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12-01-2013, 11:59 AM - 1 Like   #1
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AF Fine Adjustment for Zoom Lenses

I've been trying out the new FocusTune Ver 3 program that automates the calibration of the best AF Fine Adjustment for a lens/camera combination. Version 3 of the program provides repeatable results which leads me to have more confidence in the setting. I am in the process of doing the calibration for all my autofocus lenses with the K-3. The results for the DA*16-50 are somewhat surprising. The adjust value goes from + to - depending on the focal length.
Lens DA* 15-50
Focal Length
AF Adjustment
50 mm +5 to +6
35 mm -2 to -3
28 mm +3 to+4
21 mm -6 to -5
16 mm -3
The shape of the curve showing back focus to front focus through the range of AF settings looks about the same, However where the curve provides the best focus on the target chages from +AF setting to -AF setting as shown above.

I will be testing two other zooms, Sigma 70 to 200 and Sigma 150 to 500. Fo5 the 150 to 500 I really just use two setting 400 and 500 so it will be good to know the settings.

Since the settings are repeatable, they are probably close to correct.

So if I am doing a composed shot with f2.8 on a tripod that I plan to print I'll probably set a one time AF setting (I need to keep a card with the lens) Otherwise just set at zero and and increase DOF.

I'd appreciate others findings for AF Adjustment settings on Zoom lenses. I will also post results for the Sigma 70-200 and 150-500 if the results are significantly different in thier profiles.

For those not familiar with FocusTune 3.0 (it will be released for sale in about two weeks) check this out


12-01-2013, 12:58 PM   #2
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try it at different target distances and get more variation. one focus adjustment is simply not enough. this is awesome Sigma USB Dock quick review: Digital Photography Review
12-01-2013, 02:37 PM   #3
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QuoteOriginally posted by Patajac Quote
try it at different target distances and get more variation. one focus adjustment is simply not enough. this is awesome Sigma USB Dock quick review: Digital Photography Review
The target is fixed for the computer analysis. I vary the AF tune adjustment from -10 to 10 in 1 step increments. I take 8 or 9 exposures at each AF setting. 21 settings times 9 exposures is about 180 photos that the software automatically calibrates to arrive at the result. Bottom line - close to 200 exposures to generate the optimal AF adjustment setting.

Since the results are repeatable, run another test of 180 shots and get the same result, I feel the results are valid.

I do not know if the behavior of the DA*16-50 is typical for a zoom or if other zooms have a less erratic front/rear focus problem through their zoom range.

Perhaps the erratic front/rear/front/rear behavior is why people feel the results from this lens are less than what they would expect. The focus at open aperture would be all over the map.
12-01-2013, 03:13 PM   #4
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Can this program set the calibrated value directly into the camera, and make the camera remember these value, and automatically change the AF adjustment value according to the lens when they are zoomed to each focal length?
If so this program will be super awesome!

Or is it just the users still need to remember those value and manually change every times they zooming the lens?

12-01-2013, 04:18 PM   #5
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QuoteOriginally posted by zany225 Quote
Can this program set the calibrated value directly into the camera, and make the camera remember these value, and automatically change the AF adjustment value according to the lens when they are zoomed to each focal length?
If so this program will be super awesome!

Or is it just the users still need to remember those value and manually change every times they zooming the lens?
The K-5 remembered the set one value for each lens - so the answer is yes. FocusTune is one way, the most reliable and consistent way, to determine what the setting should be

ZOOM LENSES and FINE FOCUS ADJUSTMENT

I did a test on another zoom in addition to the DA*16-50, the Sigma 70-200 f2.8. Once again changing the focal length caused the optimal AF adjustment level to swing wildly from fron to back fcus by large margins. The bottom line - I got what I paid for and if you want somewhat reliable "right on" focus with a zoom you need to stop down and even then front and back focus varies wildly - DOF is your friend.

This seems to substantiate my limited experience using the K-3 with the DA*300 and the Sigma 150-500. The 300 delivers consistent results and with most shots I can crop and get a better image for printing than the 150-500 shooting at 400 or 500. I am sure out of hundreds of shots with a zoom a couple will come out dead on but after looking at zoom performance with FocusTune 3 I attribute that to luck. Even a blind squirrel will find an acorn.

Perhaps some of the high performance zooms such as the Nikon 200-400 f4 or the Canon EF 200-400 f/4L IS USM deliver consistent focus results but then yu get what you pay for, the Canon is about $12,000 and with the Sigma 150-500 the price is 1/10 of that. Right now Pentax does not support any high performance zooms.
12-02-2013, 01:32 AM   #6
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QuoteOriginally posted by RockvilleBob Quote
The target is fixed
What I ment was that you set your target to another fixed distance and get even more variation in "correct" AF adjust setting. near vs far will get you different results

One af adjust setting is not enough for zoom lenses and this new usb gadget from sigma is great because you can set focus parameters on the lens itself.
12-02-2013, 05:46 AM   #7
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QuoteOriginally posted by Patajac Quote
What I ment was that you set your target to another fixed distance and get even more variation in "correct" AF adjust setting. near vs far will get you different results

One af adjust setting is not enough for zoom lenses and this new usb gadget from sigma is great because you can set focus parameters on the lens itself.
Thanks, I miss-read your comment.
At this point I think I am not going to look at additional zooms, just primes, for two reasons: I have the waterfront covered (16-50; 70-200; 150-500) and I feel that with primes I get superior results even if I need to do major cropping. From what I can tell the K-3 brings out the best in the DA*300 while the performance of my 150-500 Sigma zoom stays about the same. The result is I get better images heavily cropping the 300 than I do with the extra reach of the Sigma 150-500 and less cropping. Setting the af fine adjustment with a zoom seems result in the lens (the ones I have) having strange unpredictable focus behavior. At an AF setting of 0 the lens consistently front focuses. Set the AF to a point that can give better results according to tests and the lens jumps around wildly from front to back focus at the same AF setting. I sense the best thing to do with my two Sigma zooms is just leave the af adjustment at 0, get consistent focus (albeit front focused) and use DOF/aperture to be the blind squirrel getting the acorn.

I plan to do one addition test: using the Sigma 150-500 at 400 with the af adjustment for 400 versus the 300 with its af adjustment. Just looking at the shots I've taken so far and the AF testing at 400 on the zoom, the DA* will come out ahead but it would be nice to have the extra reach for bird shots.

Sigma appears to have taken big strides to produce good lenses with innovative technology including their USB dock. I am looking forward to their new telephoto lenses planned for 2014 introduction and hope the 300 and 400 2.8 lenses are within reach of my budget next year Sigma working on super telephoto primes It would be great to have one of those with the Pentax 1.4TC. Let's hope Sigma makes those lenses in a Pentax mount.

I will also keep my fingers crossed that the new Pentax zoom approaching 400mm is a high end performer. I will not buy the Pentax long zoom unless I can run FocusTune 3 on the lens to insure it delivers consistent results through the zoom range or at a minimum shows stable behavior at a fixed focal length setting. As I have mentioned both the DA* 16-50 and the two Sigma Zooms I have, all have erratic focus behavior when af adjustments are applied. As you zoom, the optimal af adjustment jumps all over the range, and at any fixed zoom setting the variation from front to back focus with a fixed AF setting is too big to make using the af adjustment viable.

Many professional sports photographers rave about the performance of the $12,000 Canon 200-400 lens. I am guessing it has consistent focus ability and consistency through its range. The zooms I have are less than 10% in cost and for that price difference I'll just have to be content being the blind squirrel who occasionally finds an acorn.
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