how te exposure meter works

[L K Keerthi Basu]

dear friends,

 

How a exposure meter works and its mechanism. Why the f-numbers are calibrated with root 2 variation.

[Guest Sean McVeigh]

  How a exposure meter works and its mechanism.  Why the f-numbers are calibrated with root 2 variation.

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Light meters use a photosensitive element like a phototransistor or photodiode to measure the amount of light presented to the meter and convert it to an electrical signal. The electrical signal is probably passed through a log-amp or variant thereof instead of being presented linearly. Anyways.. what you get in the end is typically an EI number, which when combined with an exposure speed and an emulsion speed gives you an F-stop number. The exact equation escapes me at the moment.

 

The reason the numbers are given in multiples of roots of 2 is that the amount of light falling on the sensor is directly related to the area of the aperature opening. Remember from high-school math that area is related to diameter or radius by a square law (A=Pi * r^2 for example). Working the other way around, as the area doubles, the radius/diameter only increases by a factor of the square root of 2. Likewise, as the area quadruples, the radius doubles (hence, 4x more light = 2x the aperture diameter). This is where the F-stop ratios come from. They relate directly to aperature diameter, not the area.

 

Or something thereabouts :)

 

-Sean