Light values for photography and exposure
Digital exposure using the histogram method:
now most digital cameras display RGB histograms after the photo has been taken - that is a graphical display of how much of the image is recorded at each brightness level for each of the 3 digital color channels Red, Green & Blue.
the main exposure problem in digital cameras is the loss of detail in over-exposed regions which cannot be retrieved by later processing (although if you shoot in RAW mode, some of it can be retrieved).
thus, most digital photographers prefer to check the histograms to ensure none of the main image peak goes past the right side of the histogram - although, inevitably with high contrast situations where there are 2 peaks, the right peak for very bright sky, reflections or the light source itself, will usually be to the right of the limit of the histogram and is thus ignored, while the right edge of the left peak representing your subject does not reach the right limit.
some cameras such as the Canon 1D Mark III even display this RGB histogram in live preview mode allowing you to optimise your exposure manually before taking the photo, rather than just adjusting your exposure and re-taking the photo.
light values is a way of estimating or determining the amount of light ambient on a scene which then enables one to predict what combination of aperture, shutter speed and ISO one will need for a given scene by using the S+L=A+T exposure system.
let's say you want to take photos of paintings in the Louvre and you have only the one day in your life to do it.
given that flash photography will be practically useless, and tripods not feasible, one will generally need to rely on hand-held photography, so will your camera/lens combination cut it in taking sharp photos without camera shake?
you could just buy the most expensive camera and hope it will do, but the alternative is if you know the light value that you are likely to be working with (which I give below), then you can determine your requirements more accurately.
given my calculations for paintings in the Louvre of -1 to +2 light value, this will mean that you can use 200ISO, f/2.8 and shutter speed will need to be 1/4 to 1/15th second which is too long hand held unless you have an image stabiliser or you use a higher ISO or wider aperture. Thus most consumer cameras would not be adequate to get good results with low noise.
this same principle can be applied to a multitude of situations such as a sports event or model photography using ambient light from the sky.
a professional photographer using a Canon 1D Mark III with a Canon EF 70-200mm f/2.8 L IS lens at 200mm:
can usually get away with using f/2.8 at 1/50th sec handheld with IS on (ie. A+T = 3+6 = 9) and thus the light levels he can shoot hand held will be determined by the ISO he selects:
ISO 100 => LV = 4 eg. sports ground with night lighting
ISO 400 => LV = 2
ISO 800 => LV = 1 eg. average living room lighting; heavy cloud,
ISO 1600 => LV = zero
ISO 3200 => LV = minus 1 eg. twilight
ISO 6400 => LV = minus 2
conversely, let's say he is shooting a sports ground under night lights with a LV of 4 and shooting at f/2.8, what is his fastest shutter speed to stop the action:
ISO 800 => EV = 8+4 = 12 => T = 12-3 = 9 which equates to 1/500th sec
ISO 1600 => EV = 9+4 = 13 => T = 13-3 = 10 which equates to 1/1000th sec
ISO 3200 => EV = 10+4 = 14 => T = 14-3 = 11 which equates to 1/2000th sec
ISO 6400 => EV = 11+4 = 15 => T = 15-3 = 12 which equates to 1/4000th sec
a portrait photographer using a Canon 1D Mark III with a Canon EF 85mm f/1.2 at f/1.2 for its DOF:
full sun at ISO 100: LV = 10, S = 5, A = 0.5 => T = 10+5-0.5 = 14.5 which equates to 1/20000th sec but as min. shutter is 1/8000th, this will be 1.5 stops over-exposed, thus need a ND or polariser filter but will not be able to use fill-in flash at that shutter speed.
the difference of 1 for a light value equates to 1 f-stop difference.
if you double the distance of a light source, this will result in the light reducing to 1/4 which equates to 2 f-stops less or 2LV less.
camera manufacturers often give exposure range for their light metering, auto focus capability, etc in EV ranges, to determine exposure value (EV) from LV, just add 5 to LV assuming you are using ISO value of 100 for determining EV.
The S+L = A+T exposure system:
An EV (exposure value) is equivalent to a combination of f/ratio, shutter speed at a given ISO value (usually 100ISO), with a value of 0 being 1 second at f/1.0., thus EV = S+L = A+T.
