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Old Sep 22, 2009, 1:10 PM   #1
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Default Canon Mount Sigma 24-70 hsm

After picking up the pieces of a broken lens, I'm replacing the sigma 24-70 IF DG EF HSM lens with the same lens.

I have read that this lens sometimes has a front focusing problem.

I'm not sure how often that happens with the lens. But since more than one or two people has raised this issue, I am curious.

Is there a simple set up and test for a 24-70 lens on a full frame camera to see it if front focus or back focuses?

I've seen some rather complicated testing procedures and those looked a bit like brain surgery to me. And believe me, when it comes to camera-speak, I am not fluent.

Thanks for your ideas

Faithfully Yours;

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Old Sep 22, 2009, 1:18 PM   #2
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Stagger some books, cereal boxes, CD/DVD cases, etc., at different distances from the camera on a table. Use a tripod, and make sure lighting is good (i.e., light from open windows in the daytime if indoors).

That way, you can look for your focus point being sharpest, with text on the book covers (or CD/DVD cases, cereal boxes, etc.) in front or in back of your focus point getting softer as expected. I'd make sure to check the lens in bright enough light (as AF accuracy can suffer in dimmer lighting). Use the center focus point and make sure to use a tripod with the lens at wider aperture settings for a shallower depth of field.



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Old Sep 22, 2009, 2:00 PM   #3
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Okay, here's what I did.

The box on the left is 66 inches away from the front of the lens.
The box in the center is 72 inches away from the front of the lens.
The box on the right is 78 inches away from the front of the lens.

Shot at F2.8 70mm focal length.
Center dot focusing (auto focus of course!)

DOF Master calculates DOF:
Near 69.84 inches
Far 74.28 inches

I took the photo at the largest size I could, then looked at it greatly enlarged. The center box was the most readable. The box on the left was second easiest to read and the box on the right was the worst.

Should I redo this test? If so what should I do differently?
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Old Sep 22, 2009, 2:05 PM   #4
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Here's a crop of the box on left and center box
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Old Sep 22, 2009, 2:14 PM   #5
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That looks pretty good to my eyes (center box sharpest).

But, I'd get a bit closer (so that you'll have a shallower depth of field for a given focal length/aperture) and repeat the tests with closer spacing between the boxes (so that you have more subtle differences between subjects that are closer or further away from the camera).
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Old Sep 22, 2009, 2:26 PM   #6
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Jim, this is great. Not only are your directions easy to follow, I actually understand why you having me do this.

Holy schmoly, you might make a photographer out of me yet.
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Last edited by FaithfulPastor; Sep 22, 2009 at 2:30 PM.
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Old Sep 22, 2009, 2:33 PM   #7
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That looks great to my eyes from what I can see of the downsized image.
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Old Sep 22, 2009, 2:41 PM   #8
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Looks like it is focussing pretty much correctly. There are certainly no major issues.

As you are using a 5DMkII you can use the microadjustment to fine-tune your focus. If you place things on more of a slant, or use a ruler you can get a more precise view.
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Old Sep 22, 2009, 4:02 PM   #9
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yea, looks like there are no real issues.

http://photo.net/learn/focustest/

pretty easy to follow imo and will let you test things out in a little more detail.

there are many links to this, i just took the first that came up, pretty much all the same idea. as craig mentioned a ruler set at 45 degrees works fine too.
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Old Sep 23, 2009, 8:03 AM   #10
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I used a ruler, 18 inches away at f2.8.
The center point focus was the number 18, it fit into my focusing box of the camera nicely.

The 17 appears to be slightly sharper than the 18.

In the second photo, the ruler was 24 inches away.
The 18 is still the focal point. The 18 looks sharp, but the 17 still looks slightly sharper.

What do you think?
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