Quote Originally Posted by Steve Smith View Post
My father has just given me a Weston Master III meter in very good condition which seems accurate. Well, it was calibrated recently (1998!).

It comes with the dome for ambient measurement and also with a disc which appears to be a neutral density filter. When this is fitted, the numbers revert back to the same set as when the door is closed and it gives a similar reading to one taken with the door closed.

Can anyone tell me what it is called and what it was used for? i.e. in what circumstances would you use the ND filter instead of the closed door. Different angle of light acceptance perhaps?


You've nearly worked it out for yourself! The incident light attachment (Invercone) will not fit on the meter with the high-range baffle closed, so for a high-range incident reading, you open the baffle, insert the ND filter (which will not stay in place by itself) and fit the Invercone on top. For a low-range incident reading, it's baffle open and Invercone alone with no ND filter. From the Master IV onwards, Weston changed to the more logical and simpler solution of an Invercone that fits either with or without the baffle in place.