Introduction

Photographing an eclipse of the Sun is fun and easy. However, you will need to use a special Solar Filter to protect your eyes and your camera.

A solar eclipse occurs whenever the Moon's shadow falls on the Earth. This can only happen during New Moon when the Moon passes between the Sun and Earth. Although there is a New Moon every 29 1/2 days, there are usually only 2 or 3 solar eclipses each year. That's because the Moon's orbit is tipped 5 degrees to Earth's so the Moon's shadow misses Earth during most New Moons. (see: Solar Eclipses For Beginners)

Watching and photographing an eclipse of the Sun is a relaxing activity since it progresses at a leisurely pace. The eclipse begins as a small notch slowly appears along one edge of the Sun. During the next hour, the Moon gradually covers more and more of the Sun's bright disk. If the eclipse is a total one, the last remaining minutes of the partial phases can be quite dramatic and beautiful. The crescent of the Sun grows thinner as the Moon's shadow approaches. The abrupt darkness of totallity is stunning and quite unlike you've ever seen. And the incredible solar corona is simply the most awe-inspiring naked-eye sight in all of nature. Certainly the most a remarkable sight (see: The Experience of Totality).

Cameras

Over the past decade or so, digital cameras have completely replace film cameras in virtually all aspects of photography. Solar eclipses can be captured easily with all types of digital cameras. The simpler Point and Shoot cameras have a non-interchangable lens with a single focal length. Better models are equipt with a 3x or larger zoom lens. The most versatile (and expensive) camera is the DSLR (digital single lens reflex). These cameras allow you to replace the kit lens with any number of other lenses from wide angle to super telephoto. You can even connect a DSLR directly to a telescope so that the Sun fills the entire frame. No matter what kind of camera you own, one or more of the following techniques can be used be used to shoot a solar eclipse.

Lenses and Image Sizes

A solar eclipse may be safely photographed provided that certain precautions are followed. Almost any kind of camera can be used to capture this rare event; however, a lens with a fairly long focal length is recommended to produce as large an image of the Sun as possible. A standard 18mm lens on a DSLR yields a minuscule image of the Sun, while a 200mm telephoto or zoom produces an image four times larger (see: Field of View Table). A better choice would be one of the small, compact catadioptic or mirror lenses that have become widely available in the past decades. The focal length of 500mm is most common among such mirror lenses and yields a great image scale for capturing solar eclipses .

The sensor size of most DSLRs falls into on of two categories. The Full Frame Sensor (measuring 24 x 36 mm) is used in professional or upper end DSLR cameras. The less expensive Crop Sensor (measuring 16 x 24 mm [Nikon] or 15 x 22 mm [Canon]) is used in the less expensive consumer DSLRs. Either category can take excellent eclipse images, but the sensor size plays a determining roll on the apparent size of the Sun as seen with various focal length lenses.

As a general rule of thumb, the relative size of the Sun’s image appears 1.5 times larger in crop sensor DSLR compared to the image in a full sensor DSLR when using the same focal length lens. For example, a 500mm lens on a crop sensor DSLR produces the same relative image size as a 750mm lens on a full frame sensor DSLR (see: Solar Eclipse Image Scale). Another issue to consider is the lag time between digital frames required to write images to a DSLR's memory card. It is also advisable to turn off autofocus because it is not reliable under these conditions; focus the camera manually instead. Preparations must be made for adequate battery power and space on the memory card.

If full disk photography of partial phases of the eclipse is planned, the focal length of the optics must not exceed 2500mm on a full frame DSLR (1700mm on a crop sensor DSLR). Longer focal lengths permit photography of only a magnified portion of the Sun’s disk. In order to photograph the Sun’s corona during totality, the focal length should be no longer than about 1500mm with a full frame DSLR (1000mm with crop sensor DSLR); however, a focal length of 1000mm (700mm crop sensor) requires less critical framing and can capture some of the longer coronal streamers. The figure below shows the apparent size of the Sun (or Moon) and the outer corona in both full frame and crop sensor formats for a range of lens focal lengths. (see: Field of View Table).

