[rogerstg]

When I first watched that part I was not convinced by it, it sounded like a bit of witchcraft rather than a rational approach.

I felt the same, but after using it and thinking it through, I realize that it IS the rational approach. The traditional method relies on hope. Hope that the sample number of test photos spans the range that the camera thinks it's achieved focus. With Dot Tune, one actually determines the range of what the camera finds acceptable focus and calibrates to the midpoint of that range, thus increasing the chance the camera will achieve optimum focus on a subject.

---------- Post added 12-14-21 at 09:24 AM ----------

I'll have to go back and have another look at the video on this last step needed to overcome not finding a limit

I don't recall that being in the video. I figured it out based on what we are trying to achieve.

[Larrymc]

I tried it. Didn't work for me.

I've tried the Dot tune method several times and foolishly go back thinking I did something wrong but it has never worked for me. I bought the Focus tune software and targets, it works but good grief it is time consuming and so tedious. Now I just throw a Focus tune target on a tripod and align the camera and check the focus and I'm done.

[c.a.m]

I recently calibrated my AF lenses on my K-3 Mark III (FW 1.31) using a detailed quantitative method, arriving at baseline settings as described in this article: https://www.pentaxforums.com/forums/53-pentax-dslr-camera-articles/435297-k-...djustment.html

I put five of the eleven lenses through the Dot-Tune calibration procedure, using the same target set-up as the baseline methodology. All of the resulting AFFA values were within one or two steps of their baseline values, and generally well within the acceptable focus-quality ranges.

Target distance 200 cm

Optimal AFFA Settings: Baseline, Dot-Tune

• FA 43mm Limited: 0, -2
• DA 20-40mm Limited at 40mm: -3, -2
• DA* 50-135mm at 50mm: +2, +1
• DA 18-135mm at 50 and 90mm: +2, +2
• Pentax-F 50mm f/1.7: -1, -1

Although a couple of the lenses produced a surprisingly wide AFFA range under Dot-Tune (e.g., DA 20-40mm -8 to +5), the median values were pretty much where they need to be. Interestingly, for four of the five lenses, the Green Hex was illuminated over a much greater range than what I would deem as giving 'acceptable' focus (i.e., 'sharp enough') when viewing an image at 100% on my 24-inch HD monitor. I presume that the camera's focus detection algorithm assumes a deeper field of view.

Would I use Dot-Tune instead of my trusted -- and tedious -- numerical approach? Perhaps. My tests showed that Dot-Tune has the potential to offer a relatively easy method. However, I note the following issues:

• Achieving the sharpest critical reference focus in Live View is a challenge, especially for lenses that are not amenable to focusing manually. For example, my DA 18-135mm exhibits an annoying play in the focus action and a shift of the target in the LV magnified display when changing focus directions; it's difficult to 'rack' the focus to the sharpest position.

• The Dot-Tune method depends critically on the reference focus. I found that at least three runs were necessary for each lens to give confidence in the method, each one requiring the manually-focused reference position to be set.

• Live View autofocus is not necessarily an accurate and precise replacement for focusing manually.

• Deciding on a reliable, positive focus confirmation (i.e., Green Hexagon ON) is somewhat subjective. While most of the AFFA settings within a range produced almost-instantaneous and robust confirmations, sometimes the Green Hex was just momentarily hesitant to illuminate.

• The DT method doesn't give an indication of any potential focus-direction bias. One of my lenses needs to be focused from the infinity side of a subject distance, so Dot-Tune would not be suitable.

• Dot-Tune doesn't account for any lens-specific mechanical actuations during a focus action, which might be important for certain lenses. Regardless, it worked well for the five lenses I tested.

• The technique does not provide an indication of the sensitivity of the relative focus quality in a series of AFFA settings, nor does it give a clear idea of the AF consistency from shot to shot. While this type of information is not essential to normal picture-taking, it can be useful in diagnosing problems with lenses or assessing the AF characteristics of a camera.

• Dot-Tune seems to fail when the AFFA range extends to its limit, i.e., -10 or +10. I'm not convinced by the tentative work-around suggested by the Dot-Tune creator. My Sigma 70-200mm needs an AFFA of -7 but its AFFA confirmation range would extend beyond -10. I'll need to look into this complication further.

Despite these concerns, I am encouraged by the results of my limited testing. Dot-Tune could save considerable time and tedium. Because I have characterized the AFFA behaviour of my current lenses, I could possibly use Dot-Tune for those lenses whose calibrations are 'well behaved', saving the others for my detailed approach. I tested only a small subset of focal lengths at one target distance, so further comparisons are needed, for example, wide-angles and longer distances with telephotos.

I might also investigate the application of Dot-Tune to some of my manual focus lenses, which can also take advantage of the camera's focus confirmation.

- Craig