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candela vs luminance


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#1 Dafvid Skoogh

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Posted 25 May 2009 - 05:18 PM

Hey Guys!

I've been wondering about this candela measuringthing... how come we measure candela (or foot candles) when we talk about exposure? Talking about contrasts, I understand the good thing about measuring candela, but not with exposure.

The Luminance is the unit you describe how much light passes through an aperture. The candela does only describes how much light there is in a specific place and direction. But because a lens is always stealing some light, the candela can't be the right unit for this purpose...

Therefore, you should check the candela, when dealing with contrasts and the Luminance, when dealing with exposure.

Or have I got it all wrong? I've heard that the science behind exposure is one of the hardest things to understand when it gets to filming so maybe that's the reason I don't understand :P

Thank you for your response!
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#2 David Mullen ASC

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Posted 25 May 2009 - 05:26 PM

Except that a film camera is not a video camera, so there is no device inside the camera to measure luminance after it passes through the lens aperture, not while the lens is on the camera. So you measure scene brightness (either reflectance or incident) and calculate the aperture based on the shutter speed and ASA of the stock -- which brings up another problem with measuring luminance inside the camera, in theory besides the light level being changed by the glass transmission and aperture, it's changed by the shutter angle.
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#3 Dominic Case

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Posted 25 May 2009 - 06:00 PM

I've heard that the science behind exposure is one of the hardest things to understand

I'd certainly agree with that. The number of subtly different things to measure, and the number of similar-sounding units to measure each one, ensures confusion. I always reach for my notes before embarking on any discussion.

Briefly, the standard candle or CANDELA measures the total light emitted by a source.

It is different from the FOOT CANDLE which is a unit of ILLUMINATION. In metric, it's a metre candle, or lux. It's the illumination of a surface a foot away from a light source of one candela. This is the one that depends on distance - move further away from the source, the candelas of the source remain the same but the foot candles of the surface are less. Foot candles are what you measure with an incident light meter.

(BTW, illumination is also measured in lumens per square foot (or metre).

LUMINANCE measures the brightness of a surface. It depends on the illumination (foot candles) and on the reflectance of the surface. Measured in FOOT LAMBERTS, but also candelas per square metre, or nits. That is what a spot meter will give you.

So in effect, you can use candelas to measure the lighting ratio of your scene (key to fill etc) but if you are interested in the brightness ratio of the scene, you'll need a spotmeter and foot lamberts. This will tell you how many stops down the black curtains are, compared with the white lace ones for example.
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