A solar filter must be used on the lens throughout the partial phases for both photography and safe viewing. Such filters are most easily obtained through manufacturers and dealers listed in Sky & Telescope and Astronomy magazines. These filters typically attenuate the Sun’s visible and infrared energy by a factor of 100,000. The actual filter factor and choice of ISO speed, however, will play critical roles in determining the correct photographic exposure. Almost any ISO can be used because the Sun gives off abundant light. The easiest method for determining the correct exposure is accomplished by running a calibration test on the uneclipsed Sun. Shoot a roll of film of the mid-day Sun at a fixed aperture (f/8 to f/16) using every shutter speed from 1/1000s to 1/4s. After the film is developed, note the best exposures and use them to photograph all the partial phases. With a digital camera, the process is even easier. Just shoot a range of different exposures and use the camera's histogram display to evaluate the best exposure. The Sun’s surface brightness remains constant throughout the eclipse, so no exposure compensation is needed except for the narrow crescent phases, which require two more stops due to solar limb darkening. Bracketing by several stops is also necessary if haze or clouds interfere on eclipse day.

Certainly the most spectacular and awe-inspiring phase of the eclipse is totality. For a few brief minutes or seconds, the Sun’s pearly white corona, red prominences, and chromosphere are visible ( The Experience of Totality). The great challenge is to obtain a set of photographs that captures these fleeting phenomena. The most important point to remember is that during the total phase, all solar filters must be removed. The corona has a surface brightness a million times fainter than the photosphere, so photographs of the corona are made without a filter. Furthermore, it is completely safe to view the totally eclipsed Sun directly with the naked eye. No filters are needed, and in fact, they would only hinder the view. The average brightness of the corona varies inversely with the distance from the Sun’s limb. The inner corona is far brighter than the outer corona; thus, no single exposure can capture its full dynamic range. The best strategy is to choose one aperture or f/number and bracket the exposures over a range of shutter speeds (i.e., 1/1000s to 1s). Rehearsing this sequence is highly recommended because great excitement accompanies totality and there is little time to think.

Exposure times for various combinations of ISO speeds, apertures (f/number) and solar features (chromosphere, prominences, inner, middle, and outer corona) are summarized in the Solar Eclipse Exposure Guide above. This guide was developed from eclipse photographs made by the author, as well as from photographs published in Sky and Telescope. To use the guide, first select the ISO speed in the upper left column. Next, move to the right to the desired aperture or f/number for the chosen ISO. The shutter speeds in that column may be used as starting points for photographing various features and phenomena tabulated in the ‘Subject’ column at the far left. For example, to photograph prominences using ISO 400 at f/16, the table recommends an exposure of 1/1000. Alternatively, the recommended shutter speed can be calculated using the ‘Q’ factors tabulated along with the exposure formula at the bottom of the table. Keep in mind that these exposures are based on a clear sky and a corona of average brightness. The exposures should be bracketed one or more stops to take into account the actual sky conditions and the variable nature of these phenomena.



Exposure Bracketing Sequence

The sequence above was made by shooting a series of bracketed exposures

ranging from 1/1000 to 1 second (ISO 200, f/9)

total solar eclipse of 2006 Mar 29 from Jalu, Libya.

(click to see more photos)

It should be pointed out that the exposure table above is only a guideline for planning purposes. The brightness of the corona may vary from one eclipse to the next based on the relative point in the sunspot cycle as well as the current activity on the Sun during the eclipse. Because of the high dynamic range in the brightness encompassed by the corona, there is no one single exposure that is "correct." The best strategy is to bracket widely during totality to shoot a large range of exposures. I typically shoot at ISO 200, f/9 and will use shutter speeds ranging from 1/1000 down to 1 or more seconds.