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?
You don't say which exposure mode you were using, nor whether you had Auto ISO enabled, but you need to check the aperture, shutter speed and ISO values for each of the shots to see what was really going on.
Metering mode was matrix. The camera was not moved, the light source was constant, focus point was not moved. Light source was a mix of indoor lighting and light from 2 nearby windows. All I did was change the f stops on the camera.
This afternoon I went outside and took hand-held pictures of an almost blue sky. ISO was lowered to 100, but the same f stops described above were used. There was a drop off in brightness going from f/2.8 to f/1.8, but not as much as that seen with the indoor shots.
I repeated this outdoor shooting by taking pictures of a lush hemlock bush...again, there was brightness fall-off between f/2.8 and f/1.8 that was somewhat more pronounced than that seen with the blue sky.
Are you talking about an overall full frame darkening or a more pronounced vignetting with the central part of the frame unchanged at larger apertures?
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