If you did some research on regular 8mm cameras, you would realize that auto exposure in most of them is controlled by an external meter. This is sometimes problematic because the intensity of the measured light could be different from the one that go through the lens. The error can be caused by a hand inadvertently covering the sensor or by a lens casting shadow on it. TTL metering system eliminates such errors by placing the sensor inside the camera housing where it can measure light that come through the lens. Zoomex S is one of the very few regular 8mm cameras equipped with this TTL auto exposure feature. In addition, you can also set the aperture manually. FYI, the earlier model Zoomex II is equipped with an external meter. That's one way of recognizing the models.