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Forum: Pentax DSLR and Camera Articles 05-24-2009, 07:44 PM  
Autofocus Adjustment Hints
Posted By Class A
Replies: 23
Views: 41,172
The K20D, K-7, and K-5 support fine adjustment of the autofocus (AF) system per lens. Some other cameras (K10D, K100D) have a (hidden) service menu that allows access to a single AF adjustment.

Here are some hints to consider when attempting to optimise the AF performance with such fine adjustments:
  • Put the camera on a tripod to avoid hand-shake.

  • Don't shoot a ruler or other objects where the AF sensor has multiple targets to lock on to. AF sensors cover a bit of area and may be slightly off-centre. Get a focus chart or another target which helps you to know where the camera focused on.

  • Make sure that the AF sensor you are using (typically the centre one) only "sees" the target. Move it around the target initially, making sure that the camera can not obtain focus. If still obtains focus then you either have not sufficiently pointed away from the target yet, or the AF sensor "sees" parts of the measurement print that it should not see.

  • Don't shoot test targets under Tungsten light if you want to fine-tune AF for daylight shooting. Tungsten light will cause the AF system to slightly front focus. This holds for every Pentax DSLR using a SAFOX AF module without a "+" in its designation, i.e., in particular for every DSLR before the K-7.

  • Try to shoot a flat rather than a tilted target. The Pentax service manual for the K10D shows a setup with a flat target and a tilted ruler. Here's a corresponding image including instructions (courtesy by rawr).

  • Do a series of tests with the lens starting from infinity and its minimum focus distance respectively a number of times. Results will not be a 100% consistent with each other across the series but you'll see a trend. Doing a single shot only may let you end up on a non-optimal setting. This is also true for calibrating a focusing screen.

  • Doing a series of shots with focus bracketing may yield a more precise result than the method that tries to place the centre of focus on a "zero line" of some ruler.

  • Try to read the focus point near to the point where you focused. If the ruler is way off the focus point used, you may optimise for a particular field curvature of your lens.

  • The distribution of the DOF before and beyond the focus point approaches 1:1 the closer you get to the subject. The often quoted 1:3 ratio only applies for a subject at a third of the hyperfocal distance.

  • Make the test using a focal distance you expect to use in real world shooting. For different focal distances, the AF adjustment may be slightly different. A rule of thumb for a general adjustment is that the distance should be roughly 25-50 times the focal length of the lens. Canon recommends the factor 50 but most lenses have quite a deep DOF at this distance; not much use for fine-tuning. If you get it right at 25 times the focal length (Lens Align recommendation) then the 50 times focal length distance should be fine as well.

  • The widest possible aperture (lowest f-ratio) will make it the easiest to check where the focus is, but in particular very fast lenses (lowest f-ratio <1.8) typically introduce a slight focus shift with aperture change. So again, you may want to calibrate, say a useful f/2.8 rather than an extreme f/1.4. This way you can achieve a compromise that works for a wide range of aperture settings rather than just a narrow range.

  • Evaluate real world results and let these govern your adjustments, rather than relying on test setups.

Note that it may not be possible to find one adjustment value that will work equally well for all focal lengths of a zoom. You have a number of choices for calibrating a zoom:
  • use the longest focal length (recommendation by Canon)

  • use the focal length you use most

  • use the focal length where sharpness accuracy will be most critical

  • choose a compromise setting that gives you the best comprise for all focal lengths

The focus chart by Tim Jackson assumes that tilted targets are not problematic. I don't like that its focus target is ambiguous, i.e. a rather wide line (bar) in the middle.

The focus chart by Yvon Bourque also uses a tilted target. It nicely avoids unwanted focus targets and has a precise point (line) of focus, but I'm not sure how well it works if the lens requires a different focus for its centre vs its perimeter.

Jeffrey Friedl's chart allows read out of focus near the target but one has to make sure to use a low enough grey level version so that the AF does not pick up the read out pattern. Also, the AF target is not as unambiguous as it could be.

The chart featured in Bob Atkin's Focus Testing article seems to be a good compromise.

If your camera supports Live View you can also try this chart which is based on provoking moiré patterns. It avoids any potential issues that tilted targets may have. However, make sure that the AF module isn't fooled by the colour temperature of the screen you are using. Validate your results using natural light. You may also use Live View in order to obtain accurate Live View focus first and then check whether refocussing with phase AF causes any lens movements. Of course the latter approach will work with any target, but take caution to nevertheless obey all of the aforementioned caveats.

A commercial solution is the LensAlign tool. It uses a flat target. Not sure how far away the ruler is from the centre area which is used to obtain focus. The LensAlign website features a "LensAlign User Guide" which you may find helpful and a "LensAlign Distance Tool" which makes a suggestion at what distance you should measure a particular lens (25 x its focal length) and what focus distribution (percentages of the areas that should be in front and behind the target point for a given distance) you should expect. The "Mk II" version of the LensAlign tool seems to be made of relatively flexible material (to support disassembly into a small flat parcel for transport). A user reports "They look ok, but there could well be some mis-alignment in all of these parts. I would be shocked if there wasn't.". If they user is right then such mis-alignment could have an impact on the precision obtainable with this particular (cheaper) version of the LensAlign tool.

There is a less expensive version called Spyder LensCal. It lacks the alignment aids of the LensAlign tool, though.

You may build your own LensAlign tool, by following the instructions for how to build a GhettoCAL.

P.S.: If you would like give me reputation points for compiling this article, please use the corresponding button at the top right of this post or a corresponding button in any of my posts below.
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