Monday, May 19, 2008

K20D AF Adjustment Chart expanded as per the constructive comments received.

Hello Pentaxians and friends,
Yesterday I introduced an AF Adjustment Chart to help in determining if any particular lens has front or back focus problems. As I often do, I posted the major portion of the article on various Pentax forums. Several Pentaxians emailed me with some good suggestions as to how to improve the chart. I first tested the chart with a lens that focused as close as just a few inches and my chart actually worked at eliminating the back focus problem I had. The lens was a Pentax SMC-F 35-70mm macro. Every time I would take a macro picture, it seemed that the actual photograph would be out of focus when, I was certain that I had focused properly. The K20D, with the AF Adjustment function was the right camera to test and adjust the lens.

I recognized yesterday that the target cross was too small for constant and accurate results. I changed the chart and incorporated a much larger center portion while retaining the overall design. Additionally, I scaled the center portion to almost the size of an 81/2” x 11” letter size paper. I still believe that it is easier to have the chart standing at 45 degree from horizontal while testing the lenses with the camera on a tripod and as horizontally leveled as possible.

If you look at the chart, the center is oval in shape when viewed perpendicularly. The reason for this is because when focusing on the chart with your camera leveled, you will notice that the oval shape turns into a circle, helping in visualizing when the camera/lens is leveled.

The oversized chart can be attached to the smaller chart with paper clips, to retain the 45 degree angle. It is at 45 deg because that way, the depth of field can be seen. If the sheet was flat, there wouldn’t be any depth to it. The Metric and English dimensions are accurate on the small chart, but the enlarged chart shown the dimensions also enlarged. Still, it does show if you have back or front focus errors.

Again, if other suggestions are emailed to me, I will first make sure that we all agree with the improvements and re-issue a once again improved chart.

Now, I assume that we all own and use Pentax equipment. My blog, the other Pentax blogs, as well as the dedicated Pentax forums should all have one thing in common. That is to help each other as Pentax users.

The reason I am bringing this up is that I experienced some rather condescending comments from some members of forums. Its one thing to share your knowledge with the entire Pentax audience, but it’s another thing to be rude and pompous toward other Pentax brothers and sisters. I won’t name any of the forums, as I am sure you already visit every Pentax sites on the internet. At the top of the list for rudeness, is a member from Hong Kong whose name you probably already know. That person should tone down a little from his self-proclaimed expertise.

A Pentax site should be embracing all Pentax users from the person with no experience that just bought a Pentax DSLR today to the most talented and experienced professional photographer in the world, whoever that is.

I wrote before that owning a Pentax DLSR was like being part of a big family. I truly believe that Pentaxians are a special loyal group. Let’s help each other become better photographers by helping each other.

Thank you for reading, and thank you for all the constructive comments.

Yvon Bourque

Above is the new chart with a much bigger and uncluttered center. It is cut and folded in the same way as the previous chart.

The picture above shows the new chart standing at 45 degree from horizontal.

Above is the chart from behind.

Above is what I saw in the viewfinder with the DA 16-45mm at full aperture and as the closest point it could auto-focus.

Above is the enlarged chart, clipped to the smaller chart retaining the 45 degree angle. It is included in the free download.

Above, you can see the enlarge chart clipped to the smaller chart from behind.

Above, you can see the oval shave when viewed perpendicularly to the chart.

Above is the test of the Pentax DA 70mm Limited. It's a very sharp and accurate lens. Notice how the oval is now an almost perfect circle?

Finally, the picture above was taken today, atop the San Bernardino mountains, to show that I am not a pixel peeper, I do take real photographs.

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