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Actually really easy to test. If you shoot in raw...
Just do a controlled test in constant light (no flickering fluorescents, not incandescent) from tripod against a large flat surface. The surface has to be in the center of the image to circumvent falloff issues (vignetting). In full manual mode off course, and no auto ISO.
Start at wide open, chose a shutter speed to get normal exposure. Grab a frame.
Stop down 1 full stop, double the shutter time. Grab a frame.
Stop down 1 full stop, double the shutter time. Grab a frame.
Stop down 1 full stop, double the shutter time. Grab a frame.
Stop down 1 full stop, double the shutter time. Grab a frame.
All images should be equally bright in the center, on the flat surface. Easy to check in your raw converter, or in PS. If they're not, your aperture mechanism isn't working as it should.
This has a very small error margin, and only with f/1.4 or faster lenses (due to pixel vignetting). On a typical f/1.2 you loose about 1/4-1/3 Ev WO with modern sensors (up to 1/2Ev with Canon sensors)
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