Photographing Solar Eclipses - Exposure Estimation
Author: Bill Kramer
Last update: Saturday, 18-Apr-2015 18:54:24 EDT
Estimating the Expsoure
The following exposure settings for cameras are just estimates. You can use them as a guide to get started. Bracket exposures to either side of the estimate by adding and subtracting exposure times. For example, if the recommendation is for 1/125 of a second take at least three pictures ranging from 1/250 to 1/60 of a second.
My basic plan is to start with a quick set of diamond ring exposures then go progressively longer as the eclipse moves to maximum. As the prominences on the other side begin appearing drop the exposures back to the fastest settings for the chromosphere then increase slightly to catch the 3rd contact diamond ring with some inner corona. But that is just a basic plan - sometimes it works out and sometimes it does not.
An easy way to estimate exposure time (t) in seconds is to square the focal ratio (f) of the camera and divide that by the product of the file speed (ASA) and the brightness of the object (B).
(f * f) = t
(ASA * B)
You already have the film speed (ASA) and focal ratio (f/stop) from your system. The following table provides the estimated brightness factors for various parts of the total solar eclipse.
- Prominences, brightest corona (512)
- Diamond ring (32)
- Inner corona (8)
- Outer corona (0.5)
For example, if you have your ASA set at 200 with a 200mm f/5 lens system and want to take an image of the inner corona you can estimate the exposure as:
(5 * 5) / (200 * 8) = 25 / 1600 = 1/64 of a second.
Most cameras do not have a 1/364 of a second setting so just use the nearest available setting as the base for the exposure estimation. In this case, 1/60 resulting in a suggested bracket of 1/125 to 1/30 of a second.
Remember: These are just estimates to get you close to a solution.