[stv]

Hi,


It have been maybe mentioned before but I have discovered a strange behavior of the AF Fine Adjustment on K-S1 with Sigma 18-200 DG. In fact, I did never succeed with AF adjustment until I tried to apply the correction in the inverse direction (not in the direction that the user manual suggests). Now it works and my Sigma gives sharp photos at 200mm.


It is connected to the fact that the focal ring on my Sigma turns in the inverse direction with respect to the Pentax’s lenses ?


Regards,
Nikolay

[siva.ss.kumar]

Interesting. But then I too have seen I have to click - to + when mounting Sigma (50-500) after a while. I mean, I had -3 some time back. Recently when I started using it again, I now see i have to use the +5. Not sure if I am making some obvious error somewhere.

Edit: Oops. This is with K-5iis. Forgot it is a different forum.

[stevebrot]

It is connected to the fact that the focal ring on my Sigma turns in the inverse direction with respect to the Pentax’s lenses ?

Good question! I am not going to say that it would not work that way, but have to mention that the tuning is for the detector's firmware and basically maps a new zero point. I also believe that the mechanical AF drive in the lens is appropriate for the mount. I.e. the drive pawl on the lens emulates the Pentax direction regardless of the direction of the focus ring. For example, my Sigma 50/2.8 EX DG Macro always spins the correct direction on initial attempt. Perhaps you misunderstand the manual? "+" adjustments should be used if the actual point of focus is behind the intended. "-" should be used if the actual is in front of the intended.


Steve

[Unregistered User 8]

Hi Nikolay, how are you testing your AF? Please tell us your set-up for testing. Your target, distance from target, lighting etc.

[stv]

Hi pjv,
I set: 200mm, AV f7.1, ISO100, 1/400s. The target was at 60-70m : a wall well lit by the sun (11am). I attach the crop with -8 AF Fine correction.
Nikolay

---------- Post added 05-27-17 at 02:26 AM ----------

I forgot to say that this shot was made nearly handheld (camera put on the loggia barrier).

[Unregistered User 8]

Thanks for posting your testing method Nikolay. From the information you supplied, I think there is a problem. With those lens settings and camera to subject distance, the Depth Of Field is almost 32 meters. Here is a screenshot from a DOF calculator to check your information and also a link to the forum Fixing Focus tutorial. You may need to re-do your testing with a different method. Good luck.


Fixing Front and Back Focus - Introduction - In-Depth Articles

[stv]

Thanks for posting your testing method Nikolay. From the information you supplied, I think there is a problem. With those lens settings and camera to subject distance, the Depth Of Field is almost 32 meters. Here is a screenshot from a DOF calculator to check your information and also a link to the forum Fixing Focus tutorial. You may need to re-do your testing with a different method. Good luck.


Fixing Front and Back Focus - Introduction - In-Depth Articles

I beg your pardon. The sample photo was 170mm not 200!


New rapid test is shown below : 200mm, AV f6,7, ISO100, 1/400s. The target was at 50-60m. Left is « -8 » AF Fine adjustment, right is zero adjustment. Each sample is one best from a series of 3 shots.


The result is obvious. I agree with your DOF estimation. My test shows that an incorrect adjustment (“0” instead of “-8”) moves the right focus by more than 13m (further, I think).


I think the best method to adjust AF should be taking 3 photos at each adjustment step (-10...+10 in the case of K-S1) and compare them. In such a way there is no mess with the adjustment direction.


Bu the way, I was surprised by the sharpness of 170mm shot compared to 200mm one at the same distance. To be continued...

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[stevebrot]

I beg your pardon. The sample photo was 170mm not 200!


New rapid test is shown below : 200mm, AV f6,7, ISO100, 1/400s. The target was at 50-60m. Left is « -8 » AF Fine adjustment, right is zero adjustment. Each sample is one best from a series of 3 shots.


The result is obvious. I agree with your DOF estimation. My test shows that an incorrect adjustment (“0” instead of “-8”) moves the right focus by more than 13m (further, I think).


I think the best method to adjust AF should be taking 3 photos at each adjustment step (-10...+10 in the case of K-S1) and compare them. In such a way there is no mess with the adjustment direction.


Bu the way, I was surprised by the sharpness of 170mm shot compared to 200mm one at the same distance. To be continued...

It appears you are front-focused with zero adjust, so a minus adjustment is correct.


Steve

[stevebrot]

It appears you are front-focused with zero adjust, so a minus adjustment is correct.


Steve

PS...I suggest doing your AF fine tuning at a more moderate distance (e.g. 2x-3x the focal length) and a wider aperture to make it easier for both you and the camera to do the job. Your camera focuses wide open and evaluation should be of a wide-open shot.


Steve

[stv]

PS...I suggest doing your AF fine tuning at a more moderate distance (e.g. 2x-3x the focal length) and a wider aperture to make it easier for both you and the camera to do the job. Your camera focuses wide open and evaluation should be of a wide-open shot.


Steve

Dear Steve,
I made many attempts to adjust AF using printed scales and different procedures, including the one published on this site. My Sigma was giving different adjustment values for focal range from 18 to 200 mm. And finally, at a real distance I had always blurred shots.


The procedure to test the whole AF Fine adjustment range from -10 to +10 is the only method that permitted to me to make sharp shots beyond 135mm (including handheld!)


Regards
Nikolay

---------- Post added 05-30-17 at 12:04 PM ----------

Dear Steve,
I made many attempts to adjust AF using printed scales and different procedures, including the one published on this site. My Sigma was giving different adjustment values for focal range from 18 to 200 mm. And finally, at a real distance I had always blurred shots.


The procedure to test the whole AF Fine adjustment range from -10 to +10 is the only method that permitted to me to make sharp shots beyond 135mm (including handheld!)


Regards
Nikolay

Correction:
The procedure to test the whole AF Fine adjustment range from -10 to +10 at the maximum focal length of 200mm is the only method that permitted to me to make sharp shots beyond 135mm (including handheld!). In fact, at smaller focal lengths the DOF is much bigger and Back/front focus is nearly invisible.

Nota: I am interested in the the 170-200mm focal length because I would like to shoot birds and small planes.