Autofocus Adjustment Hints

The K20D and K-7 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 onto. 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.
• 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 before the K-7.
• Try to shoot a flat rather than a tilted target.
• 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.
• 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.
• The widest possible aperture (lowest f-ratio) will make it the easiest to check where the focus is, but some lenses 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.
• Evaluate real world results and let these govern your adjustments, rather than relying on test setups.

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.

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.

[jimH]

How do you access the "hidden" service menu on a K10?
Thanks

jimH

Edit: Firmware 1.30

[photog]

Thank you for this explination. Does anyone know how large each division is?
Ron McDermott

[Class A]

Originally Posted by jimH

How do you access the "hidden" service menu on a K10?

Please have a look at this K10D AF Adjust thread. With your firmware version (1.30) you'll need software. Prior to that one could use a key combination used after start up. There are also ways to downgrade the firmware, involving a modified firmware which has a patched release date.

[Class A]

Originally Posted by photog

Does anyone know how large each division is?

I'm not aware of a corresponding diagram. However, you can perform the "black dot" test: Choose some uniform background and place a rather small object in front of it. Focus on that small object. Switch to MF or use some other method of avoiding refocusing and move the camera so that the small subject moves around the focus area that interests you. If metering is activated (tap on the shutter button to activate it) the camera will tell you with the green hexagon in the viewfinder when the selected AF sensor sees something in focus. By approaching the subject from all directions, you can find out how large it is and where it is placed.

[photog]

I have an addition to my recent post. Testing with a number of lenses fron my Sigma 17-70 to a couple of old F lenses, I discovered by experimentally that I actually needed -10 (full scale correction)! The magnification facility was very helpful as checking on computer from test to test becomes extremely tedious.
What sort of corrections have other members found necessary.
Ron McDermott

[opiedog]

ron,

just curious, at 17mm, does your sigma 17-70 shift focus when changing from f2.8 to say f7.1?


thanks!

jordan

[photog]

I did not specifivally test for effect of aperture changes. But I sincerely hope not! I will make tests as opportunity offers.
Ron McDermott

[Arpe]

So if one's lens is Back Focusing, does one apply a negative correction in one's camera, or positive.

[Class A]

Originally Posted by Arpe

So if one's lens is Back Focusing, does one apply a negative correction in one's camera, or positive.

The adjustment value should be positive.

[jgredline]

Originally Posted by Class A

The adjustment value should be positive.

Yep, agreed!

[mrdfred]

I have used the debug mode on my K10d after downloading the English version of the usb program to turn the debug menue on. I was worriied about using it as I didn't want to permenantly screw up my camera. Prior to using it, I tested all my lenses, DA 18-55, DA*16-50, DA 50-200, Kiron 100 f2/8 macro, at max zoom and max apprature. I tripod mounted the camera and used a shutter release to take the test shots. I moved the camera to 10X the focal length of the lens and shot 3 times at the same focal length and apprature at different targets, both tilted and flat. I took 3 shots of each target with each lens, both tilted and flat. All my lenses were back focusing at about the same amount. I found that the DA 18-50 kit lens was completely out of focus at wide open and at the widest zoom, I'm suprised the camera even achieved a focus lock. I used the focus targets from Tim Jackson, Focus Test Chart V2.00, (both tilted and flat), Jeffery's Test Chart V1.0, (both tilted and flat), and a BAR code label from my cell phone box, multiple verticle lines of varieing thickness, numbers, and text of varieing sizes on very white paper. The tilted targets showed me the direction (Back or Front focusing) my lenses were off. The flat targets were used to make the correction once I accessed the Debug menu. The Bar code target proved the most helpful in fine tuneing the adjustment. l also checked if I could return my camera to the default settings, and was pleased to find that the debug adjustments can be undone. Adjust to the right to bring the focus point foward and to the left to move it back.