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Partial phases of the solar eclipse of 1970 March 07
Tasco 60mm refractor (f.l.= 910mm, f/15, 1/125 sec, ISO 160).
(click to see more photos)
You'll be photographing the partial phases of the eclipse with a solar filter over your lens. To succeed, you must determine well in advance of the eclipse the proper shutter speed and f-ratio for your particular solar filter and telephoto lens.
If your camera has a built-in spot meter that covers a smaller area than the Sun's image, you can simply meter on the Sun's disk through your solar filter and use that exposure throughout the partial phases.
If your camera does not have a spot meter (and most don't), you will need to run a simple exposure test. Set your equipment up on a sunny day. Load your camera with the same kind of film you will use for the eclipse. Use your solar filter to center the Sun carefully in your lens or telescope focus. For telephoto lenses, open the aperture to its widest setting. Shoot one exposure with every shutter speed you have from 1/15 through 1/1000 or 1/2000. Take notes so that you can identify the best exposure after your film is developed. Write down the best exposure and tape it to your tripod or the side of your solar filter. It should include the film speed, f-number, and shutter speed--for example, "Sun: ISO 400, f/8, 1/125." In this way the best exposure will be handy when you photograph the eclipse. The exposure doesn't change during the partial phases because the Sun's surface brightness remains the same throughout the eclipse.[14]
And now a warning. Your best exposure was determined on a sunny day. If the eclipse day has thin clouds or is hazy, you will need a longer exposure to compensate. 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. Try your planned exposure and several longer ones. Film is cheap and eclipses don't happen often.
[14] Some photographers like to expose one extra stop during the thin crescent phases because the Sun's limb is a little darker than disk center. This step is important only if you shoot slide film because it has a smaller exposure latitude.
Solar Corona of 1991 Total Eclipse
During the Total Solar Eclipse of 1991 July 11, the Sun's corona
displayed long streamers that are characteristic of sunspot maximum.
This composite image was made by combining a number of images with a computer.
For more on the technique, see Composite Eclipse Photography.
(click for more photos)
The brightness of the solar corona changes tremendously as you move out from the edge of the Sun's disk. The inner corona shines as brightly as the full moon, but the outer corona is over a hundred times fainter. The challenge is to capture both the brightest and faintest parts of the corona. Unfortunately, this variation in brightness is impossible to record in any one exposure because film just doesn't have the dynamic range of the human eye. (That's why you should look at totality with your eyes and not just with your camera. In real time, only your eyes can see the exquisite detail of this celestial event in all its glory. See The Experience of Totality)
The good news is that you can photograph some aspect of the corona with almost any exposure you make. There is no one "correct" exposure. Nevertheless, here are some guidelines for where to start.
Several factors determine the length of time the camera shutter is open to get the proper exposure on your final photograph. A table accompanying this chapter provides recommended shutter speeds for various eclipse phenomena using a range of ISO film speeds and lens f-numbers. Each eclipse phenomenon (diamond ring, prominences, corona) has a different brightness value and this value too must be considered in order to get the proper exposure of that aspect of the eclipse.[15]
Bracket your exposures on both sides of the ideal exposure and take several photographs at the same settings to help assure success. If you use a film with an ISO too high, you may discover that your camera does not have a fast enough shutter speed to allow the proper exposure. Determine all of your camera settings before the eclipse so that you know in advance what film you will need with your equipment.[16]
Even if your SLR's exposure is completely automatic, or if you simply don't want to hassle with exposure settings, you can still get good pictures of the diamond ring and totality with your camera set on automatic exposure. Load your camera with ISO 400 color negative film and grab some shots on auto-exposure. You will probably overexpose the inner corona and prominences, but you will still have some fine souvenirs of the event. Best of all, you can devote most of your time to watching the eclipse rather than fiddling with camera settings. Simplicity is especially recommended if you are a novice photographer or have never seen a total solar eclipse.
[15] For film ISOs or f-numbers not listed, use the following formula to determine your exposure.
E = f2/(A x B)
E = exposure or shutter speed (seconds)
f = focal ratio
A = film speed (ISO)
B = brightness value (see table below)
Brightness Value
(approximate) Eclipse Feature
256 Partial and annular phases (using solar filter)
100 Prominences and innermost corona
32-128 Diamond ring effect
32 Inner corona
4 Middle corona
0.5 Outer corona
For example, taking a picture of prominences on the Sun during totality (B = 100) using a telephoto lens at f/14 with 400 ISO film would require an exposure of:
E = 142/(400 x 100)
E = 196/40,000
E = 0.0049 second or 1/204 second
Normal SLRs don't have an exposure of 1/204 second, so use the closest shutter speeds, which would be 1/125 and 1/250 second.
