Not sure if I'm mis-reading this a surprised reaction, but if so, using light reflected from the film inside the camera body _is_ the standard TTL technology.
https://mir.com.my/rb/photography/hardwares/classics/nikonfe....
https://mir.com.my/rb/photography/hardwares/classics/nikonfe....
> Yep, it applies to almost every AE SLR ever made. Notable exceptions were cameras like the OM-4 that metered off the film during exposure, or others that meter off the closed shutter curtain before exposure. But the vast majority meter from the focus screen so are susceptible to light entering through the eyepiece.