(idea) by m_turner Thu May 01 2003 at 3:10:53

A lunar eclipse happens when the Earth passes between the moon and the sun such that the moon passes through the shadow of the Earth. These happens a bit more frequently than the solar eclipse variety because the Earth is larger and casts a larger shadow for the moon to move through - more 'slop' with the effective up/down movement of the Earth moon system (the moon's orbit is tipped 5 degrees with respect to the Earth sun orbit) and distance between the Earth and the moon (which in a solar eclipse would result in a annular eclipse (annular meaning ring) when the moon is too far away).

To a person viewing a lunar eclipse the full moon (lunar eclipses only occur with a full moon) the moon will go from a full moon to less and less visible as if the moon was rapidly changing its phase. When the moon is totally obscured by the Earth and is within the shadow it often appears red. This red is from the light of the sun going through the atmosphere of the earth and then hitting the moon and then bouncing back to our eyes. Part of the red component is from dust within the atmosphere and part of it is from the refraction of light in the same way that a sunset appears to be red. Following totality, the moon will progress back through its phases as it leaves the shadow of the earth and becomes full again.

How dark the totality is was described by a French astronomer named Dajon who came up with a 5 point scale for lunar eclipses.

L = 0
Very dark eclipse - moon almost invisible especially at mid-totality
L = 1
Dark eclipse, gray or brownish in coloration. Details distinguishable only with difficulty.
L = 2
Deep red or rust-colored eclipse. Very dark central shadow, while outer edge of umbra is relatively bright.
L = 3
Brick-red eclipse. Umbral shadow usually has a bright or yellow rim.
L = 4
Very bright copper-red or orange eclipse. Umbral shadow has a bluish, very bright rim.

As with all astrophotography there are two approaches to photographing the lunar eclipse. The easiest method is that of the star trail where the shutter is left open for a long period of time. With this type of photograph one needs to have a slow film - the moon full moon is rather bright (it is a sun light object and thus follows the sunny 16 rule of thumb - 1/125th of a second at f/16 with 100 speed film). For a full lunar eclipse this is about 3 hours (see table below). One approach to handling this is to change the aperture during the exposure - going from f/16 during the moon and its phases to f/5.6 during totality to account for the dim light. This photograph produces a curved barbell shape where the moon is full on each side and the color moves from overexposed white to a reddish and back to overexposed white. Some of the sufficiently bright stars may show up as streaks.

An alternate method is that of multiple exposures - where the moon is photographed several times over the course of the night on a single frame. Each image will contain the moon at that time. As the Earth turns and the moon moves, the moon will move and expose a previously unexposed area of the film. It is important to have the same time interval between exposures with this method to get even spacing of each of the multiple exposures. To account for the changing amount of light with each phase of the moon one changes the shutter speed and/or aperture. 

  ISO        f/stop
   25            1.4    2.0    2.8   4.0   5.6   8.0  11.0  16.0  22.0
   50            2.0    2.8    4.0   5.6   8.0  11.0  16.0  22.0  32.0  
  100            2.8    4.0    5.6   8.0  11.0  16.0  22.0  32.0  44.0
  200            4.0    5.6    8.0  11.0  16.0  22.0  32.0  44.0  64.0
  400            5.6    8.0   11.0  16.0  22.0  32.0  44.0  64.0  88.0
  800            8.0   11.0   16.0  22.0  32.0  44.0  64.0  88.0 128.0
 1600           11.0   16.0   22.0  32.0  44.0  64.0  88.0 128.0 176.0

