Lens Testing for beginners?

Started Feb 3, 2013 | Questions thread
OpticsEngineer Veteran Member • Posts: 5,745
Re: Great answers - many thanks

You certainly are on the right track about realizing that knowing how to use the camera settings will make the biggest difference in getting good pictures.

Since you are an engineer, I would like to share that the lenses are made of many optical elements. Each element has tolerances on tilts and decenters. The effect of any one of these would be to make a lens that is more sharp on one side than the other, but by just a tiny amount. For an average lens, the random variations will result in a lens that is more or less uniformly sharp up/down/left/right. But on some rare occassions, it can happen that all the tilts and decenters happen to line up so you get a lens that clearly less sharp on one side.

For very expensive lenses, the elements are toleranced so extremely few bad lenses get made. As you would expect, tighter toleranced parts cost more. On less expensive lenses, a larger fraction of bad lenses will get made. Although it is a very small fraction, such lenses definitly do get made. This is deliberate on the part of the lens manufacturers (for the lower priced lenses). They plan that the customer is the quality control department and they expect a certain amount of returns. (This is true for binoculars too.)

For that reason, it is worthwhile to perform the test that was described earlier where you put some newspapers on a wall and take a few photos. Then examine the images for uniform sharpness up/down/left/right. Keep in mind that many lenses are designed to have more sharpness in the center than the edges. That is normal. You are looking for the abnormal, which is for one section to be noticeably fuzzy. This is the test that I do within the first few days of buying a new lens when it is still easy to return them to the store I bought them from.  (Out of ten lenses I have bought, nine were good by this test and one was bad, but I actually kept it because it was a kit lens that came with a camera for not much money and it was an interesting specimen for me to keep for professional reasons.)

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