By Birger Andresen,
Trondheim Astronomiske Forening
Deciding which exposure time is ideal for your combination of lens, solar filter and film should be done as early as possible. Try it out and see what works best.
Some cameras have a spot meter that measures the light on a smaller area in the image. In that case, simply attach the solar filter, measure on a point on the solar disk and use the exposure time the camera gives for the speed of the film (ISO value).
The alternative is to shoot a number of photos with different exposure times and develop these well ahead of the eclipse. Try taking these photos when the Sun is about as high above the horizon as it will be when the eclipse occurs. Remember taking notes of which exposure times you use so you’ll know which exposure time belongs to which image on the film. If you use a camera lens, remember opening the aperture to its widest setting (for example f/5,6 – and not f/22 - for a 200mm f/5,6 telephoto lens) since this gives shorter exposure time and the depth of field doesn’t matter in this case. Table 1 gives an approximate estimate for what will work well with different films and f-stops.
| Table 1: Guidelines for exposure times for the Sun when it is not obscured by an eclipse and for the partial phase of an eclipse, photographed with a full solar filter with medium density, good weather conditions and the Sun relatively high on the sky. |
| f/ |
Film speed
(ISO-value) |
| |
32 |
64 |
100 |
200 |
400 |
| 2.8 |
1/1000 |
1/2000 |
1/4000 |
- |
- |
| 4.0 |
1/500 |
1/1000 |
1/2000 |
1/4000 |
- |
| 5.6 |
1/250 |
1/500 |
1/1000 |
1/2000 |
1/4000 |
| 8 |
1/125 |
1/250 |
1/500 |
1/1000 |
1/2000 |
| 11 |
1/60 |
1/125 |
1/250 |
1/500 |
1/1000 |
| 16 |
1/30 |
1/60 |
1/125 |
1/250 |
1/500 |
| 22 |
1/15 |
1/30 |
1/60 |
1/125 |
1/250 |
| 32 |
1/8 |
1/15 |
1/30 |
1/60 |
1/125 | |
The table gives guidelines only, and is based on film cameras. The values will be even less certain for digital cameras.
On the day of the solar eclipse you want to shoot the eclipse with the best exposure time from the test, if the conditions are more or less the same as during the test. Also, bracket your exposures with one or two stops over and under the ideal exposure. It makes you fairly sure to get something decent. When more than 90% of the solar disk is covered by the Moon, and especially if you use slide film, you may consider exposing one stop longer than the time you believe will be ideal. The reason is that the solar limb is somewhat darker than the rest of the Sun, since we there look sideways towards the Sun through a thicker layer of atmosphere than when we look through the center of the Sun. We call this effect solar limb darkening.
 |
A photo taken through a telescope without filter. The clouds gave a thick enough cover to allow for a 1/2000 second exposure time through a telescope with aperture f/8,5. Also shorter and longer telephoto lenses work well when the cloud cover is like this.
Photo: Magnar Fjørtoft
|
The exposure time must also be changed if the weather conditions are different during the eclipse than during the test, it must be increased if there’s a layer of haze, for example. A light haze may require an exposure one or two shutter speeds slower than normal, while thicker clouds could call for three or more shutter speeds slower, as would a thick haze if the Sun’s near the horizon.
Do not save on film during a solar eclipse. It happens so rarely it’s well worth using some extra film that day. Also make sure you have fully charged batteries, preferably even spares.
FOCUSING
Remember to use glasses or contact lenses when focusing a telescope. Also telephoto lenses must usually be focused, since you can’t trust that the lens is really focused on infinity. even when set on the infinity mark. Also, set the camera on manual. You should decide the exposure time. The aperture opening should be maximal.
 |
This photo was taken by Gitte Rydberg during the eclipse in Australia on December 4th 2002. A partial obscured sun low above the horizon is an impressing sight.
Photo: Gitte Rydberg |
