PART-1. Autofocus Adjustment for the Pentax K20D, Custom Setting No 35

Dear Pentax users and friends:

Please note that the chart was sent to Pentax USA for comments. Although they have asked me not to mention any names, here is the email received.

Hi, Yvon.

Actually the chart worked just fine for me. A co-worker also tried it. He’s an old hand with the popular one on DPReview, but he says your chart seems just fine. He and I both liked that it folded out to a 45° angle block, which seemed to make the process pretty easy.

Thanks for working on this and making it available.

Best regards,

~~~~~ PENTAX Imaging Company

Download the Charts.

Autofocus Adjustment for the Pentax K20D, Custom Setting No 35, and how to check the Auto-focus accuracy

AUTOFOCUS ACCURACY, BACK & FRONT FOCUSING PROBLEMS

I personally never had any auto-focusing problem with any lens or camera I ever owned. It seems that these days, members of every blog and forum sites are talking about checking and adjusting the auto-focus on their lenses or DSLR. I think there is a perception that many lenses or DSLR cameras are having front or rear focus problems. In reality, I am inclined to think that novice photographers, “Pixels Peepers”, and inquisitive hobbyists, influenced by the available internet information, are finding problems where there is none. If a particular lens consistently gives out-of-focus results, when all other lenses used with the same DSLR are okay, you might have a focusing problem with that particular lens. If most of the pictures taken with the same DSLR, but with various lenses, are out-of-focus, you might have a DSLR camera-focusing problem.

If you think there is a focusing problem, several auto-focus testing charts are available from forums, blogsites and websites. I probably downloaded most of them and found inaccuracies with all of them. They are either too small, or too cluttered, or have measurements that are not to scale, etc. My own chart, published on my blog site earlier on, was too small and too cluttered for lenses with a minimum focusing distance of more than six to eight inches. One thing charts seem to have in common is that no one is ever sure of which exact point of the chart the camera is actually focusing on.

Back or front focusing problems are more notorious with subjects that are within a short depth of field, such as macro pictures or selective focusing pictures, and with the lens used at its widest aperture. The three images below illustrate this. The middle picture is the way it should be as the cat’s eye was the focusing target. The picture on the left shows a front- focusing problem, and the picture to the right shows a back-focusing problem.

That alone would not mean that the lens or camera has a focusing problem, it could be the photographer’s error. However, similar results time after time could be the first hint that you might have a focusing problem. Perhaps a logical next step would be to test the lens in question under a controlled environment. A controlled environment could be the inside of a building where there is no wind, with the camera set on a tripod, and with good lighting. An auto-focus testing chart could be used, preferably one approved by the manufacturer if they have one available.

Of all of the DSLR cameras manufactured by Pentax, only the K20D has the option of adjusting the front / back focusing from within the camera (Custom Menu No.35). All auto-focus adjustments on other Pentax models should be made by a Pentax trained technician. Several forums members have published articles, explaining procedures to modify DSLR Firmware. Doing so could void the warranty and damage the camera. It is better to leave specialized work to specialists.


PROPOSED SOLUTION

I do not believe that Pentax has an AF checking chart available to the public. That is the primary reason I took it upon myself to design one. After reading every blog and information about the subject on the internet, after reading comments from the readers of my blog site and threads on various forums, I deducted that one chart could not do it all. Three charts were designed because the minimum focusing distance varies from lens to lens. The smaller chart (Chart–1) works well for close-up lenses and lenses that have macro capabilities. The medium sized chart, (Chart-2) works well for normal lenses, say 30mm to 100mm, which have a minimum focus distance needing a target a little bigger than Chart-1. The third chart (Chart-3) is for lenses that cannot focus very close. One could keep enlarging the last chart, but I believe that the three charts attached herein will be functional for the majority of lenses.

Chart No. 1

Chart No. 2

Chart No. 3

Download the Charts.

There are various opinions whether a chart should be viewed at 45° from the lens centerline axis, or at 30°, or any angle or even flat. The reality is that it should give good results at any angle between 30° and 60°. Chart-1 and Chart-2 were design for viewing at 45° from the lens’ centerline axis. The measurements on the viewing surface were corrected for accurate reading when viewed at a 45° angle. The Charts could have been designed for viewing at 30°, giving more viewable depth of field for both front and back. However, I opted for the 45° because the charts could fit on a 8 ½” x 11” sheet of paper (Letter size).

I chose a focusing area shaped as a circle. The bottom half is black and the top half is white. When viewed perpendicularly or flat, it has an oval shape, because the chart is meant to be viewed at 45°. When viewed at 45°, the focusing area appears as a perfect circle. That also helps verifying that the lens is at a 45° angle..

Chart No. 2 viewed perpendicularly or flat.

Chart No. 2 viewed at 45 degree angle

CAMERA SETTINGS

Set the camera to:

* Autofocus single.
* Leave the Shake Reduction off if you use a tripod (recommended).
* Use the Aperture Priority mode (Av).
* Set the lens aperture to its maximum.
* Set the AF point to center.
* Use a remote for the shutter release or use the 2-second timer to avoid any movement.

Go to : PART-2 consisting of "How to perform the test", "Understanding the results", "A word about auto-focus sensors", "The K20D custom setting No. 35 explanation".

Again, thank you very much for reading my blog.

Regards,

Yvon Bourque