I took pictures of a still object with camera on tripod, shutter release cord, mirror lock up, etc. using a Zeiss 50mm Makro Planar ZF.2 and a Nikon 85mm f1.8 G lens.
For the Zeiss lens, the only variable that changed was the f stops: they were f/11, f/8, f/5.6, f/4, f2.8, and f/2.
I notice that from f/11 to f/4 the brightness of the images was about the same and that the histograms shifted slightly, but not materially. However, images taken at f/2.8 and 2.0 were darker than the preceding f stops and their corresponding histograms shifted to the left, with f/2 being a somewhat darker image than f/2.8.
When I repeated this using a Nikon 85mm f/1.8G lens, the image darkened and the noticeable left-shift of the histogram occurred from f2.8 to f/1.8.
Can anyone suggest reasons why images get darker at the lower/lowest f stops compared to higher f stops?