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03-13-2013, 12:05 AM   #1
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Front and back focus

Hmm , I never seem to be able to get a perfect in focus shot with my K5 - Sigma 28-105mm lens

Then I heard you could calibrate , or adjust front or back focus ...

Here are some results of the experiments I did ..

First I tried adjusting some negative value ...

Then I tried positive value ..

Looks like positive is the way to go , but how would this affect the lens :

So I tried first @ 28mm to see what the effect was

Then I tried 105mm

Hmmm , is +2 the best overall setting ... Could be

03-13-2013, 05:36 AM   #2
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It certainly looks to be the case, but I'm sorry to say that your ruler test is not very reliable. There are lot of folks who seem to be doing this in the community so one would be easily misled.

You cannot reliably point the K-5 at the ruler and have it reliably guess what part of the ruler (the horizontal line, the vertical line, the lettering, or the numbers) you are pointing at. This is because the focus points on the K-5 (and probably all other Pentax DSLRs) are much larger than the little LED indicators in the viewfinder would otherwise indicate.

The commercially available charts, and the free charts online, use a much larger focus target, along with a finely graduated ruler. Which is good. The problem with the online charts, is that they only work at closer distances, less than a maybe a few feet (depending on the lens, etc), for the same reason: Beyond that distance, the center focus target area starts getting small. Ideally you would test this using an appropriately sized focus target at realistic, typical working distances. The commercial targets have a distinctly vertical target which will probably work at much better distances. But in any case, you need to follow the instructions carefully.

Trial and error will tell you if your settings are optimized. The flower has some close parts, and some far away parts, but it certainly looks like you could be dead-on.
03-13-2013, 06:35 AM   #3
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Trial and error , which is why I used a subject ( flower ) and yes , its certainly not a perfect subject for such ( calibrating ) as it has bits sticking out all over the place ..

I did read the guide to doing this , and I took several test shots ( 5 for each setting with ruler ) , same with the flowers , I took a lot of pictures , Im just showing in this thread what was a typical .

Im currently running +2 and hope to do more test shots tomorrow , and compare against previous shots to see if the focus is sharper ..

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