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05-18-2013, 12:54 PM - 1 Like   #76
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Distant subject like scenery is great for testing sharpness, the problem is to find a perfect target that will provide meaningful sharpness at all 4 corners. The way I tested my lenses for alignment is to use the stucco wall of my house to fill the lens as much as I can with careful camera alignment with bubble lever, always shoot raw, on tripod with 2s timer. The lens must be perfectly perpendicular to the wall. Both manual & auto focus and take at least a few shots at every aperture (until f8 at least but you can go lower if you have enough patience). You would be surprised how unreliable both manual and auto focus are when viewing the results at 100%. Repeat the same test with portrait orientations (both left & right). That means for every aperture, you should have at least 3 perfectly taken samples. With these 3 samples, you can see if all 4 corners are equally sharp. The first sign of misalignment is some of the corners might appear not only diffused but seems like being pulled outward (if it makes any sense to you). Don't be surprised if one of the three samples is perfect but the other 2 are not. It means some of the glasses are being pulled by gravity due to less than perfect built quality (something most AF lenses suffer as they have to be made loose enough for AF to work).

This way, you can tell 1) how bad the misalignment is wide open (but don't be too critical because perfect specimens are rare species), 2) how much the misalignment could be combated as the lens is being stopped down, and 3) whether even sharpness is possible at f8 (the lens is a goner if not, imho). Now, if one of the wide open samples has more than just blurry corners, like one side (by whatever % of the entire image) of the image is blurry, it's a lemon. This is especially true with Pentax DSLRs because the unseen part of the FF image will only get worse. Hope I make sense to you.

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