Quote Originally Posted by polyglot View Post
Excellent addition to the app; this is what it really needs.

This is a good idea except that the sensor will be outside of the image area of the lens, which means you're metering the wrong thing. However if you wanted to modify an LF camera for example, you could take the Minolta approach of metering off the film - google for "OTF meter". Requires a little sensor and very simple optics inside the camera body to average the intensity of the light diffusing off the film during exposure and when there's "enough", ends the exposure. Minolta used that for TTL flash metering with no preflash and I believe another 35mm manufacturer (Olympus?) used it for metering long exposures under varying light; both were in the 1980's.
Although I read somewhere that Minolta invented the technique for flash exposure but Olympus had it first in their OM-2 and it did long exposure as well like you said. Most cameras with TTL flash auto exposure used this technique until the introduction of the digital SLR then the cameras would rely only on preflash. In the 90's a number of cameras used both off the film plane and preflash.