abpho wrote:
What is the difference between an M plan, CFI plan and a BD plan? Is this only pertinent to Nikon?
M Plan are for metallurgy. This means that they use a 210mm tube length rather than 160mm (so you set your bellows a bit longer) and they are designed to not need a cover slip (which is good).
CF means "chromatic free" and implies that the objective can be used without a matching eyepiece; all the oprtical correction happens in the objective (which is good, for macro).
CFI is the infinity form of CF.
BD is brightfield/darkfield. In terms of using a BD objective for macro, the practical implications are a) it may use a different sized scrw mount b) you may need to block up the illumination light path.