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05-15-2017, 01:06 PM   #1
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Fine Focus Adjustment off center points

Hi All, I just completed fine focus adjustment on my KS2 using the center point and my awesome 35mm f2.4. Everything is razor sharp now. However all other points are now out of focus. I was planning to use the Select Point method for street shooting but that is impossible now. Any suggestions? Is this a repair/warranty situation or a design a flaw? In fact I can manual focus better than use an off center point.

05-15-2017, 01:23 PM   #2
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Hi Shawn

Only the central phase-detect AF point is accurate to f/2.8 sensitivity - the others are, I believe, at f/5.6 and therefore could show focusing errors at faster lens apertures. It's also best to set AF fine adjustment at typical working distances. Is this what you've done?

It's also possible that the DA35 f/2.4 suffers from a degree of focus shifting and field curvature (though I haven't personally noticed this or looked for it with mine). A such, off-centre areas may focus correctly at f/2.4 (the lens is always wide open when focusing through the viewfinder), but when the lens stops down to the aperture you've set (I'm guessing you're using f/8 for street shooting?), the plane of focus and any field curvature may result in out-of-focus results. Try focusing with Live View... If that works, my guess is that it's a combination of focus shifting and possibly field curvature.
05-15-2017, 01:26 PM   #3
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The first thing that comes to my mind is a possible DOF situation. To focus at a close distance and wide open with your 35mm lens creates a shallow DOF. To assess relative sharpness across the frame, you'd need a perfectly flat surface and the camera/lens on tripod, trained at exactly right angle to the surface, and at various apertures. Even then, every lens is going to perform better for sharpness at its central area compared to edge area to some degree.

Beyond that, there are issues the lens may have, such as possible field curvature, or possible off-centering.

You could set up your tripod for some brick wall tests at greater distance using say f/8 or f/11 aperture, then vary your focus to see if it is possible to get both center and edge focused. If not, try a different lens in the same way.
05-15-2017, 03:10 PM   #4
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QuoteOriginally posted by BigMackCam Quote
Only the central phase-detect AF point is accurate to f/2.8 sensitivity - the others are, I believe, at f/5.6 and therefore could show focusing errors at faster lens apertures. It's also best to set AF fine adjustment at typical working distances. Is this what you've done?
The cross sensors are pretty reliable even with fast lenses, although I've noticed anything outside the center three columns to be less reliable. But the non-cross points are...shaky. I usually can't get those to work well, especially if I have an ND filter on there.

05-15-2017, 03:11 PM   #5
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Thank You. Very useful information. I shoot mostly at night wide open so the DOF and focus point accuracy mentioned above becomes critical. I guess Focus & Recompose it is. Actually this method even for off center subjects yields very shaper images!
PS. I will test and see if there is an overlay of adjustment values that allow all focus point adequate accuracy and post results soon.
05-15-2017, 03:59 PM - 1 Like   #6
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QuoteOriginally posted by BigMackCam Quote
Only the central phase-detect AF point is accurate to f/2.8 sensitivity - the others are, I believe, at f/5.6
This ^ ^ ^

...though the issue is not accuracy, per se, but the ability to detect an out-of-focus state. The f/5.6 points will detect OOF with the same precision at say f/2* as at f/5.6. Similarly, an f/2.8 point will detect OOF with the same precision at f/1.4 as at f/2.8. That being said, on multiple attempts using identical technique both types will average to the same point of focus.

The implications when doing AF fine adjust are worth considering:
  • Don't compare the f/5.6 points to the f/2.8 points unless you are willing to do a statistical sampling
  • When doing an adjustment, don't be satisfied with a single confirmation attempt
  • When evaluating AF, do so de-focused from near and far lock on the focus ring. The system's notion of acceptable focus is a range and may vary depending on the start point.** Again, multiple attempts will yield better results.

Steve

* similar rate of missed focus at any aperture f/5.6 and faster

** This is easily demonstrated with a traditional fast manual focus prime using the focus confirmation feature. The green hexagon stays lit even as you move the focus ring past the initial point of focus to a point where it goes out. The true in-focus result for the subject is somewhere in between.

Last edited by stevebrot; 05-15-2017 at 04:19 PM.
05-15-2017, 09:49 PM   #7
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Current center focus point setting +9. I caliberated for right most point and the value turned out to be +1. I set the adjustment to mid point+5.Then I tested the left most point. It was way way off. So back to the previous center point setting of +9. I also tested with 50mm f1.8. Much less deviation there. I guess my 35 mm may be decentered a bit.
11-04-2018, 04:00 PM   #8
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Just found this thread. My DA 35 F2.4 also focuses terribly with off center focus points. I guess itís liveview or focus-recompose from now on for this lens.

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