Question for the group about Mamiya C-series lenses. Legend has it that the chrome shutter ones with yellowish coatings are single coated while the black shutter are multi-coated (I've seen both yellowish and purpleish coatings). Here's my question: What was Mamiya doing single coating lenses in the 1970s? Wasn't multicoating ubiquitous by then? Is it possible that the chrome lenses really are multicoated and this is just rumor? I have certainly had excellent results with my chrome 80mm, enough so that I sold the black one and do not regret it (it was actually worse; perhaps sample variation).
To be clear the yellowish and purplish colors are the colors of reflected light, like if you shine a flashlight into the lens and look at the reflection. Typically single-coated lenses will show yellow or purple-tinted reflections while multicoated lenses may show a variety of colors including green, red, yellow. I don't know the reasons for the different color reflections well, but keep in mind that the reflection off a coated surface is at 1% or less intensity, so it's not affecting the transmitted spectrum significantly.
The coating (single or multi) should not significantly color the transmitted light or image. If anyone has a strong yellowish cast on images shot with a single-coated lens, it is possible that the coating or lens is damaged (eg there are certain 60s-70s lenses containing thorium oxide that can be radiation damaged and acquire a yellow or brown tint, which can be cleared by UV exposure).
Multicoating on 35mm SLR and MF lenses becomes common to ubiquitous by the mid-late 1970s, but in the early to mid-70s there are still plenty of single-coated lenses. Mamiya is not unique in that. I'm skeptical of interpretations based on the color of the shutter, the "C" letter on the lens is perhaps a better indicator.
The reflection off one uncoated glass-to-air surface is about 4%, for a single coated glass to air surface is about 1%, and for multicoated perhaps 0.5% or less. This compounds with the multiple surfaces; for example for a 3-group lens (like a tessar or triplet), the uncoated lens has ~ (0.96)^6 = 78% transmission, a single coated lens has ~ 94% transmission, a multicoated lens has ~97% transmission. The problem with uncoated glass is not the lower transmission per se, but that much of the 22% reflected light is bouncing around inside the lens making flare. You can see that the difference between uncoated and single-coated is much bigger than the difference between single and multi-coated.
Also, clearly, the number of groups matters. As you get to more complex lenses, multicoating becomes more important. Especially zoom lenses, mostly for 35mm. So many people will use a single-coated lens of relatively simple construction (like a Tessar or a double-Gauss normal lens) and never feel the need for anything more. However, if someone is doing commercial interior photography with a wide angle lens like a Super Angulon/Biogon type and has lots of light sources in the frame all the time, maybe they want the later multicoated version.
