How to tell if I got a good copy of 18-55mm lens?

Started Apr 20, 2013 | Questions thread
OpticsEngineer Veteran Member • Posts: 6,221
Re: How to tell if I got a good copy of 18-55mm lens?

Here is what I do.  (other people have good ideas too.)

First I make a target board that is lots of text printed out on laser printer, glued to a large foam board.     In the middle, there is a big X and the words "Focus Here".  Aperture full open, making sure line of sight is perpendicular to the target.   Camera on tripod, image stabilization off. Take a photo.  One time I do manual focus.  The next time I do autofocus with the focus set to spot center.  (because I am also looking for any problem with the autofocus.  I do a few cycles of focusing on something else and then going back to the "focus here" on the target board.)

Then look for variations in how sharp the letters look center/left/right/up/down.    The reason for that is lens problems are often are the result of decentered elements.   Those cause patches of fuzziness on one region of the lens.   I do not worry about the image getting fuzzy toward the edges or corners as long as it is similar in the same regions across the lens.

A lot of people recommend then turning the camera upside down and repeating the photo.   A suspected problem should flip sides in the image appropriately, but I have never tried that myself.   This check would make sure problem was not that the target board was not actually perpendicular to the line of sight of the camera.

My E mount 18-55 is fuzzier on the right hand side than the left but I never bothered to return it.  I find it kind of interesting and it did not cost that much since it was a kit lens.      I always have in mind that someday I will get around to testing it a bit more thoroughly but never seem to get around to it.  The thing is I nearly always use my NEX7 with the Tamron 18-200 zoom and it is a really good lens for me (the Sony SEL18200 is a bit sharper though.)

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