Don’t Assume a New Lens is Good!
It’s a brand-new lens, right out of the box. It should be perfect optically. If only real life worked that way! It does not.
Periodically check your important lenses; they can go out of whack for no apparent reason (this happened 3 times with my Nikon 17-35). Nearly always, the factory service can fix the problem, but why start with a bummer? Of course, buying used entails additional risks of abuse by the previous owner.
The best way to verify performance is to shoot two samples of the same lens and/or shoot against a reference lens of the same focal length, keeping everything locked down, exchanging only the lens; this rules out everything but focus error (assuming you’ve done everything else right).
Digital sensors, which are perfectly flat and also very high resolution, mercilessly show anything less than perfect optical alignment. A mosaic like the one seen above (actual pixels crop) makes a perfect target for spotting such problems. Be sure to focus perfectly, and align the camera to the subject squarely, and be aware of both horizontal and vertical. However, tilting the camera up is OK, so long as you look for symmetry, and have reasonable depth of field. Choose a target that allows near-infinity focus to make the job easier.
In my personal experience, somewhere between 20-30% of lenses are optically “off” in some way or another right out of the box. I’m referring to both Canon and Nikon based on experience with 40-50 lenses. While I don’t write up every bad experience, you can read about several experiences in Brand-new Blur. Some problems are really awful; others are more subtle.
Zoom lenses and shorter focal length lenses seem to be the worst offenders; I've had no problems with high-end telephoto lenses. Finally, do not forget the lens mount itself.
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