(This article is a supporting part of our ongoing testing of low-light camera metering reliability)
The Canon 5Ds R has relatively poor (but still within the range of our test!) low-light metering ability. It can meter properly to EV-1 and then abruptly fails. Unlike many cameras, it does not exhibit a smooth rolloff. The “peak” in the orange line between EV0 and EV-1 is an artifact of the interpolation and does not represent actual performance. EV-1 is double the light of a full moon and corresponds to a twilight level with only a couple stars visible. At scenes darker than EV-2, its images are over one stop darker than those auto-metered at EV0. Canon reports that its metering limit is “EV1 at 23°C/73°F with 50mm f/1.4 lens at ISO 100.” Our test shows that the camera performs well darker than that.
Like all DSLRs, the Canon 5Ds R stops down the lens to take the photo, but leaves it wide open the rest of the time. In our test, its metering performance improved when used with a lens faster than f/2.8. Some DSLRs do not improve when using lenses faster than 2.8.
The Canon 5Ds R has no flexibility for when to increase the ISO vs shutter speed in Auto ISO mode. The maximum allowable ISO can be set, though.
The plot shown above was created using the usual EF-mount Rokinon 50mm f/1.4 at f/2.8. The aperture-dependent metering phenomenon was tested by setting that lens to f/2 and f/1.4.
Integrating Sphere & Camera Metering Test Project