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02-14-2011, 07:16 PM   #1
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Aperture-dependant focus shift....?

Hey all.

So I was play around with my DFA 100/2.8 WR when I noticed something rather odd - it would change its focus depending on what aperture I set it to. Note that this isn't a case of the point of focus changing upon stopping down while taking a photo, but just what the setting was - the lens hadn't stopped down at all at this point.

For example, I had it on my tripod pointing at a focus chart (point where there was only one line it could focus on), and set the camera to Av. I have a split screen installed so I could watch for any shifts (in addition to hearing it ). At 2.8 to 4, it remained constant, then at f5.6, it refocused (and was subsequently slightly off). The same happened again at f7.1

Anybody know what this problem is, and any way I can fix it? :/

02-14-2011, 07:40 PM   #2
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Can't say I've ever noticed anything like that. Then again, I haven't tested it in that manner either. Seems strange though. I've maintained in the K5 FF issue, and still do, that it shouldn't matter What you have the Aperture Set to because the lens will always focus wide open (in the case of an auto aperture K mount lens).

02-14-2011, 07:44 PM   #3
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Before branding this a problem what do the results look like?

it may be that due to being internally focused the focus point could change as a function of aperture a little and this would be most noticeable at close focus
02-14-2011, 08:04 PM   #4
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Sorry for the slight inconsistencies in composition; as luck would have it, the first time I tested it in (mentioned in my OP), my tripod fell apart on me when I was putting it back (a segment of a leg fell out). Lovely V-day for me.

As a result, these're all taken handheld.

f2.8


f5.6


f5.6 (focussed at f2.8, letting go of focus button (which I have set to the AE-L button) and then stopping down to f5.6 after the fact.)


f7.1


02-14-2011, 11:15 PM   #5
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My DA14 does this. At f5.6, the camera would FF it a little. When I test it manually, FF it slightly does indeed produce sharp images at small aperture. That means at f2.8 & f5.6, they have different focus. I know this has to do with aberration but not sure if my lens was faulty, or merely due to poorly corrected design. You will find many discussions on this matter by googling "aperture focus shift".
02-15-2011, 12:03 AM   #6
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Yes, I've noticed this with my DA50-200 (WR) - in fact I have mentioned this before during the course of one of the discussions in this forum. The camera will adjust focus when changed from wide-open to f11, and vice-versa. By turning off AF, I have ascertained that the minute change in focus does actually improve image sharpness (marginally), so it seems that there is purpose in what the camera's doing. (My K-m was tripod mounted for this test, with SR off.)

My conclusion is that the reason for this is to compensate for imperfections in the lens's spherical aberration correction: it's an often overlooked fact that the correct focus for a lens may well change when the lens is stopped down.

It seems to me the Pentax engineers are aware of the stopped-down behaviour of their lenses, and program in the necessary steps to compensate.
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