All units of light measurement are not suitable for photography because we need to adjust according to Film Speed. An unit calculated by f-number and shutter speed, EV (Exposure Value) is used. From measurement in EV, we can fix the film speed and find the corresponding f-number and shutter speed, e.g. with EV 13 and film speed ISO 100, we can use f/8 and 1/125 sec. or f/5.6 and 1/250 sec. These choices have the same exposure because the f-number and shutter speed have the same scale for exposure, difference between f/8 and f/5.6 is the same as the difference between 1/125 sec. and 1/250 sec.

There is no difference between a meter and the metering system in a camera for measurement. They have a different output only. Meter gives EV, some design may change the EV into a pairing of f-number and shutter speed or a pre-selected of one and the corresponding of the other. Camera metering system is used for automatic program, e.g. with Portrait mode, the priority is the largest f-number with the corresponding shutter speed. With aperture priority set at f/11, if the reading is EV 13, film speed ISO 100, the automatic shutter speed will be 1/60 sec. In manual mode, the reading will show by needle or LED showing the up or down or match against the setting.