What is light? There is the anthropic definition of light which is: that which interacts with objects and the eye to allow vision. There is also a more rigorous physical definition that uses terms like electromagnetic radiation and aggregate photons. No matter what terms you choose to describe it, light is a luminous, radiant phenomenon that interacts with matter in definite, predictable ways. It is also a magical medium that is key to the art of photography.
Photography works with image, texture, and relative brightness. The quantity of light emanating from a surface is recorded on developed film as differing optical densities of the emulsion. This modulation of density across the frame in turn meters the amount of light that passes through the film to make a print or a projected image on a viewing screen. The amount of light emanating from the object being photographed, combined with the intended qualities of the desired final image, determine the camera settings that need to be used to obtain that image. Assuming that you know what you want the final image to look like, knowing how much light you have to work with, and then how to adjust the camera to turn that light into the final image, gives you the power to complete the task before you to a high degree of accomplishment.
I have a 40 watt incandescent light bulb in my broom closet that is rated (or so labeled on the box) as having a light output of 445 lumens. Those are the units in which the total radiated light from a luminous source is measured. Light from such a source radiates in all directions. If it radiates in all directions evenly, it is termed an isotropic emitter, the term isotropic meaning “evenly and uniformly in all directions “.
Now assume we have a light bulb that radiates 4 p (that is 4 x 3.1416) lumens isotropically (i.e. evenly in all directions). While that would be an extremely dim light bulb, choosing that brightness allows us to follow the light in terms of unit brightness. The angular extent of a spherical surface, seen from the center of the sphere, is 4 p steradians. (One steradian is the unit of area on a sphere with a radius of one, like the radian is the unit of length on the circumference of a circle with a radius of one.) Thus a bulb radiating a total of 4 p lumens radiates one lumen onto one steradian of the unit sphere. In such a situation, an observer looking at the light bulb would see a luminous intensity (or brightness) of one candela in the SI system, or one standard candle in the old English system. Light emanating from that light bulb falls onto an object located one foot away with an intensity of one foot-candle. Light intensity falls off as the square of the distance from a light source, so an object located two feet away from the same light bulb would have light intensity at one-fourth foot-candle falling on it.
Now assume that I place my 40 watt incandescent bulb into the lamp standing behind my chair. The pad of paper I am writing on is located four feet from the lamp. Thus the light falls on my paper pad with an intensity of
(Note: I still use the old units instead of SI. If you need SI, then be prepared to multiply foot-candle by 10.764 to obtain lux (meter-candela) and candle-per-square-foot values by 10.764 to obtain nit (candela-per-square-meter)).
Light that falls from a source to an object is termed incident light. The quantity of such light is termed illuminance. It is customarily represented by the symbolic variable I, and is measured in units of foot-candles in the old English system, or meter-candelas, otherwise known as lux, in the SI system. One lux is also called one lumen per square meter, which relates it back to ultimate source brightness.
Light that falls onto an object is reflected to a certain degree. There are various ways in which light can be reflected, however for our purposes we will only consider specular and diffuse reflection.
Specular reflection is reflection from a mirror-like surface, in which an in-falling light ray is reflected away very much as it came to the surface. The term “specular” refers to a “speculum”, which is a polished metal surface used as a mirror prior to the invention of modern metal-coated glass mirrors. You know you have a specular reflection if you can see a virtual image of the light source hanging there on the reflecting surface. A perfectly specular surface will reflect in-falling light at the same angle relative to the plane of the reflecting surface, but in opposite direction. In optical physics, this is called Snell’s Law.
Diffuse reflection is reflection that scatters the in-falling light ray, so that the reflected light energy is leaves the surface in many directions. There are various degrees of diffuse reflection. A weakly diffuse surface scatters light more-or-less in some specific direction, but unlike a specular reflection the light is not a single ray anymore. A strongly diffuse reflection scatters in-falling light in all directions rather uniformly. The hypothetical perfect diffuse reflector is termed a Lambertian reflector, which scatters in-falling light in all directions evenly, dividing the incoming beam across an entire hemisphere.
