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I just bought me a Nikon D7100 with 18-140 kit lens. When I used focus test chart page 18 to test my camera and lens I got one of following result at 35mm/f4.2.

I used Nikon's ViewNX2 to open the raw image and the red box is the focus point displayed in the tool.

enter image description here

Do you think the lens is front focused? Is it normal?

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    For more reliable results, focus on markings on a sheet that is strictly perpendicular to the optical axis, and place a separate rules at a tilted angle next to it. Tilting the whole chart is unreliable. – Szabolcs Jan 1 '15 at 21:10
  • @Szabolcs good suggestion! – hardywang Jan 2 '15 at 0:19
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It appears your test chart and camera may not be properly aligned with each other. Until that is corrected your results will not be valid. If, as appears to be the case, the left side of the chart is farther from the camera than the right side, then it is possible that the portion of the chart displaying -10mm is the same distance from the camera as the point where you focused as indicated by the red box.

If the target and camera are correctly aligned, then the photo indicates some front focusing. But you might get better results if you increase the light in the scene which appears to be underexposed. Low light can affect AF accuracy.

Also keep in mind that there will be a slight variation in the AF performance of your camera and lens from one frame to the next. Try a series of several frames with the same settings. Manually focus to infinity before actuating the AF for some. Manually focus to the minimum focus distance before actuating the AF for some. Then observe how which direction the focus mechanism is moving affects the results.

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Maybe. This kind of test is hard to get right. It does look focused more to the front, especially if it really did grab that area as the focus point. However, how far away are you testing from? Your lens/camera combo may front-focus in this range and not so much at normal shooting distances. For that reason, and because the diagonal paper is hard to get aligned just right, I recommend one of the other ways to test focus as outlined in What is the best way to micro-adjust a camera body to a particular lens? — either the Moire Fringe Method, or (best and easiest, in my opinion), the Contrast-Detect-vs.-Phase-Detect Adjustment Method.

Note that front-focusing isn't a camera body thing, or strictly a lens thing either — it's the combination of the two, and especially with zoom lenses, a lot of compromise can be involved. It may front focus at 18mm and back-focus at 140mm — and if you make an adjustment for the former, the latter may get worse. And as noted above, it may be different at normal focusing difference than it is up close for a focus test.

  • I tried moire fringe method, basically it is very hard for me to tell the difference between manual focus and auto focus. – hardywang Jan 2 '15 at 0:21

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