As I know, for any lighting condition, there is a recommended/indicated/correct exposure value for the camera to render the object as middle gray.

The indicated exposure value can be calculated from the scene luminance and illuminance using the exposure equation described in Relationship of EV to lighting conditions .

If the calibration constants of two camera are the same, the indicated exposure values of these camera at the same lighting condition are the same. Canon and Nikon use the same value for these constants. Other manufactures use slightly different values.

For any lighting condition, the cameras with the same exposure value will receive the same exposure (i.e. the amount of light hitting the sensor measured in lux second).

So I guess the required exposures for all cameras to render the object as middle gray are the same (or slightly different in case the calibration constants are slightly different).

My first question: Is my guess right?

Since the exposure equations were used for film camera many years ago. I think this "required" exposure is quite universal for all cameras, whether they are film or digital.

My second question: What is the value of that "required" exposure (in lux second)?

  • \$\begingroup\$ My earlier answer (here) about stops between white/gray/blackpoint might (partially) answer your question... \$\endgroup\$
    – agtoever
    Aug 18, 2015 at 19:48
  • \$\begingroup\$ Just as in the film days, when different speed films required different EVs to render medium gray, in the digital age the base sensitivity of the sensor plays the same role as film speed. \$\endgroup\$
    – Michael C
    Aug 18, 2015 at 20:38

1 Answer 1


The exposure formula is:
The aperture, f/# = 1/ square root of the speed (was ASA, now is ISO).
For example: The f/# would be f/10 for ISO 100

Shutter speed = 1/Candles per square foot.
For example: 1/250 for 250 candles per square foot.

Use footlamberts for incident light. Use candles/sq. ft. times 10.76 for meter candles (lux)

These are the values that were used to produce a 0.3 density grey above base + fog density with standard sensitometric processing.

I believe that the electronic equivalent is 10% over dark current.


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