Reflectance is customarily represented by the symbolic variable r or “rho”, which is the Greek version of the letter r. It is given a subscript s when we are discussing specular reflection, or a subscript t when discussing total reflection. For our purposes, we will not add a subscript when discussing diffuse reflection alone. Reflectance is a unit-less variable, being a scaling factor that takes values ranging from zero to one. A perfectly black surface would have a reflectance of zero, while a perfect mirror would have a reflectance value of one. A standard photographic tool is the Kodak Gray Card, which has a reflectance of 18%, which is considered “middle gray”.
Reflected light is termed luminance , and is represented by the symbolic variable E. Luminance is calculated by multiplying the illuminance by the reflectance. Luminance that is measured in only one direction is measured in foot-Lamberts. The luminance in all directions from a perfect Lambertian reflector is obtained by dividing the foot-Lambert value by p (the Greek letter pi, or the ratio of the circumference to the diameter of a circle, given the approximate value 3.1416). Thus diffusely reflected luminance, or Surface Brightness, is obtained by the formula
and is in units of candles per square foot, or caldelas per square meter (a unit also called the nit) in the SI system.
Remember my 40 watt bulb in the lamb behind my chair? Assuming that my paper pad has a reflectance of 72%, the surface brightness of my paper pad turns out to be 0.51 candles per square foot.
Things tend to respond logarithmically to an increase in light level. For example, if one step in visual brightness increase as perceived by the eye is defined as two times the brightness, then two steps is actually four times the brightness. The camera works in the same way, where a one-stop adjustment in either lens relative aperture (f-stop) or shutter speed provides twice as much light. Logarithmic relationships can be hard to deal with, so it is convenient to define a linear scale for light intensity when dealing with light and cameras.
Many years ago, camera manufactures settled on the Exposure Value scale. The EV scale is based on the fact that specific combinations of f-stop and shutter speed provide the same exposure on the film, within the limits of the film exposure reciprocity law. Thus exposures of 1/30 at f/16, 1/15 at f/22, 1/125 at f/8, etc, all give equivalent exposure, and they correspond to EV 13. While this provides a convenient way of talking about camera settings, EV numbers cannot be used to talk about light levels without including the speed of the film. Many exposure meters are calibrated in EV values, with EV=10 corresponding to a measured luminance of 10 candles per square foot, but this nomenclature is only correct when a film speed of ISO 100 is being used.
Recently, several attempts have been made at establishing a Light Value (or Luminance Value, to be more precise) scale to put incident light or surface brightness in terms of a linear scale that corresponds to the Exposure Value scale. I strongly favor the LV scale that is based on LV = EV when ISO 100 film is used, since this then allows meters that have been calibrated in terms of Exposure Value to be used as Light Value meters. When this is accepted as the convention, the whole business falls together neatly into the following relations.
Light Value can be defined in terms of Surface brightness (in candles per square foot) as
Exposure Value can be defined in terms of Light Value and Film Speed as
EV = LV + EI Adjustment
Exposure value can be defined in terms of shutter speed and lens relative
aperture (f-stop) as
The equation that puts all the pieces together in terms of Light Value (LV), film speed adjustment, and the aperture and time components of EV, is this:
LV + (film speed adjustment) = EV = EVA + EVT
(Note that this closely follows the exact film exposure formula, E x S = (1/t) x r2, where E is the subject Zone V surface brightness in candles per square foot, S is the film ISO speed rating, t is the exposure time in seconds, and r is the lens focal ratio, or f-stop. The terms in the exact formula are logarithmic, and so are multiplied together instead of being added together like the linear terms of the LV/EV relation.)
This convention maintains all of the loose definitions that have been used over the years. For example, an LV=10 surface brightness has a value of E=10 candles per square foot. Similarly, the adjustment for film speed is zero stops at ISO 100, a one stop reduction for ISO 50 film, and a one stop increase for ISO 200 film. And the EV scale gives the same camera settings it has always done, with the base value of EV=0 equivalent to an exposure of 1 second at relative aperture f/1.0. Thus a change in LV of 1.0 corresponds to a one stop change in brightness, and a change in EV of 1.0 corresponds to a one stop adjustment of the camera.
