The precision of a hand-held spot meter is generally measured in degrees, but the in-camera spot meters in advanced cameras are usually measured in percentage of the frame. This is because the actual angle of view of the spot meter changes depending on the focal length of the lens you have on the camera. This feature calculates an approximation of the metering angle for a given camera and lens.
This calculation is closely related to the angles of view calculation, but the process of reducing the problem to an AoV calculation is tricky enough to justify a separate feature.
Don’t let the name limit how you use this calculator. It should also be useful for answering other questions, such as those having to do with a camera’s autofocus sensors.
This formula gives the approximate diameter of the camera’s meter d given the imaging frame’s area A and a percentage of that frame p. It’s approximate because we don’t really know the shape of the sensor. It’s probably some sort of rough oval, but to keep the calculator simple, we assume that it’s circular. This should be close enough to the truth that the calculator is still useful. This approximation lets us figure the sensor’s radius by running the familiar πr2 formula backwards to calculate the sensor’s radius from the percentage of the frame area Ap, then multiplying by 2 to get the diameter.
Once we have the sensor’s diameter, we’ve reduced it to an angle of view calculation. Our ideal round meter’s diameter goes into Equation 6.1, “angle of view” as d, and the focal length goes in as f, giving the meter’s approximate angle of view.