Beginner help, why does it look grainy at low ISO
I took pictures at a motorsport event with my E-M10 Mark II and 40-150mm F4-5.6 R. When I zoom in, it looks grainy. Is it diffraction from stepped down lens? I forgot to buy an ND filter unfortunately. On the other hand, the 2nd picture is at perfect aperture range of lens and still looks grainy. It can't be ISO because it's very low, right? Is it just the cameras/lens limit? Or is something wrong with it (or my settings)? All shot in RAW.
1st picture (grain best visible on grass in background, but also on paint when I look at it in OM Workspace. Reddit compresses jpg heavily so it doesn't show on the paint here):
- 100mm
- 1/160
- F13.0
- -0.3 EV
- ISO 200
- Mode S
2nd picture (grain easily visible on black paint):
- 120mm
- 1/250s
- F5.3
- -0.7 EV, set +0.4 EV in OM Workspace
- ISO 400 (Auto, forgot to set manually)
- Mode A