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Autofocus Adjustment Hints
Posted By: Class A, 05-24-2009, 07:44 PM

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.

Last edited by Class A; 09-11-2011 at 06:54 AM.
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04-17-2013, 06:50 PM   #16
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I've long suspected that attempting AF adjustment myself would be a total disaster. Now I have no doubt. I admire those of you who have the patience and discipline to pull it off.


Last edited by dadipentak; 04-22-2013 at 03:52 AM.
04-21-2013, 10:16 PM   #17
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So, after numerous tests I came up with an annoying experince. The focustest charts indicates. That my Tamron needs a -10 adjustment, however in real life situation a -7 gives the best result. My Sigma 70-300 needs -5 by test chart, in real life it's rather -10. So? Is the camera faulty? Are the lenses faulty?
05-28-2013, 08:10 PM   #18
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My K100D and K20D produce much sharper images after adjusting the focus. I had to calibrate my K100D even for manual lenses, since I use camera's red blinking square to confirm focus in manual mode. There is only one global setting in K100D's debug menu.

Since it is the camera which determines if the image is in focus how can it vary between lenses?
What happens when you set your AF adjustment to say +10, does it really move the sensor accordingly?
05-29-2013, 06:38 AM   #19
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QuoteOriginally posted by TropicalMonkey Quote
Since it is the camera which determines if the image is in focus how can it vary between lenses?
I don't know a 100%, but one reasonable explanation is focus shift, i.e., the phenomenon that the plane of sharpest focus shifts with changes of the aperture.

The AF sensor sees the world at f/5.6 (only the K-5 II(s) has a faster f/2.8 AF area) but the image is taken at f/1.4, for instance. Due to spherical aberration -- the amount depends on both the lens model and copy -- the image captured at f/1.4 needs a different focus then one captured at f/5.6. Modern cameras compensate for that. Modern lenses contain compensation tables that tell the camera by which amount to adjust the focus depending on the set f-ratio.

QuoteOriginally posted by TropicalMonkey Quote
What happens when you set your AF adjustment to say +10, does it really move the sensor accordingly?
No, there are no physical changes.

The adjustment amount only introduces a bias with respect to the phase matching. An adjustment of "zero" means that both images seen by the AF sensor (through different areas of the lens) must match with zero phase difference for the image to be in focus. A non-zero amount means that the phase difference must have a certain amount (specified by the adjustment value).

05-29-2013, 06:55 AM   #20
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QuoteOriginally posted by szmatefy Quote
The focustest charts indicates. That my Tamron needs a -10 adjustment, however in real life situation a -7 gives the best result. My Sigma 70-300 needs -5 by test chart, in real life it's rather -10. So? Is the camera faulty? Are the lenses faulty?
These observations do not imply a fault of any kind.

It is unfortunately normal for the optimal adjustment to depend on zoom settings, distance to the subject, etc.
One value will never be optimal for all situations.
The goal is to find a compromise value that works for well for all the typical usages of a particular lens.

The new Sigma lens dock allows users to fine-tune the AF adjustment of a lens per USB much more specifically than this is currently possible through the camera with just one value for a particular lens. It allows different adjustment values depending on the zoom setting and subject distance.

05-29-2013, 08:24 AM   #21
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QuoteOriginally posted by Class A Quote
The AF sensor sees the world at f/5.6 (only the K-5 II(s) has a faster f/2.8 AF area)
Good information! Since the DOF is much shallower at that aperture, does it mean that K-5 II will produce sharper image compared to K5? Or does it just mean that it needs less light to auto focus?
05-29-2013, 01:56 PM   #22
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QuoteOriginally posted by Class A Quote
The new Sigma lens dock allows users to fine-tune the AF adjustment of a lens per USB much more specifically than this is currently possible through the camera with just one value for a particular lens. It allows different adjustment values depending on the zoom setting and subject distance.
That's very cool--except of course that you have to know what adjustments are appropriate and that's the hardest part, imo.
05-29-2013, 11:49 PM   #23
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QuoteOriginally posted by TropicalMonkey Quote
Since the DOF is much shallower at that aperture, does it mean that K-5 II will produce sharper image compared to K5? Or does it just mean that it needs less light to auto focus?
I believe that the baseline length of the AF system (i.e. f/5.6 vs f/2.8) only affects the precision potential. The K-5 II(s) better sees slight amounts of misfocus.

I understand that the light sensitivity depends on the optics and other construction details. Note however, that the K-5 II(s) sports a -3EV sensitivity whereas the K-5 only claimed -1 EV. I can personally confirm that the K-5 II AF light does not come on even in very dim lighting.

05-30-2013, 08:42 AM   #24
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Thanks "Class A". It is easy to understand contrast detection AF, so while reading more about Phase detection AF I came across the following link, which seems to contain a lot of good information. What is Focus Shift?
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