LensAlign from RawwWorkflow . . .

Started Dec 20, 2008 | Discussions thread
J A K Forum Pro • Posts: 15,833
Bob, this should clarify what is going on.

AThomas772 wrote:

I am interested in your front and back focus test setup. Since I am
not too mechanically inclined I have a few questions to clarify how I
set this up for my own testin. Following are my questions.

1. Where do I focus my "One Shot" focus spot. Do I focus on the "18"
ont the ruler since that is the center and how the front and back
focus in measured? If not on the "18" on the ruler, then where?

You focus on the block with the AF target with the cross; this is the block you have to be perpendicular to. The ruler is at a 45 degree angle with respect to the block with the AF target and of course is at 45 degrees with respect to the optical axis. If you take a straight edge along the block with the AF target it intersects the ruler at 18 inches; DOF is measured relative to the 18 inch mark and hopefully, if everything is okay, you will be symmetrical about this mark.

2. Assuming I am correct in #1 above (Focus is on the "18") what is
the purpose of the design on the small block of wood (the target)and
where do I get this design if it is important?

As I explained above the small block of wood with the AF target taped to it is what you aim at.

3.I will be using a 5D II and testing a 24-105mm lens. How far from
the target should I set my camera to test the lens at 24MM, 50mm,
105mm, etc?

If you set the distance from the sensor plane (marked on your body) to the front of the small block of wood to 12.2 times the focal length you are testing the magnification will be -1/10; this is where I measure AF performance. Of course I confirm AF performance by testing at longer distances. So, if you are testing your 24-105 I would set the focal length to 105mm and the measure off a total distance of 12.2*105mm.

4. I will also be testing a 70-300 DO. How would my test for this
lens be different than for my 24-105mm.

Similar to the setup for the 24-105, set the focal length to 300mm and measure off a total distance of 12.2*300mm. Please be aware there is nothing magical associated with the 12.2 number ... I've just memorized it as the multiplier that gives me a magnification of -1/10 regardless of what lens I'm testing.

These are my questions. Sorry to be so naive on this matter and Thank
you in advance for your help.


Bob, no sweat, no apology is necessary, I'm more than happy to help you out.

If you go to the link below you will see the big picture of what the fixture looks like; note, this is a shot with a different AF pattern to test AF. You can create any AF target you want via Photoshop, print it out, and tape it to the small block. Okay, now pay close attention ... if you read the text at the link below there is an equation in case you want to know what the total distance is for any magnification you choose for testing AF accuracy.



Joe Kurkjian

Galleries: http://www.pbase.com/jkurkjia


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