Subject   Q   Shutter
Full      8   1/4000 1/2000 1/1000 1/500 1/250 1/125  1/60  1/30  1/15
Umbral    7   1/2000 1/1000 1/500  1/250 1/125 1/60   1/30  1/15  1/8
U 0.25    6   1/1000 1/500  1/250  1/125 1/60  1/30   1/15  1/8   1/4
U 0.50    5   1/500  1/250  1/125  1/60  1/30  1/15   1/8   1/4   1/2
U 0.75    4   1/250  1/125  1/60   1/30  1/15  1/8    1/4   1/2   1s
T L=4    -3   1/2     1s     2s     4s    8s   15s    30s    1m   2m
T L=3    -5    2s     4s     8s    15s   30s    1m     2m    4m   8m
T L=2    -7    8s    15s    30s     1m    2m    4m     8m   15m  30m
T L=1    -8   30s     1m     2m     4m    8m   15m    30m   ---  ---
T L=0   -11    2m     4m     8m    15m   30m   ---    ---   ---  ---
The formula for the above is t = f2 / (I x 2Q) where:
  • t = exposure time (sec)
  • f = f stop
  • I = ISO (film speed)
  • Q = brightness exponent
This is available at http://www.mreclipse.com/LEphoto/LEphoto.html and developed by F. Espenak in March 1996. To use the above chart, select the film speed and f stop used in the top part of the chart. This provides a column to look at for shutter speed in various lighting conditions. The full moon is the first line, where the moon first touches the umbra (part of the shadow) and then various steps along the way (1/4 obscured to 3/4 obscured). The totality levels are then described.

For just one photograph, especially with "point and shoot" cameras or "Push Here Dummy" (over the weekend I heard a conversation: "So, what camera are you shooting with?" "I've got a PHD" "Huh?" "Push Here Dummy") one may wish to consider loading a high speed film. 1600 speed film at f/16 can capture the phases of the moon up to totality with shutter speeds of 1/125th of a second and faster (full moon at 1600 speed film, f/16 is 1/2000th of a second) - I've used 1600 speed film to capture pictures of the Milky Way on a clear night in 15 seconds.

For 35mm format: 

Focal length  Field of View (diag)    Size of moon on film
  50 mm        27°   x 40°   (46°)      0.5mm (1/72 w ;  1/48   h)
 105 mm        13°   x 19°   (23°)      1.0mm (1/36 w ;  1/24   h)
 200 mm         7°   x 10°   (12°)      1.8mm (1/20 w ;  1/13   h)
 400 mm         3.4° x  5.1° ( 6°)      3.7mm (1/10 w ;  1/6.5  h)
 500 mm         2.7° x  4.1° ( 5°)      4.6mm (1/8  w ;  1/5.2  h)
1000 mm         1.4° x  2.1° ( 2.5°)    9.2mm (1/4  w ;  1/2.6  h)
2000 mm         0.9° x  1.4° ( 1.25°)  13.8mm (1/2.6 w ; 1/1.75 h)
2500 mm         0.7° x  1.0° ( 0.99°)  22.9mm (1/1.5 w ; 1/1.04 h)
Please note that at 2500 mm, the moon barely fits in to the height of the frame. Another important aspect to realize is that the Earth turns and the moon moves. To capture the entire eclipse from one end to another one must consider that the moon moves about 13° per hour. Using the diagonal above, we come up with the following table: 
Focal length  Field of View (diag)    Max duration
  50 mm        27°   x 40°   (46°)      3h 30m
 105 mm        13°   x 19°   (23°)      1h 45m
 200 mm         7°   x 10°   (12°)         55m
 400 mm         3.4° x  5.1° ( 6°)         27m
 500 mm         2.7° x  4.1° ( 5°)         22m
1000 mm         1.4° x  2.1° ( 2.5°)       11m
2000 mm         0.9° x  1.4° ( 1.25°)       5m
2500 mm         0.7° x  1.0° ( 0.99°)       4m
This should give you some idea of how much of the eclipse can be captured with a single frame and a single lens. This does not mean you have to use a 50mm lens - spectacular photographs can be made going from umbra contact to totality or from totality back to the full moon using a longer lens. This should also give you an idea of how fast the moon will be moving through the frame and thus the slowest reasonable shutter speed for a particular film.

Lunar Eclipses from 2001 - 2010

http://www.mreclipse.com/LEphoto/LEphoto.html
http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html

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I like it! 4 C!s