I have recently made a 4x5 camera and found myself confused with bellows extension, effects of filters, and testing homemade lenses. I decided a through the lens meter would help me resolve these issues. After much trial and even more error I managed to construct two working prototypes. The principles are as follows if you wish to give it a go. Two basic parts are required. PROTOTYPE 1 1) An old 4x5 plastic cut film holder. 2) A lux meter one with the sensor on the end of a curly wire (cheap as possible) plenty of these on ebay. From the Lux Meter remove the photodiode and its attached curly wire from the sensor. It looks like a tiny glass square. Mount this photodiode centrally in the cut film holder. Put film holder in camera Swirch on lux meter. Calibrate with light meter. Draw table of results. PROTOTYPE 2 (MY PREFERRED OPTION) Replace photodiode with solar cell. I found one on ebay which was almost exactly 4x5. This amazingly gave me exact Lux readings.