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Seth Baldwin

Converting Lux and Footcandles to F/T Stops

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Hey, this is probably a stupid question,

 

Does anyone know what value of foot candles or lux accurately equates to one stop of light. Obviously standard light metres can do this math for you,

But I'm seeking to better understand this math.

 

Cheers

Seth

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Each f-stop change is a doubling or halving of the amount of light exposing the film, and so would doubling or halving the number of footcandles.

 

The old rule is the 100 fc at 24 fps / 180 shutter using 100 ASA film gives you an exposure of f/2.8.

 

An f-stop by itself does not equal any amount of footcandles objectively without knowing ASA and exposure time.

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Footcandle and Lux have to do with the light going to an object you film, not with the light reflected by it.

 

We discern two measurements, the incident and the reflected light measurement. When measuring the incident light you know already something about the object’s reflectance. You replace the situation by the metering instrument which carries a white cup on the cell. Cup and cell are adjusted within the instrument’s circuit.

 

In many cases you replace the object by a grey chart that has a known reflectance. Often it is fifty percent, the so-called 18-percent grey chart. That’s a little perplexing, you get half the incident light from a surface that is 18 percent dark. The reason for this is that the black of the printed dots isn’t dead black and the white of the carton isn’t perfect white. Now you make on overall reading of the light coming from the object towards the camera.

 

F(ocal) and T(ransmission) stops are relative values. An f stop is the diameter of the iris opening calculated against the focal length of the lens. F/5.6 means 100 mm in the case of a 100 mm focal length lens divided by the square root of 32 (√32 = 5.6568), that is 17.677 mm. Photographers and cinematographers actually only care about the figure 5.6 because the openings of the different lenses vary and thus are not interesting. A T stop is a value given by the lens manufacturer who makes measurements of the light transmitted by the lens. Contrary to the purely geometric f stop figures T stops include light loss by the glass elements and everything else with a given lens. F stops are considered accurate enough for amateur filmmaking.

 

To finalize the calculation you mean you need to know the film sensitivity or exposure index as it’s also called, the exposure time given by the shutter angle of your movie camera, and the frame rate. For example, at 24 frames per second and 160 degrees shutter opening the exposure time is [(360°/ 160°) × 24] ‒1 = 1/54 second.

 

To answer your question: One diaphragm stop corresponds to double or half a Lux value but they are not directly related.

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