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