PART-2. Autofocus Adjustment for the Pentax K20D, Custom Setting No 35

PART-2. Autofocus Adjustment for the Pentax K20D, Custom Setting No 35

HOW TO PERFORM THE TEST

To see Part 1, click here.

Download the Charts here.

The two smaller charts were designed so that they can be cut and taped together in a way that they can stand at a 45° angle when placed on a table, desk, or flat surface. The camera / lens is then leveled and preferably positioned on a tripod so that the lens’ centerline is aligned with the center of the chart. Place the lens as close as you can to the center of the chart target while it can still focus on the intersection of the black / white section, then back it off just a little. Position the large chart flat (horizontally) and the camera / lens angled at 45° from the chart.

After choosing the chart that will work best with your lens, and once the chart and camera / lens are position correctly, start by trying to focus on the white area of the chart. The lens should not be able to focus correctly and it should be “hunting”. If it achieves focus while pointing to the middle of the white area, it means you could actually be in focus with the lines surrounding the white area and you need to use the next larger chart.




Next, move the camera / lens angle slowly downward until it auto focuses. At that point, you know that you are focused on the intersection of the black / white portion of the chart. That is the center of the chart. The measurements (Metric on the left and English on the right) are at zero on both sides of the chart horizontal centerline. Take the picture and look at the results. The center target line (00) should be perfectly clear while the top and bottom target lines and text should get increasingly and proportionally out of focus. The top target lines represent the back focus area while the bottom target lines represent the front focus area. Repeat this test several times to make sure you have done the test correctly. If the center target line is in focus, but the front and back target lines are not proportionally getting out of focus, there shouldn’t be any serious problems with the lens. Remember that this test is done with the lens opened at the maximum aperture and that the depth of field will increase with smaller aperture. Most of the lenses have a “sweet spot which is usually 1 to 2 stop smaller than the maximum aperture. Therefore, you should only be concerned if the center target line is not in perfect focus while either the back or front lines are.


UNDERSTAND THE RESULTS

The perfect lens should return a result similar to the image below:


A lens with acceptable Auto Focus should have the center target line in focus, even if the back and front focus lines are not equally or proportionally out of focus.




A lens with front focus problems will look like the image below while a lens with back focus problems will look like the image below it.




A WORD ABOUT AUTO FOCUS SENSORS



Sensors are either vertical, horizontal or cross type. Pentax K10D and K20D have 9 cross type sensors and two vertical sensors. Vertical sensors detect the sharpest horizontal contrast within its area of coverage and lock the focus on that point. Alternatively, the horizontal sensors detect the sharpest vertical contrast within its area of coverage and lock the focus on that point. You guessed it; the cross type sensors detect the highest vertical or horizontal contrast and lock the focus on that point. Since we use the center focus point of our camera, which is a cross type sensor, our chart was deliberately designed without any vertical lines in the center so that the focus can only lock at the intersection of the black and white portion of our target. Note that in your viewfinder, the little red square indicates the focus area but the cross sensor is not necessarily dead center. It merely shows you the area of the sensor, but is not an exact focus point as the sensor will lock on the point of maximum contrast.

DIFFERENCE BETWEEN 45° and 30° CHARTS


The above charts, one at 45° and one at 30°, illustrates why the depth of field is proportional to the angle of the chart.

K20D CUSTOM SETTING NUMBER 35 – AF ADJUSTMENT

In the Custom Setting menu, navigate with the four-way controller to AF Adjustment, Setting number 35. Use ▲▼to turn the AF Adjustment On or Off. When choosing On, press ► and select if you want to adjust the lens attached or all the lenses. It seems that choosing all lenses would indicate that the camera needs AF Adjustment as choosing the current lens would adjust the AF for that lens when attached to the camera. The camera can retain settings for up to 20 lenses. Using the rear rotary dial, you can adjust the values from minus 1 to minus 10 and plus 1 to plus 10. You can reset the camera to the factory defaults anytime by navigating down ▼ to Reset and confirm Reset.

DISCLAIMER

Although the Charts have given me good results, you should fully understand how they help analyzing the Auto Focus accuracy. Read the above paragraphs several times until you completely grasp the procedures. You can adjust the focus of all Pentax lenses within the K20D, as they are non-permanent changes. You can reset the Auto Focus to factory defaults at any time. As for the other Pentax camera models, you should only have a Pentax trained technician adjust the Auto Focus of your lenses or DSLR cameras.

Thank you very much for reading,

Yvon Bourque