The S+L=A+T exposure system uses this concept, but expands on it to create a value for each of the 4 variables of Speed (film speed), Light intensity, Aperture & Time (shutter speed) such that an integer
difference in any of the values equates to 1 f stop difference, and the sum of S+L equals the sum of A+T which equals the EV at ISO 100:
Speed: ISO film rating of 100 has a value of 5, each time you double the ISO value, you increase this value by 1, conversely, halving it drops the value by 1, thus ISO 1600 = 9. (S = log2 (0.3 x ISO))
Light: bright sunlight = 10, for more see below (I use abbreviation LV for this) also LV = log2 (brightness in foot-Lamberts)
Aperture: f/4 = 4, each f-stop change gives an integer change in the value, thus f/8 = 6 as it is 2 stops different to f/4. A = log2 (f-stop2)
Time: 1sec = 0, each time you halve the shutter speed, you decrease exposure by 1 f-stop, and you add an integer to the T value, so that 1/125th = 7 and 1/1000th = 10. T = log2 (1/exposure time in seconds).
THUS, bright sunlight at 100ISO = 5 + 10 = 15EV, and thus any combination of A & T that sum to 15 can be used such as f/5.6 and 1/1000th sec.
of course you will need to adjust settings to take into account loss of light from any filters used, and if using film with long exposures, adjust for reciprocity failure (see below).
don't forget, I have a free exposure calculator to help you calculate this here.
Examples of light values:
|Light value (approx.)
add 5 for EV at ISO100
|Example ambient lighting scenes|
|9||subjects backlit by sun with light background to give a silhouette (try 7-10LV); photos of the moon +/- 1stops;|
|7||outdoor shade (light from midday blue sky)|
|6||overcast sky (6.5LV ~1000lumen/sq.m); subjects backlit by sun with dark background (try 5.5-7LV) - may need a warming filter;|
|5||light from blue sky at sunset; crescent moon; ie. landscapes immediately after sunset;|
|4||sports ground night lighting (3.5-4 eg. ISO 3200, f/2.8, 1/1600th sec); indoor lighting (3-5); Paris' Dome Church ceiling in daylight; Church stained glass window in daylight; twilight 30-45min after sunset of mid-toned subject - bracket +/- 2 fstops; neon lights;|
|3||under a 300W reading lamp|
|1||average living room lighting; heavy cloud, rain (0.6LV ~100lumen/sq.m); slit lamp for corneal flourescein;|
|0||the Louvre paintings (most are -1 to +2, eg. Mona Lisa 0.5, although some are very dimly lit at -2.5 to -2)|
|-1||night scenes of street lighting on buildings/streets (-2.5
to +0.5 eg. 1/15th sec, ISO 2500, f/4); twilight (~10lumen/sq.m)
St Peter's Basilica interior (1/15th sec: 1600ISO, f/4 if using Canon or 400ISO, f/2 if using Olympus)
|-2||light from a candle at 1 foot = 1 foot-candle; church cathedral interiors (eg. Notre Dame -3 to -1); fireworks;|
|-3||lightning +/- 2 stops;|
|-4||light from a candle at 2 feet = 0.25 foot-candle; distant view of city skyline lights;|
|-5||light from a candle at 1 meter = 1 lumen/sqm = 1 meter-candle = 0.09 foot-candle; deep twilight;|
|-7||night scene lit by full moon overhead (~0.27lumen/sq.m); photo of bright stars (~0.0001lumen/sq.m for -1.4 magnitude)|
|-11||photo of deep sky objects such as comet tails, nebulae (~0.0000001lumen/sq.m for 6th magnitude objects) - see astrophotography|
Zone system for reflective light metering:
|VIII||80%||+3||off white; show some details;|
|VII||70%||+2||light gray; shows distinct texture.|
|V||18%||0||mid-gray = setting for light meters|
|III||13%||-2||black skin; shows distinct texture in prints|
|II||9%||-3||very black skin; shows some details;|
|I||2%||-4||black velvet; no details;|
A few more concepts:
Very brief duration light sources:
What is the true ISO?:
Reciprocity failure compensation for most films:
Dealing with high contrast scenes: