What are modern exposure measurement systems?
Virtually all modern small and medium format cameras have some form of an integrated system to measure the luminosity of the subject's light and thus evaluate the correct exposure.
This signal is when you have manually adjusted an appropriate combination of f-number and shutter velocity, or it.
Automatically sets:
the correct shutter speed for an opening that has been adjusted;
the appropriate opening for the shutter speed that you set;
an appropriate combination of shutter and aperture adjustments selected by the system as part of an integrated program.
A tiny light-sensitive measurement sensor can be placed in the front of the camera housing pointing directly at the subject near the lens, or sensors can be positioned inside the camera, sampling the light that came through the camera's lens. In both locations, the sensor must measure through any filter you can use on the lens.
An internal sensor system is more precise and works just as well when changing lenses. All modern exposure measurement systems require battery power.
It is generally completed with the same power circuit for autofocus, shutter synchronization, and film player.
You may need to adjust the sensitivity of the film you are using (its ISO speed), even if many 35mm cameras do so automatically, read a barcode onto the cassette through electrical contacts in the film supply compartment.