Photography - writing with light.
There are many books that deal with exposure. A good many of them, and a few oft-recommended here, deal with a lot of knob-twiddling and don't explain the background reality behind exposure. The reality began with two gentlemen in the 1800s. Their names were Ferdinand Hurter and Vero Driffield. They studied how photosensitive material reacted to exposure to light and subsequent processing.
Now that we mentioned light, let's discuss light. Visible light is that light that falls within the electromagnetic spectrum. This spectrum contains all the forms of radiation. The portion of that spectrum that we as humans and photographers are most familiar with is electromagnetic radiation whose wavelength falls between 400 and 700 nanometers, approximately. Violet is on the 400 side and red towards the 700 side. The wavelength determines what color we perceive. Yes, I'm overlooking people who are color blind.
This light falls into two general categories, incident and reflected. Those subjects that we photograph are illuminated by incident light. The photograph records the light reflected by the subject we are photographing. There are a variety of ways light is reflected, diffuse and specular are two of the most familiar. Fortunately, we photographers have ways of measuring this reflected light in a useful manner.
The most useful manner in which to measure light we want to photograph is by measuring the reflected light. Measuring incident light neglects measuring the actual light that is going to form the photographic image. Though, measuring incident light can have its benefits. The reflected light is our main concern.
As we all know, white light contains all of the colors that are visible, and probably some that aren't. But, we won't worry about those. Let's stick to the visible light. For simplicity, we are going to generalize here. Before we go any further we need to define what it is we are measuring. We are measuring the luminance of a subject or scene that we wish to photograph. This luminance is, in reality, measured in candles-per-square-foot[1].
How much luminance a surface has is determined by the amount of incident light falling on it and the reflectance property of that surface. This reflectance is expressed as a percentage. A very dark surface may reflect less than 5% of the light falling on it. Whereas a white surface may reflect 95% of the light. Fred Archer and Ansel Adams formulated a way to divide this range of brightness into Zones. For photographic convenience, this range is divided into 11 Zones. They are 0 for black through X for pure white. Zone V is the middle of the scale, referred to as middle gray.
Now a word about light meters. Most light meters return a value that places the scene they measure in Zone V. Light meters have no way of knowing whether they are pointed at a black shirt in a shadow or a white shirt in bright daylight. The exposure they indicate will place the subject in middle gray or Zone V. This is an important concept to keep in mind.
Now, with this knowledge in mind, how is a photographic exposure determined? The key exposure is going to be thus:
The key f-stop is the square-root of the ISO.
The shutter speed is going to be the reciprocal of the number of foot-candles^2 (1/number of foot-candles^2) of reflected light.
Let's put this to practice using a spot meter, one that can measure a very small area. Additionally, this meter is going to give us a foot-candles reading of luminance. The photographic exposure value is determined by the number of foot-candles^2 and the ISO. Just as an experiment, I took a lit candle in a breezeless room, set my digital camera to ISO 100 and placed one foot from the lit candle. From the above, the f-stop should have been f/10, but I set my camera to f/11, as that was marked. The shutter was set to 1 second. 1/1 foot-candle. I then measured the RGB values of the recorded image. They were very close, but slightly below 127, as expected. This verified that the above relationship works[5].
So, let's measure a theoretical scene and know that we are using an ISO of 100. The scene measures 250 foot-candles^2. Knowing this our basic exposure is 1/250sec at f/10. This exposure will render whatever we measured as middle gray. Here's where some photographers may have a problem. Let's say we are going to photograph a sunlit hillside covered with snow. Daytime sunlight can vary between 5000 and 10,000 foot-candles[3]. Snow reflects approximately 80% of the light falling on it[4]. So the bright sunlight reflecting off snow can be 8000fc at its brightest. So, our exposure, again using ISO 100, is f/10 at 1/8000sec. However, the results are going to be disappointing, as the snow will be rendered a dismal gray and not the white one would expect.
Taking this into account, a proper rendering of the snow would require increased exposure when using color film or digital cameras. This will place the snow areas in the appropriate Zone. This is referred to as ETTR/EBTR (Expose To The Right/Expose Beyond The Right) Now, a bit of cautionary advice. Digital cameras do not seem to have a one to one relationship with Zone exposures. One f-stop may equate to, for example, 1 to 1-1/2 Zones, or a little more/less. The only way to determine how one's camera will react to additional exposure is through very meticulous testing. Keeping in mind that when one changes lenses, the amount of light transmitted through one lens at a set f-stop may not be exactly the same amount of light through another lens at the same f-stop. Remember. f-stops are a ratio of lens diameter (aperture) and focal length. The transmission qualities of the glass are not considered in determining f-stops. This incidentally is why motion picture lenses use T-stops (Transmission) instead of f-stops.
Incidentally, when using black and white negative film, the darkest area of the scene is measured and placed in the appropriate lower-numbered Zone. Then, processing will produce the appropriate negative densities to render the highlight values one desires in the print. With our digital cameras, the brightest part of the scene is placed through exposure and processing controls the darker areas of the scene.
The good part is that our cameras do all of this calculation for us. But, what lies beneath what they do is what we just covered above. Measure that snow-covered hill and add the correct additional exposure and the resulting image will look as one would expect. White snow with details, those being slight variations in the brightness. Of course, for this discussion, this is a very simplified example. Processing the resultant image is critical and an intimate part of producing a photograph.
How one meters a scene is very important. Spot metering provides the most accurate for setting exposure appropriately for a given scene. Average metering can provide a good exposure setting, but there may be inherent issues with highlights not containing details because of the overexposure of those areas.
So, there is the very basic foundation of how exposure is determined. There are other methods or "rules of thumb" approaches. They work but not as precisely as actually metering the scene's appropriate areas to determine one's f-stop and shutter speed.
[1] Candles-per-square-foot are derived from foot-candles of incident light in the following manner: If 100 foot-candles fall upon a diffusing surface of 50 percent reflectance, 50 foot-Lamberts will be reflected, and this quantity divided by pi yields about 16 candles/ft-squared. There are other units but they do not relate directly to exposure.[2]
[2] The Negative - Ansel Adams.
[3] orchidculture.com
[4] thenakedscientists.com
[5] No, this wasn't a standard candle nor did I use an NBS measuring device for the distance. However, what was used was close enough to verify the process.
Photography - writing with light. br br There ar... (
show quote)