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08-10-2012, 05:56 PM   #1
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minimum focuis distance da15 ltd

I just checked the lens database and read that the minimum focus distance is 18 cm. I recently photographed some flowers and I was almost touching them. Does this mean my lens is back focusing?

08-10-2012, 06:07 PM   #2
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Focus distance is measured from the sensor - there's normally a little symbol on the top of your camera of a white circle with a horizontal line through it. This shows where the sensor is located.
08-10-2012, 06:24 PM   #3
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Thanks for the quick response. I was so close that the leaf of the plant touched the lens. I'll just have to check for f or b issues.
08-10-2012, 08:55 PM   #4
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As noted focus distance is measured from the plane of the sensor NOT the end of the lens. I do not have the 15mm but based on the given length plus hood the end of the hood is approximately 14cm from the sensor plane. That's just an estimate, you can measure it yourself. That leaves you 4 or maybe 5 cm distance to the flower to be within minimum focus.

So to answer your question this is NOT an indication of FF/BF issues at all. That simply cannot be tested in that manner. A back focus or front issue is when the auto focus thinks it has a lock but that the position of the lens elements is actually slightly different than what the AF system thinks. The actual error would be measured in MM not CM, at least at that distance. And the result would be an OOF image not an image that can be taken closer than you think is possible.

This is not to say that your lens does not have FF/BF issues, just that your test cannot prove it one way or the other. If your image was in focus then you do not have a FF/BF issue. One of the quickest ways to mess up your focus is to start fooling around with the AF fine tuning without a proper test. You will almost always just make things worse. And the only way to test is on a tripod, with a perfectly aligned camera and proper focusing target, which is not a bunch of flowers. And to repeat the test numerous times before even thinking of making an adjustment.

08-10-2012, 10:19 PM   #5
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Thanks for the detailed explanation. 4-5cm would seem very close especially when looking through the eye piece. I know this is not a test for f or b focus therefore as I was having the sensor cleaned on my new K30 this morning at the Pentax service centre I asked if they would check the 15 for focusing accuracy. One hour later I collected my camera and lens. They stated that they had adjusted the lens which was back focusing. They did not use the in body focus adjustment facility.
08-11-2012, 07:55 AM   #6
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The DA 15 is a very capable lens up close and a stopped down shot will still look very good in distances closer than the official minimum. Adjustments for FF/BF shouldn't change the minimum focus distance of the lens. The DA 15 has great DOF. I know that nobody else has ever done this but I once forgot my camera was set on MF and just walked around at a party taking pictures and not really noticing that my AF was not working. Lucky for me, I had the DA 15 on the camera and almost all my shots were keepers.
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