Sigma 30mm f1.4 - problems with focusing and sharpness. Need advice

Started Mar 13, 2011 | Discussions thread
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Sigma 30mm f1.4 - problems with focusing and sharpness. Need advice
Mar 13, 2011

Hello everyone,

I need some advice. I recently bought a Sigma 30mm f1.4 (non-HSM) lens. It is a used lens but still has 2 years of warranty left. I have problems getting sharps shots. The camera focuses on an object, the red light blinks in the viewfinder but the final outcome is really bad. My first thought was that my lens is affected with FrontFocusing or BackFocusing. However I have some doubts. If the object I focus on is in-focus in the viewfinder shouldn’t it also be in-focus on the sensor? In my opinion yes, but perhaps I am wrong.

What is more about 1 out of 15 (or maybe even 20) shots are in focus. How can this be? How can a lens focus at one time wrong and at another time correct? Or maybe this 1 out 15 shots is actually an AF-failure, but because the lens is not calibrated it turns out sharp?

I have also tried manual focusing. In this case the results are a bit better but still far from good.

AF-adjustment in the camera menu does not allow me to improve the outcomes.

My camera is a Pentax K20D. I bought the lens from a K-7 user (for him this lens worked fine). I made some additional tests on a K200 and K-x - in both cases the results were bad.

Below are a couple of sample images. The red arrow indicates where the focus was set.

Increasing the f-number helps a bit but only due to the fact that the DOF is greater. I am worried because never before did I have such problems with any lens.

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