Light Meter for Film Photography
Through-the-Lens Light Metering
When you look through the viewfinder of a 35mm film camera with a through-the-lens (TTL) light meter, you will see the meter to the side or bottom of the frame. You will set your aperture according to the depth of field you desire or the shutter speed if motion photography is the result. The light meter will automatically show where your aperture or shutter speed exposure is when you half-press the shutter release button. Next, you change the aperture or shutter speed to match the initial setting.
With the metering system, one light will indicate your shutter speed while the other indicates the aperture. You must simply choose the shutter speed or aperture you wish to use and then change the settings of the latter so that the red light matches up with the other. From this point, you can also change the aperture settings one or two stops for underexposure and overexposure. Overexposure is particularly helpful with winter photography.
TTL light metering is seriously flawed when conditions are extremely bright (beach and winter photography) or extremely dark (night photography). Because a TTL metering system is limited to the apertures and shutter speeds of your film camera, you will likely encounter times when you do not have a slow enough or fast enough shutter speed to correctly expose the film. When this happens, the arrows of the TTL meter will light up, indicating that you 
need to raise or lower your settings accordingly. This will, of course, affect depth of field and motion as you compensate to lower or raise the aperture or shutter speed.
If your 35mm film camera has an Auto shutter speed setting, or aperture-priority, you can still expose when the light meter is above or below the extremes. As this means you are relying completely on the camera to make the right choices, this would not be recommended, but it can be done. For the best light meter readings in extreme conditions, it would be advised that you purchase a handheld light meter to measure accordingly.
