TTL view-finding, focusing and metering are standard. I have been working on taking the TTL concept to the next level with through the lens flash. As a proof of concept I am working to modify an SLR for TTL flash. My approach is to mount a flash to project its light through the viewing system, bouncing the light off the mirror and through the lens. Conceptually one envisions the output of the flash progressing in reverse through the view-finding system and out the lens to the subject, then returning to the camera with the mirror up and thus onto the film/sensor. Unfortunately, due to the great speed of light, this system is unworkable. Modifying this approach I plan to replace the mirror with a 45 degree prism. This means that the prism's glass has to be selected so that the reflection is not total but acts as a beam splitter. This has an unwanted side effect. Since the reflector is a beam splitter part of the outward bound light can return to bounce off the underside of the splitter and onto the film. Obviously all of the light that is transmitted through the splitter (i.e., not reflected toward the subject) on its outward journey must be absorbed. Thankfully, physics provides the answer in the form of a cavity, also known as a black body. The physicists among you will immediately realize that cavities absorbs all light. In my prototype I will lose TTL view-finding, focusing and metering, but this is only a proof of concept model. Once I have the basics down I'll build another camera which retains these features.