To see the full natural sharpness of your lens/camera you should take a photo in Live View. Take the same scene in Live View and also through the Viewfinder (like you have been) and post of few results if you can. The comparison with Live View will tell you if you really have an issue with your lens.
I made a big test to see is there any mechanical vibration due to mirror flip or the so called shutter shock issue. Photos are without any edition.
- Camera Nikon D5600, lens is AF-P 18-55 f3.5-5.6
- Wired shutter remote MC-DC2
4 Methods:
- Viewfinder, viewfinder + shutter delay (mirror lockup)
- Live View Mode, Live View Mode + shutter delay (mirror lockup)
Test was on sturdy tripod, shutter speeds from 1/2000 - 1/10s, only ISO was changed until 1/50s, below 1/50s lighting was changed in the room.
- Focus Mode: AF-S
- AF Area Mode: Single, center focus point
- Aperture: f8
- Focal length: 55mm
- VR: OFF
- Noise reduction: OFF
- Jpeg size: 6000x4000 (24mpixel)
- Jpeg type: Fine
- Camera sharpenig: 8
Viewfinder : DSC_2025-DSC_2034 1/400s - 1/50s
Live View Mode: DSC_2051-DSC_2060 1/400s - 1/50s
The conlusions are (best way to detect the vibration where the lines start goes from the middle of the square center of the photos, also number 18 )
At Viewfinder Mode a very small vibration enter at 1/320s and goes down continuously to 1/100s, at 1/80s almost gone, but 1/60s again visible and stay there down to 1/10s. After I repeated with VR ON. Result is improved a lot, vibrations was only recognizable at 1/200s, 1/160s, 1/125s, 1/100s, 1/60s, 1/50s, 1/20s the rest are CLEAR.
At Live View Mode the vibration also enter at 1/320s but smaller, worse is 1/60s again visible, like in Viewfinder Mode. I can tell that Live View Mode is almost CLEAR from vibration except 1/60s.
Above 1/320s it means from 1/400s is completely gone, all the photos doesn't have any vibration up to shutter speed 1/2000s this is 100% guarranteed both Viewfinder and Live View Mode.
Viewfinder and Live View Mode with shutter delay 90% of the vibrations are gone.
Worse speed is 1/60s that was always visible during all the 5 tests.