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KAJed
Registered: Aug 13, 2009
Total Posts: 475
Country: Canada

Hey guys, I've been having some issues with my 35mm I bought last year around this time. I thought it might be a front/back focus issue so I tried out a test.

It looks to me that its front focusing by a little and might be why I'm not seeing sharp images.

The lens is just nearly 1 year old, and I have it on my XSi. I'm in Canada and if it is something that would require calibration I wonder how it works up here in the north.

Looking for opinions on the test as I've never actually done it before.







omarlyn
Registered: Feb 19, 2004
Total Posts: 4066
Country: United States

I'm not familiar with that test rig of yours but where is your focus point...is it the line where the little arrows are pointing to? If so, it's possible the actual AF point is picking up the signature just below it and skewing the results. That's why I hate these (and similar) focus test rigs as it's too easy to misplace the focus point. Focusing should always be on a completely flat area that is absolutely 90 degrees perpendicular to the camera/lens in both axis. When you introduce an angle that is too close to the area of focus, it could effect the focusing and invalidate the results.

HTH,
Omar



dcains
Registered: Oct 09, 2005
Total Posts: 7377
Country: United States

Diagonal focus test charts don't work. I'll let you guys argue about it amongst yourselves.



BrianO
Registered: Aug 21, 2008
Total Posts: 8547
Country: United States

KAJed wrote: ...Looking for opinions on the test as I've never actually done it before.

The vertical axis on your camera needs to be exactly parallel to the vertical axis of the test target; in your example it's not. That's why the text at the top is out of focus while the text at the bottom is in focus.

The camera also needs to be square with the target. Again, yours isn't, which is why in the fourth paragraph the word "It's" is more out of focus than the word "the" at the end of the same line.

Lack of alignment makes the test invalid, so you can't really tell if you have a front- or -back-focusing issue.



BrianO
Registered: Aug 21, 2008
Total Posts: 8547
Country: United States

omarlyn wrote: I'm not familiar with that test rig of yours but where is your focus point...is it the line where the little arrows are pointing to?

It shouldn't be. The focus point should be up on the first or second paragraph of text on the box, well away from the bottom plate.

BTW, Tim Jackson redesigned his chart many years ago, doing away with the box completely. The redesigned one is simpler to use, and not as critical on minor misalignment issues negatively impacting results.

Visit his home page to download the newest version, and to read his response to those who say his test chart can't work.

http://focustestchart.com/chart.html

I myself use a system that sort of goes back to the version with the box, but it's not a homemade paper device.