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

SD19194 wrote:

This only hit me the other day after reviewing many older photos and then doing a ton of practice shots. My Canon 50mm STM (newer model) simply isn't sharp at f1.8. So many times in the past I've gone down to this aperture hoping to get those crisp shots and have had to delete the photos thinking something must have went wrong whether it were camera shake or subject movement or the wrong shutter speed, etc. Now I'm starting to think I just have a rather lackluster lens that advertises f1.8 but really is best suited for f2.5 or 2.8. Has anyone ever heard anything with this lens with regards to this issue? Or is it at all possible I've been doing something wrong all along when going down to this aperture? It should be noted that when I go to 2.5 or bigger, things are almost always fine.

Also, the test shots I've done at f1.8 have been with a timer with the camera on a table to completely rule out hand shake. I also changed the AF point (red dots) to just one red dot in the middle for the test shots. Still, the photos lacked super sharpness in the middle.

Why rely on AF to see if the lens is sharp? If you want to see how sharp a lens is, just shoot some concrete at an angle of about 60 to 70 degrees with a fast shutter speed at base ISO in sunlight, and see if anything is sharp. Whatever is sharpest in the frame (which covers a range of distances), is the sharpest the lens is at that point in the frame.

Anyway, you seem to have missed the memo that inexpensive super-fast lenses are never sharp wide open. They are fast not for sharpness from low diffraction, because the aberrations are much larger than the diffraction blur, wide-open. They are fast to get faster shutter speeds and/or more exposure, or for greater background blur or what people call subject isolation.