Intensity, Exposure, and Time

• Light is flowing energy. It can be useful to think of it as flowing in packets called photons.
  
• The apparent brightness of an object to your eye depends on the rate of energy flow to your eye. This rate of flow is called luminance, or intensity. In the metric system it is measured in lux (roughly the brightness of a normal candle at a distance of one meter). Intensity can be viewed as the number of photons per second that reach your eye from the subject.
  
• The degree of film or digital sensor exposure depends on the total amount of light energy that reaches the flow. (i.e. the total number of photons).
  
• Exposure equals the intensity of the light multiplied by the number of seconds the film receives it (i.e. the photons per second flow rate times the number of seconds).
  
• Exposure = Intensity x Time, or
• E = I x T
• (Exposure is sometimes also denoted as H)
  
• The area of the camera's aperture regulates the intensity. (i.e. cutting the aperture area in half, cuts the intensity in half; doubling the aperture area doubles the intensity).
  
• The shutter speed regulates the exposure time.
  
• For most films, the relationship E = I x T does not hold when the exposure time is very slow or extremely fast. Typically, most films require more exposure than the E = I x T formula indicates. This is called reciprocity failure. It can cause color distortion since it usually effects each color component differently.
  
• For digital cameras, the relationship E = I x T does hold for long exposures, so reciprocity failure is not an issue

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Expressing Aperture diameter using f-stops.

• F-stop expresses the diameter of the aperture as a fraction of the focal length.
  
• Aperture diameter = (focal length) / (f-number)
  
• F-stops are commonly written as f/number, e.g. f/16. Think of the "f" as representing the focal length, and the aperture diameter as f divided by 16 (f/16).
  
• This is why larger numbers correspond to smaller apertures.
  
• Apertures are expressed this way to account for the fact that an increase in focal length requires a proportional increase in aperture diameter to achieve the same intensity of light.

The standard sequence of f-stops: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, etc.

• The f-stop sequence starts (in theory) at 1, although few consumer lenses have an aperture that large.
  
• Each successive f-stop decreases the area of the aperture by a factor of 2, which means the diameter decreases by the square root of two, or roughly 1.4 (with some rounding to whole numbers). Note that every other f-number increases by a factor of 2.
  
• Many lenses will have the largest aperature at a point between standard f-stops, such as 1.7 (between 1.4 and 2) or 3.5 (between 2.8 and 4).
  
• Digital cameras typically step in fractions of (1/3 or 1/2) of f-stops.

The standard sequence of shutter speeds: B, 30, 15, 8, 4, 2, 1, 2, 4, 8, 15, 30, 60, 125, 250, 500, 1000, etc.

• The shutter speeds to the left of 1 are measured in seconds, those to the right of one are fractions of a second (e.g. 125 = 1/125 second).
  
• Each step up on the scale reduces the time by a factor of 2 (with some values rounded for convenience).
• B stands for bulb, and holds the shutter open for as long as the shutter button is pressed.
• Rule of thumb for hand-held shooting with a digital SLR: Keep the shutter speed at or above 1/(1.5 x focal length). For example, when shooting with a 60 mm focal length, keep the shutter speed at 1/90 sec or faster.

f-stop, Shutter Speed, and Exposure Value.

• Each f-stop marks an aperture area which is half the preceding step and double the next.
  
• Each shutter speed marks an exposure time which is half the preceding step and double the next.
  
• Considering aperture area and shutter speed in combination, if one is doubled and the other halved, then the exposure remains the same.
  
• A combination of aperture and shutter speed that gives the same exposure is said to have the same exposure value (EV).
  
• Exposure values have been assigned to combinations values shown in the table below.
  

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• Some hand-held light meters indicate exposure values rather than a particular combination of aperture and shutter speed.
  
• The system can be confusing since the higher numbers indicate less exposure.
• The system developed in the era of film photography, when ISO sensitivity could not be changed from frame to frame.
  • In digital cameras, ISO sensitive offers a third degree of freedom.
  • For a given subject, there are many combinations of shutter, aperture, and sensitivity that result in the same image tone.

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Light Meters

A conventional light meter aims to set the exposure so that the average tone is middle grey. This generally works for "normal" scenes where there is a mix of light and dark elements, but can also give unexpected results.

This image shows a white column and a very dark door in the background (on the right).		Close-up of column, using exposure recommended by meter. The meter takes the white column to middle grey. To make the column look lighter, it is necessary to increase the exposure.
		Close-up of door, using exposure recommended by meter. The meter takes the dark door to middle grey. To make the door look darker, it is necessary to decrease the exposure.

• The exposure value that takes a surface to middle grey is a useful way to measure and compare the brightness of subjects. (e.g. if the brightest surface of interest meters at f/22 and the darkest meters at f/2.8 at the same shutter speed, then you can say that the scene has a range of six stops).

• Modern cameras typically have multiple metering modes:
  • Center weighted: Base exposure on a weighted average of light intensity in the frame with more value given to the center area.
  • Evaluative (multi-segment, multi-zone, matrix): Base exposure on average, range, and position of intensities in a matrix of areas (somewhere in the range 8 to 16) innthe frame.
  • Spot:
    Base exposure on a small circle (about 3%) in the center of the frame. Used to manually determine the metering required for specific areas in a scene.
     
• It is important to know what your light meter is doing, and how to outsmart it when the situation is not "normal".

• Most cameras have exposure compensation (Av+/-) which lets the shooter to tell the camera to over- or under-expose by a specified number of stops.
  • Learning to use this is essential to shooting in unusual light.

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Exposure modes

• Modern cameras typically have the following exposure modes:
  • Aperture preferred (A, Av): The shooter sets the aperture, and the camera decides the shutter speed.
  • Shutter preferred (S, T, Tv): The shooter sets the shutter, and the camera decides the aperture.
  • Program (P): The camera decides both shutter speed and aperture.
  • Manual (M): The shootern decides both aperture and shutter speed.
  • Presets (e.g. Portrait, Landscape, Action, Night, etc.). Presets are for people who don't understand principles of exposure. You will not need presets.
• Some Cameras also have options to set sensitivity automatically.

Next: Film and sensor characteristics