The Sony FE 55mm f/1.8 Sonnar shows a stunning performance wide open through ƒ/2.8.
But alas, there is seen starting at ƒ/4* a major degradation, which appears to be shutter vibration. The problem shutter speed is documented in the above series.
A confirming diagnosis comes from the same series shot with a different lens (Leica 50/2 APO) showing a similar degradation at the same shutter speed and aperture. It’s incredibly disappointing: we are talking 55mm here. Checking the problem shutter speed with previous work suggests that 35mm might not be immune.
A cross-shoot using the 24-megapixel Sony A7**, e.g. should show zero degradation at any shutter speed, which is in fact the case even at 180mm and 280mm (personally tested) and even 800mm (a NYC pro I trust for that 800mm figure). So on the A7, one can expect to see perfection across the aperture and shutter speed range.
Another confirming diagnosis is to obtain both perfect and impaired results at the same aperture at different shutter speeds; this can be done with a series of ND filters or a continuously variable lighting source (which I don’t have at present). And that is tomorrow’s project, with ND filters.
Taking the landscape orientation 180mm focal length results showing a ~1 pixel blur (e.g., camera displacement by about 5 microns), we can deduce a 55/180 fractional blur = 1/3 pixel, an idea based on some angular rotation causes by the shutter shock which of course would affect a long lens more in pixel terms. About 1/3 pixel looks to be about what I’m seeing with the 55/1.8 on the A7R at the problem speeds. It’s like a very strong anti-aliasing filter on the Red Barn scene.
UPDATE: I have confirmed a loss of sharpness under ideal (for the camera) studio conditions at ƒ/4, varying lighting intensity, but keeping the aperture constant and the focus on manual. Field conditions cannot be better than this studio setup, only worse. Proofs will follow; I am preparing a detailed analysis to present.
See also the dolls study.
* Aperture ƒ/4 should offer the very best performance of all, and ƒ/5.6 nearly so.
** The Sony A7 offers a vibration-free electronic first curtain shutter.
Physics are physics
General discussion on principle; mass has its own influence and the camera system (camera + its internals + lens + mount) is far from a rigid block.
A shock acting against a mass will vector that mass. Kick your car and it moves. Kick a bus and it moves*. Not very much, but a force acting on a mass will not be denied.
* For politicians, kick the can down the road.