[Wingincamera]

I wasn't sure if I sould post this in the Lens section or KP section, so I decided to post here. I have a Tamron 70-200 F2,8 lens that needed a +10 adjustment for the fine focus adjustment when I used my K-S2 body. About ten months ago I acquired a KP and had not rechecked the fine focus on any of my lens. I have an event coming up where I was going to use the 70-200mm for the first time since getting the KP. Upon checking it I discovered the focus seemed a little off, so I decided to recheck the fine focus adjustment. While with the K-S2 it required a +10, with the KP I had to change it to a -1. This was at 180mm. I checked it at 80mm and for me, it was acceptably sharp but at the focal length I would probably be using my DA70. I will be re-checking all my lens with the KP, which I should have done when I first got the KP. This is a reminder to recheck the fine focus when acquiring a new body.

Just for info I recently acquired the *55mm F1.4 lens and it acquired a -1 adjustment on the KP.

[stevebrot]

You have discovered what is expected (sort of). Not only does AF fine adjustment vary according to lens, but appropriate correction varies across cameras. Copying the numbers from camera A to camera B probably won't work as well as starting from scratch on B. BTW...I am assuming you are aware that AF fine adjustment does not change anything on the lens, right?


Steve

[kh1234567890]

Not only does AF fine adjustment vary according to lens, but appropriate correction varies across cameras.

Isn't that the point of having separate two additive parameters - global for camera lens mount tolerances and individual for each lens tolerances ?

[stevebrot]

Isn't that the point of having separate two additive parameters - global for camera lens mount tolerances and individual for each lens tolerances ?

Hmmmm...I am not sure the two are additive (that is disputed), but what you seem to be suggesting might work. I have my doubts in that part of the discussion is the actual performance characteristics of the PDAF detector when matched to a particular lens. It is not a calibration as in dialing in bias. It is a correction adjustment.


Steve

[kh1234567890]

I have my doubts in that part of the discussion is the actual performance characteristics of the PDAF detector when matched to a particular lens.

That should be taken care of when a particular body is manufactured and presumably whatever correction is needed will be stored in the body. Hence stories of 'recalibration' requiring both the lens and body to be sent back to Pentax.

My feeling had always been that people get themselves in knots trying to use the 'fine tune' for something that it was never meant for. In my case the difference between all my lenses, on both my old K-7 and my current K-5ii, is pretty constant at +2. As I've cured my LBA, I've not had a need to fiddle with fine tune for quite a while now.

[stevebrot]

That should be taken care of when a particular body is manufactured and presumably whatever correction is needed will be stored in the body. Hence stories of 'recalibration' requiring both the lens and body to be sent back to Pentax.

There is a lot of confusion about how PDAF works and why adjustment might be needed. I will just leave it at that.

My feeling had always been that people get themselves in knots trying to use the 'fine tune' for something that it was never meant for.

Agree totally. Most AF Fine Adjustment done by users on this site is done to correct problems that don't exist and many have been convinced of the need by well-meaning forum members that suggest AF Fine Adjustment as a first-line response to any and all issues with AF or soft images.


Steve

[Larrymc]

Isn't that the point of having separate two additive parameters - global for camera lens mount tolerances and individual for each lens tolerances ?

Global and individual lens fine adjustments are not additive its one or the other but not both. Say for instance that you have a global set to +5 and you set an individual to -5 it will not result in a net of zero fine adjustment. Some lens require adjustments and some don't I have a Tamron 200mm f/2.8 that requires a +10 and a new HD smc 55-300 PLM that is fine at zero adjustment. I think the screw drive lens seldom are spot on due to Mfg tolerances of the mechanical focus drive where as the electronic focus mechanism of the PLM lens is very accurate. I've tested mine with Focus Tune/Lens Align and that system works for fine adjustments but it need tons of light to to accurately tune the lens.

Larry

[stevebrot]

Global and individual lens fine adjustments are not additive its one or the other but not both.

That is my understanding as well, but I have neither the time nor the inclination to give it a thorough look-see. I have never had the need to do a global adjust and only have find adjustment for one of my AF lenses.


Steve

[stevebrot]

I think the screw drive lens seldom are spot on due to Mfg tolerances of the mechanical focus drive where as the electronic focus mechanism of the PLM lens is very accurate.

At issue is not the AF drive mechanism. It just follows very simple orders...direction and approximately how far. Fine adjustment is a reprocessing of the detector signal to mitigate an ambiguous image to the detector. At root, the cause is optical and my understanding is that poor alignment or on-axis element positioning is the common culprit. The analogy I have found useful is a traditional split image rangefinder where the two sides of the split are essentially dissimilar such that the best match is way off from what should be.


Steve

[Wingincamera]

...I am assuming you are aware that AF fine adjustment does not change anything on the lens, right? Steve

Yes, I am aware of that. The other lens that I had used all appeared OK, so I had not seen a need to check them.

[zapp]

AF fine tuning inlucdes so many parameters that most camera manufacturer gave up 100% factory quality control. Use a Sigma dock and you will realize that you can set AF fine tune for different focus distances, focal length, ... aperture has an effect as well. The camera is set to a certain adjustment, the focus screen can be out of alignment. The user is dommed to check his/her settings for each camera/lens/... setup.
Nobody has asked the user whether he wanted to do this job for the manufacturer, but here we are.

[pschlute]

AF fine tuning inlucdes so many parameters that most camera manufacturer gave up 100% factory quality control. Use a Sigma dock and you will realize that you can set AF fine tune for different focus distances, focal length, ... aperture has an effect as well. The camera is set to a certain adjustment, the focus screen can be out of alignment. The user is dommed to check his/her settings for each camera/lens/... setup.
Nobody has asked the user whether he wanted to do this job for the manufacturer, but here we are.

I suggest that the main reason why AF Fine adjustment has become a feature of high level DSLR is down to the fact that higher MP , higher resolution sensors can more easily show up faults with existing manufacturing tolerances, on both camera sensor positioning and on lenses.

I for one am very happy that AF Fine adjustment is available. When getting a new camera it takes a couple of hours to pair lenses to the new camera.

Do not forget most of us when getting a new camera will use existing AF (and manual) lenses on these new bodies. If AF-FA were not available, what would we do then ?

[stevebrot]

Use a Sigma dock and you will realize that you can set AF fine tune for different focus distances, focal length, ... aperture has an effect as well.

Yes, familiarity with the Sigma dock sheds a ton of light on the fine adjustment situation. If there is a need, it is due to issues of manufacture of the lens and not a simple +/- adjustment.


Steve