Exposure Time Determination

During totality the moon is dark. It is not completely dark though. Light refracted through the Earth's atmosphere will shed some light on the subject. The moon will be a ruddy-brown-orange color and depending on the atmospheric conditions, elevation above the horizon, and relative background light; it will be either a "bright" or "dark" eclipse. As a consequence, the photographic exposures provided below are estimates at best and it is strongly recommended that you shoot a series of varying exposures that bracket the times. These guidelines were adapted from previous publications and experience.

The exposure setting you use is a function of the speed or ASA rating of the film or digital camera setting. The other main variable is the focal ratio. The focal ratio (f/) is obtained by dividing the size of the objective lens into the focal length of the optical system.

To use the numbers from the table with your own telescope/camera you must first calculate the focal ratio (f/) of your system. Do this by taking the focal length (FL) and divide that by the diameter (D) of the objective or main lens.

f-ratio = FL / D

Divide the f-ratio of your system by the f/ number from the chart. To find the f/number in the chart to use, cross match the subject you are shooting along with the film speed you are using.

Fdiff = f-ratio / f-value from chart

Now create a multiplication factor (M). This factor will be applied to the time value from the chart. Square the Fdiff value to obtain the multiplier.

M = Fdiff * Fdiff

Use the multiplier (M) against the recommended exposure time to yield the recommended exposure to use.

Exposure = M * Exposure value from chart

Note that this is just one method for determining exposure times. It is based on a Kodak publication named "Astrophotography Basics".

Example: 90mm MAK telescope with a FL of 1200mm.

f-ratio = 1200 / 90 is 13.3 meaning the scope/camera system is an f/13.3 lens setting. You cannot adjust this value unless you use a tele-extender to increase the value or use a barlow lens to double the effective focal length thereby changing the f-ratio. You can adjust the exposure. And that is what this equation is used for.

Using ASA 400 speed film, the recommended exposure and f/ settings are 1 second and f/2.8. To calculate an initial exposure estimate divide 2.8 into 13.3 to get Fdiff.

Fdiff = 13.3 / 2.8 = 4.75

Now square that value to get the M value.

M = 4.75 * 4.75 = 22.56

Multiply M by the recommended exposure to determine the initial estimated exposure to use. Exposure = 1 second * 22.56 = 22.56 seconds.

The value just calculated will work well for a very dark eclipse, and it is strongly recommended that instinct and experience be trusted since the atmospheric conditions will greatly effect the results. Bracket the exposures plus or minus 15%. If using a digital camera, check your results immediately to get the best exposures.

If 22 seconds seems high, remember, the moon will be very dark during mid-totality. For eclipses that occur away from the darkest part of the umbra shadow the exposures may be less since the surface will be illuminated somewhat. Off center eclipses tend to be brighter towards one side. To show the range of colors and shades I would bracket the result obtained above from 15 seconds to 30 seconds depending on how bright the eclipse appears in the sky.

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