This week, we discus various words used to describe and measure light in photography and videography.
luminance illuminance luminous intensity luminous flux
Luminous Flux is how much light a light source emits, measured in lumens (lm). This could be the sun, a light bulb, a flash, etc.
Luminous Intensity is how much light travels in a certain direction, measured in candelas (cd). For example, the sun sends out light all over the galaxy, but chances are we are only interested in the light hitting around us.
Illuminance is how much of that light is hitting an object, measured in lux.
Luminance is how much light is reflected or emitted by an object or scene, measured in candela per square meter (cd/m²). Luminance can either be measured for the object or scene being photographed, or for the final print photo.
Brightness, on the other hand, is a judgement call made by a subject looking at an object or scene, which can be biased. This is usually made by comparing two objects to determine which is brighter and which is darker.
To better understand the difference, lets look at a cell phone or tablet screen. Ever noticed how bright your cell phone looks inside, but when you take it outside in the sun, you can barely see the screen? A very popular model of cell phones was measured at 536 cd/m². Inside the house, the screen looks very bright. But if you bring it outside on a sunny day at noon time, the sun’s 1.6×10^9 cd/m² luminance will make that same cell phone screen look dark.
Exposure is a measure of how much of that light reaches the camera per area, which is affected by the aperture, the shutter speed, and the luminance of the scene or object. A scene or object can be considered underexposed, if it is too dark, correctly exposed, or over-exposed, if it is too bright.
Exposure Value represents the aperture and shutter speed settings of a camera, and is measured in EV. An EV of 0 represents an aperture of 1 and a shutter speed of 1 second, where 1 EV represents a 1 stop of light difference.
Interestingly, Exposure Value does not mention the ISO light sensitivity of the film or sensor, so it is very typical to describe an Exposure value as 15 EV at ISO 100, which would give a correct exposure for a sunny scene based on the Sunny f/16 rule where in a sunny scene, a correct exposure can be obtained with an aperture of f/16 with the ISO and Shutter speed being equal (f/16, 1/100 secs, ISO 100 would work, as well as f/16, 1/2000 sec, ISO 2000).
Light Stops are used to compare Exposure and Exposure Value, where each stop of light represents double the intensity of light. Stops can be used to describe the aperture, shutter speed, and ISO settings used to take a photo, and are usually used in full stops, half stops, or third stops.
Come back next week for our next Photography Word of the Week!
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