A very common problem, that takes a long time to answer well, is
“I just got this new lens, and I’m not impressed with the images.”
There are lots of things that could be wrong with a lens, or camera and lens, but in most cases, the shooter is trying take the wrong kind of picture to really TEST the lens.
If you want to know about the lens, ISOLATE the lens, eliminate the other variables.
The first thing you want to do, is take ONE, VERY SHARP IMAGE.
You don’t want to be confused by motion blur, so use a faster shutter speed. That means test it in good light. You don’t want to be distracted by noise, so use your lowest ISO, that means test it in GREAT light. You want to use your sharpest aperture, start by finding out what that is…you can Google it…it’ll likely be around f8.
You don’t want to blur your image, even slightly when you press the shutter release, so use the timer. Use a tripod. Turn the stabilization off.
You don’t want to be confused by subjects that are several different distances from the camera, so shoot something FLAT. You want to be able to scrutinize details, so choose a subject that HAS fine details.
I like a poster or image with tiny text, flat on a wall in excellent light.
And you want to eliminate the possibility of focusing mistakes, so try autofocus and manual focus and try each one several times….afterward, only consider the SHARPEST example, that will show you the POTENTIAL of the lens.
But if your AF results are very different from you MF results, you may have a problem.
Don’t complicate your life with RAW, just shoot jpeg, you should get an image that is indistinguishably sharp, without the chance for user error in developing the RAW file.
If you can make ONE image, razor sharp, you’ve proven your lens CAN be sharp. It’s possible you have focusing issues, but the lens itself, is intrinsically SHARP.