It's not uncommon to use mixed grades on cameras such as ex++/+++. This implies that the camera is excellent ++ but has attributes which are excellent +++. When I have a beautiful camera with one annoying flaw, I'll list it as - excellent +++ with a ding on the top plate.
Mint - 100% - Absolutely flawless in every respect which includes optical and mechanical condition. You won't be able to tell this apart from a brand new camera. Most realistic collectors believe no such vintage camera exists.
Mint Minus - 99.9% - This is realistically the best condition a vintage camera can be. Expect one or two very light cosmetic marks - you will have to look closely to find them.
Excellent +++ - 98-99.9% - As above, but with a few more faint cosmetic marks. Unfortunately, I've sold quite a few camera's in this condition and seen others turn around and advertise them as mint!
Excellent ++ - 95-98% - This camera normally has scattered bright marks, light paint wear and/or light wear to the leather.
Excellent + - 90-95% - This camera has fairly nice cosmetics but you can expect bright marks, moderate paint wear and/or moderate leatherette wear.
Excellent - 80-89% - Cameras in this condition will have moderately worn leatherette and/or chrome. The paint will usually have small chips and/or brassing.
Excellent Minus - 70-79% - Cameras in this category are starting to look well-used. You'll find some moderate to heavy wear on leatherette, chrome and/or painted finishes.
Everything Below Excellent Minus - I try not to handle cameras in this condition unless they're something rare or unusual. These items are usually on the verge of being quite ugly if they aren't already.
