Lack of sharpness at f1.8 on Canon 50mm STM

AnthonyL wrote:

A couple of things come to mind with my limited experience:

• Copy variations. I had the pre-STM version and it was more than sharp enough for me. Unfortunately I shot some photos at a restaurant and it being so small I left it on the **** table and only realised a few weeks later
• The specs for PDAF focussing are something like within 1 DOF - ah I found that I documented a post from some time back:

Canon spec is to focus with in one depth of field for standard lenses and with in 1/3 depth of field for lenses faster than F/2.8. All Canon cameras focus with the lens wide open regardless of what aperture you have the camera set at.
Here's a post by RD Kirk that explains it:
(quote)
"If the camera places the actual focused plane within the depth of focus range, the intended focused plane of the subject should "look sharp" on a 6x9-inch print from a distance of 10 inches. In "high precision mode" the intended plane of focus should "look sharp" on about an 11x14-inch print at 10 inches (extrapolating from the standard size given by Canon for normal mode).
This standard is important to understand. If you use high precision mode and view the image on a monitor at 100 percent of the original pixel resolution, do not expect the actual plane of focus to coincide with the intended plane of focus. It's not necessarily going to do that--it's only designed to get close enough to "look sharp" at the standard final display enlargement size and viewing distance. If it does that much, then it's doing its job as designed.
If you're operating at normal AF precision, the camera will happily settle for any point within the depth of focus range (and not necessarily the same place every time--randomness within limits is what "tolerance" means in this context).
If you need optimum sharpness at greater than the standard final display sizes and viewing distances, then yes, you'll have to focus manually. However, a lot of focusing problems are caused by users not realizing that even autofocus requires learning certain techniques, some similar to the techniques of using a manual split-image rangefinder.
(end of Quote)

• Have you tried a focus test with contrast detection/live view to see whether it is the PDAF or the lens that is the issue?

Unfortunately I wouldn’t have the first clue how to do this