Hi Wolfgang, First, you can find plenty of comparisons of image quality on sites like DPReview and image sensor quality at DXOMark so no reason to reinvent the wheel. I agree that the EM1 II has excellent image quality, that being said quality for cameras like Nikon-850, Z-7 and Sony A7R III would then be superb, stupendous, orgasmic, etc.
The difference is a result of the sensor size, the $2000.00 EM1 II has a 20.4MP sensor with a max resolution of 5184 X 3888 with a bit depth of 12. By comparison the $2000.00 Sony A7 III full frame camera has a 24.2MP sensor max res. of 6000 x 4000 and a bit depth of 14. Bit depth is a measure of the amount of information that can be registered by a computer, binary code 1's and 0's. A bit depth of 12 is more than 68 billion while 14 bit is over 4 trillion. This is simple physics, the more space you have on the sensor the larger the photo sites and the greater the bit depth.
Nikon Z7 at $3400.00 has a 45.7MP full frame sensor with max res of 8256 X 5504 while the $3200.00 Sony A7R III has a 42.4MP sensor with a max res of 7952 X 5304 both are 14 bit sensors. The reason this makes a difference too photographers is that larger max res. allows for larger prints and a greater amount of cropping along with better overall image quality. Viewed on the internet large numbers of MP's are just overkill.
One of the first things I tell students in my workshops regarding equipment is that you should be making equipment choices based on a complete system approach, not based on the latest camera body or numbers of MP's. The other is that the system should fit each persons needs and budget. Someone who makes twenty dives a year on a vacationing posts the images to Facebook won't need the level of equipment someone doing over 100 dives a year and making large prints would need.
Everything in U/w photography is a tradeoff, small body, lens and housing size V. better image quality, larger lenses, DOF, expense and more.
The argument that full frame equipment is so much bigger that it can't be contained in carryon luggage is a bit of a myth. I carry all of my Sony A7R III equipment onto the plane with little trouble or over weight issues. The photo compares the size differences between N85 EM1 II housing, Sony N100 housing and N120 DSLR housing, in the background is a Hasselblad medium formate housing which present issues.
