Thursday, June 16, 2011

Auto-Focus adjustment for Front and Back Focus problems.

. Email: brqyvn@gmail.com

Hi Pentaxian friends.

Before you spend $$$ for one of the Lens Alignment tool on the matket, spend .99cents on this Auto-focus Adjustment chart for front and back focusing problems. The results will be just as accurate. In fact, the alignment charts that are on the market were based on my charts that were available about one year before. The charts work great for any DSLRs, including Pentax, Canon, Nikon and other DSLRs with auto-focus adjustment capability.
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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.

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.
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. Outside, with no wind and overcast sky is ideal. An auto-focus testing chart could be used.

Of all of the DSLR cameras manufactured, only a few have the option of adjusting the front / back focusing from within the camera. All auto-focus adjustments on other models should be made by a 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.

I do not believe that any manufacturer has an AF checking chart available to the public. That is the primary reason I took it upon myself to design one. My charts were out about one year before some companies imitated the design and sold them, made out of plastic, for up to $100.00. The results are the same. 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 shown herein will be functional for all lenses.




Chart No. 1


Chart No. 2


Chart No. 3

There are various opinions whether a chart should be viewed at 45° from the lens center line 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’ center line 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). Note that you can have the charts printed on large paper, or as a photo, and make it the size you want.

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.


A WORD ABOUT AUTO FOCUS SENSORS

Sensors are either vertical, horizontal or cross type. 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 always 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.


The cross sensor is not necessarily in the center of the red square or dot shown in your camera viewfinder. The actual focus point can only be determined by using a chart. The moment your camera center AF sense the contrast between the black and white area of the circle, that is the point of center focus.

The center Focus point is used to test lenses' Focus accuracy using the charts.


To download the charts, check the right side bar of this site. If viewing from a mobile device, click on "View web version" at the bottom of your screen. If you are receiving this post trhough RSS feed or from email, click here to get to the actual Blogsite. http://pentaxdslrs.blogspot.com/

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