Sunday, September 29, 2013

How to Use DotTune AF Fine Tune Calibration For Nikon Cameras


Autofocus fine-tuning (calibration) basically falls into two categories: distance-scale methods and everything else. Distance-scale methods involve aiming the camera at a target and measuring the distance from true that the the actual focus deviates by. You can shoot a set with different adjustment values and pick out the best one, or you can skip all of that and use math. Under the "everything else" category, you have methods like the moiré fringe and DotTune, which don't rely on distance scales.

DotTune was first proposed by veteran DPR forum member Horshack (Snapsy on Fred Miranda) in this post in early 2013. It works by taking advantage of the fact that the familiar AF adjustment value of +/-20 is applied on the input side of the AF operation. Plainly stated, this means that the adjustment value is added to the AF system's sensory input before the processor calculates distance rather than having it added afterwards to adjust the distance value. In other words, the adjustment factor essentially "tricks" the AF system into thinking that the distance to subject is different from what it actually is.


Method


The quick and dirty method:

  1. Under live view, establish critical focus on a suitable AF-target. The target should be high-contrast and perpendicular to the camera. (I.e., this isn't like a slanted-target test)
  2. Once focus is established, switch the focus system to manual focus, being careful to not change the focus position of the lens.
  3. Check for focus in the viewfinder. This is where the "Dot" part of "DotTune" get's it's name. The focus indicator has a green dot to indicate whether or not a subject is in focus, and two arrows on either side to indicate whether or not the camera thinks a subjects is in front or behind the point of current focus. If the arrows is pointing to the left, then the camera+lens is back-focusing, so you need to decrease the value of AF tune (negative adjustment). If the arrow is pointing to the right, then the camera+lens is front-focusing, so you need to increase the value of AF tune (positive adjustment). Note: you will need to half-press the shutter button to keep the AF-meter on while you are doing this.
  4. Repeat step 3 until you get a green-dot indication from the rangefinder.

However, some time after this method started making the rounds, it was discovered that the focus indicator in Nikon cameras will light up over a wider range of focal distances compared to when the AF is engaged. It's speculated that Nikon built this quirk into the system in order to keep manual-focusing user friendly. As it currently stands, manual focusing with a lens that has a quick focus-throw ("fast gearing") can be finicky as the dot indicator will only light up within a minuscule amount of focus adjustment. If the manual operation was as sensitive as the AF operation, then relying on the focus indicator as an aide would possibly be more fustrating than it is already.  This brings us to the first modification of item 3. :

  •  Set the AE-L/AF-L as the AF-On button (Menu item f5 on a D7000/D7100). When checking for focus, use the back button to activate the PDAF system. When you aren't pressing the the back-button, the camera isn't actively trying to focus, but it is still running autofocus mode, and consequently, you will get the narrower range of operation for the AF indicator dot, making the fine-tuning process more precise.

The second modification to the basic method is a refinement for pinpointing the adjustment value. In practice, you should always be doing this final step in the following manner instead of just settling for the first instance of focus confirmation that you see:

  • Instead of merely adjusting the fine-tune value until the dot lights green, take note of the range of adjustment values which trigger activation of the focus dot. Find the midpoint of this range, which is where the true adjustment factor lies. For example, if you are getting a green-lit indicator between the adjustment values of -3 to +1, then your AF adjustment factor is -1.

The author of this technique has posted a video on YouTube, which I highly encourage you to watch in its entirety to get an understanding of how this technique works:


Compared to Other Methods


There is an obvious advantage to this method over distance-scale and moiré fringe: it requires the least amount of setup. Distance-scale requires the most amount of work in proper setup, as both the camera and the target have to positioned accurately. The camera needs to be level and if slant-targets have to be positioned precisely at 45 degrees (made easier by products such as LensAlign).

Moiré fringe is much less finicky in terms of setup; you still have to align camera and target, it's just that the method is more tolerant of either being not level. However, the moiré method requires that you display the target on a display screen (computer, laptop, tablet, TV, etc.) as it does not work with a printed target. The fringe method's single greatest advantage is that the user still has final judgment over focus confirmation. In my opinion, the moiré fringe method gives the most precise control over AF adjustment, and has the most user-friendly and precise way of indicating whether your lens/camera system is in tune or not. In comparison, with DotTune you are relying on the camera's AF detectors while working around the mechanical tolerances of focusing system.

The DotTune method really only needs a tripod and a wall-mounted AF target, and so long as the wall is perfectly vertical, the AF target can be something as ordinary as a newspaper, so long as it is high-contrast (preferably black and white). This makes the method suitable for spot adjustments in the field, something unthinkable with any other method. As with the fringe-method, DotTune eliminates the single biggest hassle of traditional distance-scale methods: having to transfer the image to a computer to judge focus distance. The fact that there are now a multiple number of ways to calibrate autofocus speaks volumes about how the user-friendliness of the traditonal distance-scale methods, and because of the simplicity of its setup, I wouldn't be surprised if DotTune becomes the standard way by which most enthusiasts set up their cameras.


7 comments:

  1. very interesting ! 3 questions : 1) is there any "distance" recommendation for the test (25x to 50x focal length for Lens Align or Focal software) ? 2) is the type of light important ? I have read that we must used the sun or spectrum-compatible source of light, otherwise, the focus could be made at the "wrong place" (as with incandescent light). 3) what about lens with strong focus shift liek the Nikkor 24mm 1.4G ( a 2K lens....) Many thanks !!! claudedumas.com

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    1. 1.) As far as I know, Nikon calibrates their lenses at 50x focal length. I have yet to discover an adequate answer on what the DX to FX conversion should be, but I do know that below 20x focal length, the quality of the results goes sharply down.
      2.) I think the more important issue is make sure that there aren't any stray IR light sources that are reflecting into the optical path, and being around incandescent lights can certainly affect that... theoretically. My opinion will probably change in the future, but so far, I am of the mind that the type of light illuminating your test target is not critical, but stray sources of IR or UV light might be.
      3) I'd just run it at 50x focal length, or 1.2m. I know that particular lens can focus a lot closer, but for me, personally, since wide-and-up-close is such an infrequent thing, I would leave the calibration at 50x, and just use live-view for something closer.

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    2. Just a followup to 2.) After having said all of that,the simple answer is whatever you do, don't calibrate with/under tungsten light.

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  2. Can you use dot tune with the Sigma dock? If so, how would you perform a calibration? Thanks....

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    1. You would use the same method. The advantage of the Sigma dock is that you can save multiple values for AF fine-tune, say at different focal lengths for a zoom, or at different focusing distances. What you would need to do is to Dot-Tune your lens at each different parameter, make a dote of which AF adjustments are needed at which lens setting, and then input those values into the Sigma dock.

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  3. Douglas - thanks for your help. I'll give it a try.

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  4. No matter what settings i use I can not get a green dot indicator. On my D810 the dot remains white. Any Suggestions?

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