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05-24-2016, 07:41 AM - 2 Likes   #1
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Autofocus precision with a variety of primes (AF-S)

While tuning AF fine adjustment, I took some notes on the AF precision which may be interesting to share. I looked atfor now.

My test setup was a cube-shaped box of about 15cm width, set on a place mat with a ripple structure perpendicular to the line of sight, with the camera center focus point/region centered on the box from 1.5m (for the F 28mm), 2m (for the 35mm and 50's) and 3m (for the 100mm lens), from a perpendicular axis. There was a very slight angle downwards, but shallow enough to not have an influence on the distance (~5cm height difference, which would mean 0.6mm in length). I only used the center focus point in medium bright daylight from the window.

Observations (PD=phase detection/viewfinder, CD=contast detection/life view):

Both Macros D-FA 50mm/2.8, D-FA 100mm/2.8:

- consistent focus in both PD and CD in both directions (coming from long and short distances)
- spot on, +/- 0 fine adjustment
- focus speed roughly equal between CD and PD with an edge to PD as it doesn't have to go past the focus point and wind back
- 100mm may be a tiny bit more precise/reproducable from 3m than 50mm from 2m

F 28mm/2.8:

- consistent CD focus in both directions
- some misses in PD (~5%), with the difficulty that it stops just short of the actual focus point, so it is hard to notice in the viewfinder, spot on in all other cases

DA 35mm/2.4:

- extremely FAST
- CD slightly overshoots, in a consitent manner: coming from long distances focus ends up slighly in front by the same amount, coming from short distances focus ends up slightly behind, both <1cm at 2m shooting distance
- PD had a few close misses (similar to the F28mm), otherwise consistent, needs +1 or +2 on my camera
- my most reliably focusing lens on K-5 disappoints in that respect on the K-1, gearing is likely a little too fast

FA 50mm/1.4

- more reproducable focus in CD than PD, but both vary quite a bit
- hard to judge because of low contrast/sharpness wide open
- focus shift make results of 1.4 non-optimal for smaller apertures
- settled on adjusting for F/2.0, needed -7

F 50mm/1.7

- noticeably overshoots in CD by <1/3 of the width of '2' on the distance scale, consitent as described above
- CD is slower on this lens than PD
- PD had a few misses, but was spot on otherwise, +/-0

SMC DA 70mm/2.4

- extremely FAST turning in CD
- overshoots in CD always by exactly the same amount, less than F 50mm in terms of rotation, but maybe similar in terms of depth
- PD had no misses, confidently locks on from both directions, needs -1 (maybe none)
- This lens focuses precisely in PD but is always slightly off in CD! So far, I thought of CD as the most precise way to focus. I'll try in low light again, when the K-1 seems to switch to slower speed.

I felt I had enough of 50mm primes for now and skipped the DA 50mm/1.8. Quickly testing the D-FA 28-105 I found no need for adjustment of PD, but at both 28mm and 50mm it was hard to tell anyway with my simple setup. For those lenses which did consistently focus using CD, I had the impression that the K-1 may favor a very slight front focus. I could never get the FA50mm/1.4 to reliably focus on my K-5, on the K-1 however it works much better. The contrary is true for the DA 35mm. On the K-5, the DA 70mm/2.4 has a tendency to readjust after initially locking on in AF-S, on the K-1 it locks on very confidently.

Last edited by JensE; 05-24-2016 at 08:01 AM.
05-24-2016, 08:01 AM   #2
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Nicely written up. Thank you for doing all this, which is excellent news as far as I'm concerned when I eventually step up to full frame. I have the D-FA 100WR, the FA50/1.4 and the DA70, and knowing I can expect good things (or in the FA50's case at least better things than on the K-5) is very cheering.
05-24-2016, 11:08 AM   #3
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I'm thinking about a K1 to pair often with my FA50. I'm concerned by your findings that it doesn't focus consistently. I wonder how much this might be lens sample specific?

My FA50 1.4 is extremely consistent on my k50. It is my most consistent lens and only ever misses below 1.8 which I always figure to be a limitation of the camera with its 5.6 sensors. It also misses often if I am intentionally inducing flair for effect but I think that is unavoidable. Otherwise even in low light it is excellent.

How do others find the FA50 1.4 on the K1?
05-24-2016, 12:17 PM   #4
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QuoteOriginally posted by mgoblue Quote
I'm concerned by your findings that it doesn't focus consistently.
Looks like the somewhat glossy print on my target box wasn't ideal. Just re-tested consistency in my office at ~4000K 250lx flourescent lights on a vertically structured bottle rack (metal frame with sides made of thin bamboo). The FA50 locked on consistently under those conditions, with PD-AF sitting fairly precisely at the dot of the 1.5m mark and the maybe 20 refocus attempts, both from below or above, worst sitting in the middle of the gap between the dots and the numbers. That is consistent with CD-AF coming from the long end. Coming from the short end, CD-AF ends up on the '5'. So for somewhat contrasty subjects with smaller than F/2.0, I will not worry at all for now even if I'm thinking now i need to push the focus point even further back to compensate for focus shift - but that may depend heavily on the light color. For F2.0 and wider shots, it's likely best to use magnified life view, potentially at the final aperture if different from F/1.4, to get better contrast and deal with focus shift.


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