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Old Nov 16, 2005, 3:22 PM   #1
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I ran some tests for backfocus. This was triggered when I noticed that my newly acquired Sigma 20mm f1.8 EX DG lens was not as sharp as I had expected it to be. So I thought perhaps it's backfocusing.

I used a couple of cereal boxes that are parallel to each other.On each of the sets, the first photos have the left cereal box to beabout half inchfurther away from the lens than the right cereal box. The second photos of each set has the cereal boxes to be about a full inch further away. I had my 20D mounted on a tripod, used center focus only. Focus point was set at the horizontal bar below the word "Protein" on the right cereal box.

Anyway, here are 6 photos:

Canon 50mm f1.8



">



Sigma 18-50mm f2.8






Sigma 20mm f1.8







So my question is...do I have a backfocus problem on my Sigma 20mm f1.8? Or is this normal? I forgot to change the shutter speed on the 20mm tests...I still had them at 1/50...which is what I used for the Canon 50mm and Sigma 18-50mm lenses.

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Old Nov 16, 2005, 7:28 PM   #2
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so you were focusing on the closest cereal box??
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Old Nov 16, 2005, 7:30 PM   #3
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Yes...focusing on the nearer cereal box...which ison the right. The one on the left is half an inch away on the first shot and an inch away on the second shot.
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Old Nov 16, 2005, 7:37 PM   #4
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it looks like it could be backfocusing..

you could also line up a bunch of screws or pegs in a slightly diagnol line.. focus on the center one and then see which one is sharpest.. may help you decide a little more..
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Old Nov 16, 2005, 7:40 PM   #5
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Ok, will do. I'll do some other tests. So so far, it looks like my Sigma 20mm is backfocusing...but not the other lenses...nor my 20D, correct?
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Old Nov 16, 2005, 7:54 PM   #6
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the others look fine..
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Old Nov 17, 2005, 4:22 AM   #7
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use a ruler at an angle... this time.
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Old Nov 17, 2005, 10:32 AM   #8
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Ok, here's a backfocus test chart I found on the 'net. Looks like it is backfocusing indeed...




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