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10-27-2010, 10:12 AM   #1
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Different focus correction for each end of a zoom?

So after learning about the moire interference technique and getting repeatable accurate focus correction values, I started to do all my lenses. One lens has a correction of +20um at 18mm, but +140um at 50mm. This is repeatable from different focusing distances and whatnot. Is this common at all or a defect?

The Moire interference technique for those inclined:
AF microadjustment for the 1Ds mark III, 1D Mk3, 5D Mk2, 7D
Using Moire' Interference Patterns to Test DSLR Auto Focus

Most accurate, can do indoors without worrying about tungsten, easier to line up 100% perpendicular yet less affected by it compared to a focus chart.

10-28-2010, 01:24 AM   #2
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I've heard of this with some lenses. I'd consider this a defect, on the other hand if it's a design or manufacturing flaw affecting all (or large series of) copies of the lens then you can't do much other than sell it and get a different type / brand.

What you could try is to set it at the middle of the zoom range (~34-35mm) and then check how much it's off on the wide and tele end, I mean can you easily notice focus being off or is it so subtle that only pixel-peeping reveals some small softness.

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