[16] Here's a strategy used by many eclipse photographers to plan their exposures. Using the film ISO and telescope or lens f-number, determine the shortest shutter speed to capture the prominences (relatively bright) and longest shutter speed for capturing the outer corona (dim). After totality begins, shoot a sequence of exposures using every shutter speed, starting with the one for prominences and ending with the one for the outer corona. For instance, for ISO 400 and f/11, the recommended exposure for prominences is 1/1000 and for the outer corona 1/2 second. The shutter speed sequence would then be: 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4 and 1/2. This is a total of ten exposures. If you want some additional insurance, you could now repeat the sequence in reverse, ending with 1/1000. With a 36-exposure roll of film, you would still have 16 exposures for the diamond ring and prominences at the beginning and end of totality.
The exposures given here have been determined after photographing more than a dozen solar eclipses, but they are only suggestions. Each eclipse is different and the corona's brightness varies. Weather conditions (haze or thin clouds) may require longer exposure times. Use the recommended exposures as a starting point and then bracket during the eclipse, especially if the weather is a factor.
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The times for key phases of each eclipse are listed for any location along the eclipse path in NASA eclipse bulletins. To anticipate these events, you should set your watch for the exact time before the eclipse begins.
Coordinated Universal Time[17] is broadcast 24 hours a day over short-wave radio station WWV, Fort Collins, Colorado, and from Hawaii on WWVH at 5, 10 and 15 MHz. Radio station CHU in Ottawa, Canada also broadcasts time signals at 3.330, 7.335, and 14.670 MHz. If you leave your radio on throughout the eclipse, you will always know the correct time without checking your watch.
Unfortunately, these time signals can seldom be picked up outside of North America and the central Pacific Ocean. A global positioning system (GPS) fills this void. A GPS receiver can provide not only correct time but also your precise location.
The GPS program was developed by the United States Department of Defense as a highly accurate way of determining geographic coordinates worldwide. The system uses a set of 24 satellites in orbits 12,545 miles (20,183 kilometers) high and with periods of 12 hours. At any time, 8 to 12 of the GPS satellites are visible from any spot on Earth. A GPS radio receiver uses signals broadcast from the satellites to determine its 3-dimensional position with respect to the satellites. This position is displayed as latitude, longitude, and elevation above sea level, along with the exact Universal Time. Military GPS receivers can determine positions to within inches. GPS receivers for civilian use have an accuracy of about 300 feet (100 meters). You can use them to assure yourself that you are standing within the path of totality.[18]
[17] Coordinated Universal Time is equal to Greenwich Mean Time to within a second or two.
[18] The price of GPS receivers has dropped dramatically during the 1990's. They are now available for under $100.
A small cassette tape recorder is very useful for recording your observations and reactions during the eclipse. Put in a fresh set of batteries before the eclipse begins. Fifteen minutes before totality, begin recording continuously. You will not only have a helpful and permanent record of your comments, but the tape will also capture the excitement of your companions.
Some advanced eclipse observers bring a second cassette recorder for which they have pre-recorded a tape of instructions that they play back through headphones during the hectic minutes surrounding totality. The tape of instructions is synchronized with their watches and gives them audio cues for the time remaining before second or third contact, times to remove or replace solar filters, camera settings, and reminders to watch for various eclipse phenomena.
If you've never seen a total eclipse, such a tape may seem more trouble than it's worth. But the onset of totality can be so overwhelming that even experienced eclipse chasers lose track of what they intended to do. Consider an account from 1842:
[19] George B. Airy: "On the Total Solar Eclipse of 1851, July 28," Astronomy, volume 1, Bernard Lovell, editor: The Royal Institution Library of Science (Barking, Essex: Elsevier Publishing, 1970), page 4.
For more information, see:
Totality - Eclipses of the Sun
Second Edition
by Mark Littmann, Ken Willcox and Fred Espenak
Order Totality from MrEclipse.com
All rights reserved. This material may not be reproduced, published, copied or transmitted in any form, including electronically on the Internet or World Wide Web, without written permission of the authors.
Contact Espenak (at MrEclipse) for more information.
All photographs, text and web pages are © Copyright 2007 by Fred Espenak, unless otherwise noted. All rights reserved. They may not be reproduced, published, copied or transmitted in any form, including electronically on the Internet or WWW, without written permission of the author. The photos have been digitally watermarked.
The photographs may be licensed for commercial, editorial, and educational use. Contact Espenak (at MrEclipse) for photo use in print, web, video, CD and all other media.
WebMaster: MrEclipse
Last revised: 2008 Jan 13