Note that my chosen method is different from the Additive Photographic Exposure System (or APES), which uses Brightness Values (Bv) based on subject brightness in foot-lamberts, and a film speed adjustment with a zero value at a speed rating of ISO 3. I would gladly use the APES if my light meters read out in light values based on that system. However, to avoid constantly adjusting the light meter readout, I have chosen to depart from the APES in my own work.
So what does this all mean in terms of actual numbers? The following LV values correspond to various brightness levels and situations (once again assuming reflectance from an 18% gray card):
|Light Value||Surface Brightness||Situation|
|-10.00||9.53674E-06||Crescent Moon: a=135, k=.2, z=60, d=nominal, LV=-9.51|
|-7.00||7.62939E-05||Quarter Moon: a=90, k=.2, z=45, d=nominal, LV=-6.38|
|-5.00||0.000305176||Gibbous Moon: a=45, k=.2, z=30, d=nominal, LV=-4.43|
|-3.50||0.000863167||Average Full Moon: a=4, k=.2, z=15, d=nominal, LV=-2.76|
|-1.80||0.002804439||Brightest Moonlight: a=0, k=.11, z=0, d=minimum, LV=-1.98|
|0.00||0.009765625||Edge-of-town scenes with light sources at least fifty yards away.|
|1.00||0.01953125||Dark street scene|
|2.00||0.0390625||Typical night street scene|
|3.00||0.078125||Brightly lit night street scene|
|7.00||1.25||My pad of paper, located 4 feet from a 100 watt light bulb.|
|10.00||10||Very dark overcast day.|
|13.00||80||Typical daylight open shadows|
|14.00||160||Side light for a few hours after sunrise or before sunset|
|15.00||320||Typical middle gray subject in bright sunlight (Sunny 16 Rule)|
|16.00||640||Light skin in full sunlight|
|17.00||1280||Most near-white objects in sunlight|
|18.00||2560||Bright ice clouds reflecting full sunlight|
|19.00||5120||Specular sun glints off of chrome, ice, or deep water|
|20.00||10240||A light bulb|
Once again returning to my lamp and my paper pad, if the surface brightness of my pad is 0.51 candles per square foot, then its brightness has an LV of 5.7. The surface brightness of a more typical working surface would be illuminated by a 100 watt bulb, having a light output of 1115 lumens. Working through the numbers, my pad of paper in such conditions would have an LV of 7.
The following table gives the correction for film speed (EV = LV + Film Speed Adjustment)
|Film Speed Adjustment||Film Speed|
And then, of course, there is the conventional tables of EV values for setting the camera. For shutter speed:
|(stops)|| h = hours
m = minutes
s = seconds
|(stops)|| h = hours
m = minutes
s = seconds
|-14.14||5.00 h||-4.91||30 s|
|-13.81||4.00 h||-3.91||15 s|
|-13.40||3.00 h||-3.81||14 s|
|-13.14||2.50 h||-3.70||13 s|
|-12.81||2.00 h||-3.58||12 s|
|-12.40||1.50 h||-3.46||11 s|
|-11.81||1.00 h||-3.32||10 s|
|-11.40||0.75 h||-3.17||9 s|
|-10.81||0.50 h||-3||8 s|
|-9.81||0.25 h||-2.81||7 s|
|-9.23||10 m||-2.58||6 s|
|-9.08||9 m||-2.32||5 s|
|-8.91||8 m||-2||4 s|
|-8.71||7 m||-1.58||3 s|
|-8.49||6 m||-1||2 s|
|-8.23||5.0 m||0||1 s|
|-8.08||4.5 m||1||1/2 s|
|-7.91||4.0 m||2||1/4 s|
|-7.71||3.5 m||3||1/8 s|
|-7.49||3.0 m||4||1/16 s|
|-7.23||2.5 m||5||1/32 s|
|-6.91||2.00 m||6||1/64 s|
|-6.71||1.75 m||7||1/128 s|
|-6.49||1.50 m||8||1/256 s|
|-6.23||1.25 m||9||1/512 s|
|-5.91||1.00 m||10||1/1024 s|
|-5.49||45 s||11||1/2048 s|
And for lens relative aperture, or f-stop:
Note that to get overall exposure value, you have to add the EV from the shutter speed to the EV for the lens f-stop:
Let’s assume that I have a camera set up to take a picture of my paper pad. We already calculated the surface brightness to be LV 7 (I switched to the 100 watt bulb so as to be able to see what I am doing). My camera has Fuji Acros 100 in it, which I rate at a speed of ISO 64 due to the way I develop it (in Rodinal 1:50). I want to use a lens f-stop of f/5.6, since that is the f-stop at which my lens has the greatest sharpness. What exposure time to I need?
