AF at Wide Angel distinctively unreliable?
8 months ago
I have a D3200 and tested with a Sigma 18-250 MACRO and my friend's 18-200 VRII. I only use AF-S with center point only.
PDAF is absolutely dead on, say from 100mm-250mm. However, at 18-50mm, each PDAF gives me wildly different focus distance as seen from the lens barrel.
On a tripod, I tried to focus on a license plate 7 meters away with the lens set at 18mm F3.5. 10 different PDAF focuses set the lens focus distance anywhere from 6m to near infinity(!). While all the images are acceptably sharp, you can definitely tell the difference by 50% zoom. With live view CDAF, it focus the lens to the 7 meter mark reliably every time. I also tried MF using the green dot indicator and surprise to find the tolerance to be quite huge at 18mm.
This PDAF inconsistency is only apparent at wide angel with medium to far subject distances. There is no tendency to go front or back focus, it just land at a difference place every time. Close distance and telephoto (regardless of focus distance) all seem to work fine.
I am wondering if this is a normal behavior for PDAF at wide angel(since the DOF tolerance is huge). If you have not tried this before, observe your lens distance scale every time you PDAF at wide angel with a moderately far subject.