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08-13-2012, 05:54 PM   #1
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AF issues with D-AF 100mm Macro Lens

Hi, I acquired a D-AF 100mm macro lens back in April from another forum member and just started seriously using it. About 95% of time focus would fail when shooting AF regardless lighting conditions. Initially thought was my technique but recently started using MF and have been getting great results. Please view photos below. The blur one on the bottom was with AF, both shot at f4.0 on tripod. Has anyone experienced the identical issue with this lens or it needs to be calibrated by Pentax? Thank you in advance for your feedbacks.




08-13-2012, 06:45 PM   #2
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The exact location of the AF sensors may not correspond exactly with the af sensor marks in the viewfinder, and it depends on which AF sensor the camera gives priority to. I'm guessing that in your photo the plane of focus was at the bottom of the batteries, the aperture was wide open, giving shallow depth of field, and the camera was at an angle to the batteries and not horizontal.

Try the photos again, but manually select your focus point, usually the centre point, and take another set of photos with AF. If there is still a difference you may have a front or back focus issue and will need to micro-callibrate the lens to the cameras AF system.

Regards

Chris Stone
08-13-2012, 07:02 PM   #3
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Thanks, Chris.

I did manually select the focus point which was the center point aiming at the upper-center part of the battery in the middle. Under most circumstances, If anything, AF seems to shift to the side(s), will this consider as front/back focus?

Last edited by mxlinn; 08-13-2012 at 07:07 PM.
08-13-2012, 07:48 PM   #4
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AF will look for the highest contrast to lock onto. In this test that might have been the edge of the battery, not the center. The other possibility is that you have a slight front focus / back focus and need to fine tune. Since you have the k-5 this is easily done.

Set up as you have with camera on tripod, make sure everything is aligned. The center of the lens needs to be perfectly horizontal to the target and aligned vertically. Any slight deviation from alignment renders the test inaccurate and will probably end up making your focus worse not better. Find a better focus target, batteries are OK for a quick test but not accurate enough for fine tuning. Several charts are available on the web (Google focus chart) or use a ruler at a 45 degree angle.

Take a series of shots to test. I take a few shots manually focused, then a few using live view and zooming in and then a few using the viewfinder AF system. When all three methods agree then you know you have everything adjusted correctly.

The live view will be the most accurate so I use that as the standard to adjust to. Once you have your shots go to the computer and compare; is there a difference in the shots using live view and using the view finder AF? If so which way? Is the focus point closer or farther away? Once you have this go into the camera menu and go to menu Custom - 4 option 26. You can set adjustments for up to 20 different lenses on the k-5. Make the adjustment, a little at a time and retest. Repeat until live view focus and viewfinder AF agree.

Please take your time and retest multiple times, this is not an exact science and you can make it worse if you just jump in and start changing things.

I use a Spyder LensCal system Amazon.com: Datacolor DC SLC100 SpyderLensCal Lens Calibration System: Camera & Photo but the homemade system will work just fine if you take your time and double check everything.

08-13-2012, 10:01 PM   #5
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Since there is nothing at all that appears to be in focus in the second shot, I would probably guess that this is a front/back focus issue. Especially since you said this was a problem 95% of the time.

The other 5% could have been just luck -- For example, if you are taking pictures of a person, the AF sensor might actually be trying to focus on a person's hair, but then the person's eyes accidentally wind up in focus due to a FF issue. As you probably know from other threads, the f/2.8 is pretty limited by the fat AF sensor. At f/1.8 or 2.0, I sometimes get one eye in focus, the other not in focus, or the top of someone's head or an ear instead of either eye.
08-14-2012, 03:47 AM   #6
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Look second pic tablecorner line on left. There is very little backfocus and that seems to be the problem in this case. Can try to correct with AF finetuning.
08-14-2012, 06:35 AM   #7
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The exif of the two images are different... exposure and time shots were taken are different. Seriously there could be any number of reasons why the images aren't tack sharp. Test and retest again.
08-14-2012, 12:10 PM   #8
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QuoteOriginally posted by creampuff Quote
The exif of the two images are different... exposure and time shots were taken are different. Seriously there could be any number of reasons why the images aren't tack sharp. Test and retest again.

Whoops, pick the wrong one. Nevertheless, took a series of 20+ photos in a spam of 30 minutes using AF and MF, the end results are the same as these two photos represent: AF - blur, MF - right on.

08-14-2012, 05:07 PM   #9
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QuoteOriginally posted by 123jippo Quote
Look second pic tablecorner line on left. There is very little backfocus and that seems to be the problem in this case. Can try to correct with AF finetuning.
Thanks, I think you have point here. Good catch.
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