Since EV = LV + ISO Adjustment, the overall EV is 7 - 2/3 or 6.33. However, I don’t want the paper to turn out a middle gray in the photograph, I want something more realistic, meaning two stops brighter, so that the resulting image looks and feels like an image of white paper. Thus I need to make an adjustment to be sure we get two stops more light onto the film. The best way to do this is to reduce the EV by another 2 stops, or to 4.33. The EVA for the lens f-stop is 5.00 (from the table above). So the EVT for the shutter speed is 4.33 - 5.00 or
-0.67. Looking that up in the shutter speed EVT table above gives somewhere between 1 and 2 seconds. If I use the formula, I calculate it to be 1.6 second. Since I can’t set my shutter to exactly that time, the smart thing to do is use 2 seconds (EVT = -1.00) and close down the lens 1/3 stop to f/6.3 (EVA = 5.33). That gives the camera setting EV = -1.00 (shutter speed) + 5.33 (f-stop) = 4.33, which is what you want. Of course, you also need to apply any correction for film reciprocity failure to the exposure time, if any is needed, since the exposure time is greater than 0.10 second. I’ll assume that the film I’m using has no reciprocity failure in this range and just go with the 2 second exposure, since the error is not likely to be large enough to notice. (Most current T-grain films have pretty good reciprocity effects out to several seconds, though testing is necessary if you really want to understand what your favorite film will do under low-light working conditions. See my Reciprocity Failure page on this site for a description of how to perform this testing using equipment most amateur photographers possess.)
So there we have it! We have managed to go from knowing the brightness of the light source and the desired result for an object being photographed, all the way to the camera settings given various factors like film speed and the desired lens f-stop. In essence, this is what your light meter does for you. However, there are some circumstances where the amount of available light is too low for your light meter to pick up. If you can determine the brightness of your light source, you can use these methods to figure out what camera settings to use to obtain the desired image. This is what I do with my moonlight landscape photography. My NightLandscape computer program calculates the amount of light available from the moon, given the moon phase, position in the sky, and other factors. I can then take that information and determine the camera settings required to get the result I desire by applying the methods discussed above. It may seem like a lot of bother, however the consistent results, and the ability to accurately pre-visualize what a scene will look like as a final photograph and then go get that desired effect on the first try, make the tedium of applying this method very much worth while. There is nothing like getting film back from the lab after a moonlight photo shoot and seeing everything just not quite right, and knowing that you will have to wait at least another month for the moon to be right to try again. Chances are that the next full moon will be clouded out, so that you will have to wait yet another month. And I typically drive several hundred miles to get to my desired scene for a typical moonlight shoot. You bet it is worth doing a little homework ahead of time so as to be able to get the results right the first time.
C. D. "Kit" Courter
Torrance, California, USA.
October 25, 2003
There are many other web pages out there on APES, EV, and some on LV. Do a Google search on these as keywords and you will have many sites to pick from.
Here are a couple of good technical pages on measuring light intensity and brightnessRadiometry and photometry in astronomy - A good technical site on measuring brightness. Though be forewarned: it is Very Technical!
Ansel Adams, "The Negative", New York Graphic Society, Boston, 1981
Gordon, Barry, "Astrophotography (featuring the fx system of exposure determination)", second edition, Willmann-Bell, Richmond (VA), 1985
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