningless bits of paper taped to a wall.
03-30-2011, 06:35 AM   #4
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QuoteOriginally posted by Wheatfield Quote
I'm gonna jump in here with a bit of heresy.
I think focus charts are generally misused and overall misleading.
Most lenses have some curvature of field, and so a focus chart is pretty much guaranteed to give a false reading. Combine this with the fact that it is practically impossible to align things to the perfection that is required to ensure that they would give a correct reading even with a flat field lens and to my mind, they are well nigh useless.
I also found when I tried using one that when I got things adjusted correctly at the rather close distances that these things are used at, that my normal focus distance was out significantly.
Here's how I adjust my lenses.
I set my camera on a tripod and focus on an object that is a more normal distance away for the lens in use.
I take an adjustment bracket series at -5, 0 and +5, and see which is better focused.
I then bracket adjust around the best focus in smaller and smaller increments until I am satisfied.
This gives me a real world AF adjust that works for making pictures of what I take pictures of, rather than nice crisp pictures of meaningless bits of paper taped to a wall.
Wheatfield is right on the money with this. I have found with my lenses sometimes that once I have gotten adjustments perfect with a focus chart, they will suddenly stop being appropriate when I shoot something that is further away, or under different lighting conditions.

If you are doing a lot of macro work with your lens, then the focus chart makes sense (in fact, the most reliable adjustments I've gotten with one have been with my 35macro), but otherwise use some more typical subjects at more typical distances under more typical light, per Wheatfield's method above. You may compromise the accuracy of your lens' AF when focusing very close as a result, but that will probably end up being the minority of your shots.

03-30-2011, 06:44 AM   #5
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Thanks guys! I'm gonna try the technique without the chart (because I don't have a printer right now ) and let you know how it goes!
03-30-2011, 06:54 AM   #6
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QuoteOriginally posted by v5planet Quote
Wheatfield is right on the money with this. I have found with my lenses sometimes that once I have gotten adjustments perfect with a focus chart, they will suddenly stop being appropriate when I shoot something that is further away, or under different lighting conditions.
Fully agreed.
03-30-2011, 08:00 AM   #7
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I tend to use 3 aa batteries in a diagonal formation, such as seen on here on numerous occasions. Sit them on a flat surface ( I put them on the outdoor glass table) mount the camera on a tripod so it sit with the lens horizontal. point the lens directly at them (horizontally) so that you can focus on the centre battery. Then take it from there. The batteries don't have to be at 45 deg as you can adjust the angle to suit a decreasing DOF if your looking for refining the focus further. Check from a reasonable distance too and check and recheck at different focal lengths. If aa are too small from a large distance use D size or improvise with something else!

Steve
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