Basic Exposure Formula

1. We need to determine a method for selecting which f/stop and shutter speed combination we will use to get normal exposures.  The method is called The Basic Exposure Formula and is discussed in greater detailbelow.  First, we have to define what f/stops and shutter speeds mean and develop an understanding of their relationship to each other.  They will be our primary tools for controlling exposure without relying on our camera metering systems.
   
   
2. There are two primary variables that control how much light the film will receive.  Actually, there really are three variables, the third being ISO, or film speed.
   
   
   1. Apertures (f/stops); see f/stop table.
   2. Shutter Speeds; see shutterspeed table.
   3. Full stop adjustments of the these variables will allow either twice as much light or half as much light to the film plane.
      
      
3. Your built in meter (assuming your camera has one) is a reflective light meter and is based on evaluating light in scenes that have a 'normal brightness range'.  If your scene does not represent a 'normal' brightness range you will notice that the resulting exposure will be darker than or lighter than the true values that originally existed in the scene – this is also why we must learn to be 'smarter' than our meters. Meters only 'interpret' light (energy) relative to the film speed we're using. A reflective light meter would 'see' (interpret) an 18% grey card (the basis for measurement in all meters; called 'middle grey') as 18% grey; but it would also 'see' or read a white snow scene as 18% grey and it would 'see' a black building as 18% grey if that building filled the viewfinder's frame.
   
   
4. To obtain a normal exposure on film without the benefit of a light meter, we must first assess if the conditions are 'normal'.  The easiest light condition to assess is a bright sunny day as most people can identify this condition accurately.  It gets trickier if we try to determine a normal exposure on a 'slightly hazy' day, or a 'very hazy' day or a 'partially overcast' day, etc.  Fortunately, there is a formula that we can apply to determine the correct combination of aperture and shutterspeed to produce a normal exposure on bright sunny days.  This is called BEF or Basic Exposure Formula (some call it the "sunny 16 rule").
   
   
5. BEF or Basic Exposure Formula is a rule of thumb for determining correct exposure for film without using your camera meter. Every film has an ISO number associated with it. The ISO references film speed or how much light it needs to create an accurately exposed image. Every film needs just the right amount of light striking the film plane for a correctly exposed image. Too much light and film is overexposed (looks light and washed out) and too little it is underexposed (looks dark and muddy). The total amount of light hitting the film is determined by two settings on your camera, the aperture and the shutter speed.
   
   
   1. Example: ISO 100 speed film on a sunny day with the subject in direct sunlight, needs an exposure combination of f/16 at 1/100th of a second to produce a normal exposure. Since we know that Santa Barbara often has some amount of haze and since we are using color negative film (which has more latitude and exposure flexibility than color transparency) we can set 1/125th as the closest shutter speed.  1/125 is a little faster than 1/100 and therefore it lets in a little less light than 1/100 (1/3 of a stop less), we must compensate for this loss of light to the film plane by selecting f/11 (a larger lens opening than f/16, larger by one full stop) as the accompanying f/stop to produce a normal exposure.  Color negative film always responds better to a little more light than normal exposure rather than a little less light.  Color slide film (transparency film) on the other hand responds better with a little less light than more light.BEF = 1/ISO @ f/16.  Here's the translation.  1/ISO means put the number one over the ISO (film speed) of the film you are using and this fraction becomes the shutter speed you should use.  @ f/16 means that f/16 is the f/stop you should use in combination with the shutterspeed you just formulated.  Example:  ISO = 125; then a normal exposure will result from a setting of 1/125 (shutterspeed) @ f/16 (aperture) in bright sunny conditions.
      
      
6. Apertures: The aperture is simply the size of the opening that lets light through the lens. The shutter speed is the time that the light coming through the lens is allowed to hit the film. These concepts will be explored in more detail later. For now though, we will not use our camera meter but will use BEF formula to determine the correct exposure for any film. You will have to figure out how to set your camera to a manual mode. This mode will bypass any information sent from your cameras internal meter allowing you to set both the aperture and shutter speed.
   
   The number for the aperture that you will set is called an f-stop. For the rest of this course, aperture refers to the size of the opening in the lens while f-stop is number that you set.  The number represents a fraction and is displayed like this: f/2 or f/8.  It translates into a fraction by replacing the f with a 1 and now we see these f/stops actually refer to 1/2 and 1/8.  It is a fraction that relates to the actual size of the lens opening.  Since we can put different lenses on our cameras (or select different focal lengths on a zoom lens) it is necessary to express the actual size of the aperture opening in relative terms; thus f/2 instead of 1/2.  To take this further, if you had a 50mm lens on your camera and you set your aperture to f/2, you would be setting an aperture diameter of 25mm (1/2 of 50mm = 25mm).  If you selected f/2 on a 100mm lens that opening would be 50mm in diameter (1/2 of 100mm = 50mm).  f/2 on a 300mm telephoto lens would have aperture with a 150mm diameter, and so on. Because the f-stop should be considered a fraction, i.e. f-8.0 can be considered 1/8 and f2 is 1/2, we can better understand why f/2 is a larger opening than f/8; because 1/2 is larger than 1/8.
   
   Viewing the Aperture Diagram we see that when the aperture gets smaller and lets in less light the f-stop number gets larger.  F/8.0 is a larger number than f /2.0 yet the aperture is smaller and lets in less light. When you convert the f-stops to fractions, it makes more sense, 1/8 is smaller than 1/2, hence f-8 lets in less light.
   
   
   
   Apertures range in f-stops from f/1 to f/64 in one-stop increments. A one-stop increment either doubles or halves the amount of light hitting the film. The whole f/stop numbers are listed below.  You will be responsible for committing these whole f/stops to memory.
   
   1 _ 1.4 _ 2.0 _ 2.8 _ 4.0 _ 5.6 _ 8.0 _ 11 _ 16 _ 22 _ 32 _ 45 _ 64 
   
   
7. Shutter speeds: Obviously the longer the shutter remains open the more light that can pass through to the film. On your camera the most used numbers are also fractions. Therefore the larger the number, the faster the shutter speed. When speaking apertures do not mention the fraction, e.g. f-8, but usually when speaking about shutter speeds the fraction is mentioned, e.g. 60 on the camera means and is said 1/60th sec. Some digital cameras with LCD's will actually write the shutter speed as a fraction. Otherwise the shutter speeds over one second, i.e. not a fraction, are in a different color.
   
   The shutter works by opening and closing for a specified length of time. The SLR camera, especially 35mm cameras, usually have what is called a focal plane shutter. The shutter speeds usually start from around 5-30seconds on the slow side up to 1/4000th of a second for certain cameras.
   
   2 s _ 1 sec _ 1/2 _ 1/4 _ 1/8 _ 1/15 _ 1/30 _ 1/60 _ 1/125 _ 1/250 _ 1/500 _ 1/1000 _ 1/2000 _ 1/4000
   
   
   © Linda Lowell