Bart,
I assume your rule of thumb refers to glossy prints (the most demanding for sharpness)? Canvas prints, due to their surface texture, should be more forgiving, no?
Hi Slobodan,
Maybe. IMHO it depends a lot on the particular structure. In my experience, canvas is also capable of showing considerable detail, especially where the structure is relatively flat (maximum depth or height) instead of on a texture slope of a fiber.
My tests of other (e.g. matte) media, in a search of potential negative effects on output resolution when printing my resolution test target on less reflective surfaces (to avoid distracting reflections), shows me that resolution doesn't suffer as much as some would like us to believe. Even on matte media (e.g. Hahnemühle Photo Rag Matte FineArt, which has some structure), it can still resolve single line spaced 600 PPI structures. The only thing canvas adds is a bit of a slope where the detail is printed, but other than changing direction, it doesn't really diffuse that much (unless it is very coarse). The surface structure adds a bit of randomness, a visual distraction that can hide only a bit of the lacking detail. IMHO, of course ...
I do know that I'm a bit more critical than most ...
Cheers,
Bart