Exposure Calculator

How To Use the Exposure Calculator

Perform the Following Steps

1. Select the light intensity that illuminates the scene to be photographed.
2. Select the film speed of the film being used.
3. For an "Aperture Priority" system (where you choose the aperture and the camera calculates the shutter speed), select the f/stop of the desired aperture -- the correct shutter speed will be calculated and displayed.
      - And/Or -
   For a "Shutter Priority" system (where you choose the shutter speed and the camera calculates the aperture), select the shutter speed -- the correct f/stop will be calculated and displayed.
   The selected and calculated f/stop and shutter speed values are also shown graphically (see "About the Graphs" below).
4. Thereafter, any change to any of the four selectable input factors (light intensity, film speed, f/stop, shutter speed) will automatically update the output values (shutter speed and f/stop).
5. Optional Settings (found at bottom of calculator):
   • You can select the amount of detail available in the calculator. Select "full stops only" to display f/stop and shutter speed values that differ from one to the next by one full stop (note: one "full stop" represents a doubling or halving of light). Alternatively, you can calculate with more detail using 1/2 and 1/3 stop increments by selecting the corresponding button; 1/2 stop values are shown in green and 1/3 stop values in purple.
   • Check the "Show Graphs" checkbox to display graphical output of f/stop and shutter speed values (see "About the Graphs" below).

About the Graphs

Graphical output of the f/stop and shutter speed values is available (Internet Explorer version 5 and higher only with Flash player). The graphs are optional and do not affect the calculations. Use the "Show Graphs" checkbox to show or hide the graphs. Due to the wide range of f/stop and shutter speed values, only the more commonly used values are graphed: f/stops between f/1 and f/32 and shutter speeds between 1/15 and 1/1000 seconds.

f/stop Graph...
The diameter and area values of each f/stop are based on a lens with a focal length of 50mm. Note that an f/stop value is the ratio of the focal length of the lens to the diameter of the aperture. Thus, f/1 has an aperture diameter of 50mm, f/2 a diameter of 25mm, f/8 a diameter of 6.25mm, etc. The percentage value shown next to the aperture area is the aperture area as a percentage of the total f/1 aperture area. The amount of light entering the camera is directly governed by the area of the aperture opening. Thus, f/1.4 is 50% or half the area of f/1 and therefore cuts the light in half -- a "full stop" less light. f/2 is 25% or 1/4 the area of f/1 and is 2 stops less light. [Note: The depicted graphical sizes of the aperture openings are accurate relative to the f/1 aperture size. Because few lenses are this fast (ie. few lenses open up to f/1 size), the graphical aperture sizes may seem smaller than expected.]

Shutter Speed Graph...
Each bar in the bar graph of shutter speed represents the duration of time the shutter remains open for the given shutter speed indicated by the red arrow. The absolute value of this duration in seconds is shown on the lower/horizontal axis of the graph. The amount of light entering the camera is directly proportional to the duration the shutter is open and therefore to the length of the bar.

Other Important Information

If given unrealistic combinations of input values, the calculator will yield values that are also unrealistic (yet nonetheless still theoretically accurate). For example, if you select a very bright light situation, a very fast film speed, and a large aperture, the calculated value for shutter speed might be some unrealistic value like 1/524,000th of a second. Selecting a slower film speed or a smaller aperture will yield more realistic results. The same is true for aperture calculations. For example, if you select a very low light situation, a slow film, and a fast shutter speed, the calculated f/stop could be an impossible f/0.008. Obviously you should adjust your input values.

The calculator does not account for reciprocity failure; for long exposures an adjustment may need to be made for this phenomenon. See the data sheet for the film you are using for more information.

F/stop and shutter speed values displayed in the calculator are rounded to the nearest convenient, significant value. Internal calculations are done using precise, unrounded values.

A site I used to help develop/check this calculator is Fred Parker's "THE ULTIMATE EXPOSURE COMPUTER" at www.fredparker.com/ultexp1.htm [off-site], which essentially contains a tabular version of the calculator as well as other helpful details on exposure.