PHOTOGRAPHING THE SUN AND PARTIAL SOLAR ECLIPSES:

CHOICE OF FILM, CAMERA AND LENS

By Birger Andresen, Trondheim Astronomiske Forening

CHOICE OF FILM

It’s important to decide beforehand how you would like to photograph the Sun and whether to use slide film or regular print film (negatives). Print film gives a decent result even if you’re off as many as three f-stops (or shutter speeds) while slide film is usually ruined if you’re off more than one f-stop. The advantage of slide film is the high quality of the colors.

Light-sensitive, also called fast, films, with ISO 400 or higher are preferable. ISO 400 films have such fine grain these days that there’s no reason to choose the slow ISO 100 or 200 films.

With a digital camera you can experiment. Try locking the sensitivity to 100 ISO if possible. 400 ISO may give unnecessary large “grains”.

The advantage of fast films is the short exposure time, which decreases the risk of blurry photos due to f. ex. vibrations in an unstable camera stand, especially during windy periods. Films from the large producers such as Agfa, Fuji, Kodak and Konica should give good results. Films which work well for regular photography usually work well for photographing solar eclipses.

Photo of a web camera. Click on the photo for full resolution. (Manipulated in Photoshop.) Photo: Carsten A. Arnholm.

CHOICE OF CAMERA AND LENS

You can pretty much use any lens and camera to photograph a partial eclipse as long as you have a solar filter which fits, but the Sun’s diameter will only be about 0.45 mm on a regular 35 mm film if you use a 50 mm lens. You can calculate the size of the solar disk on a 35 mm film by dividing the focal length of a lens by 110. So a 500 mm telephoto lens will give a 500 mm/110 = 4.5 mm solar disk, while a telephoto lens/telescope with focal length 2500 mm will give a 23 mm solar disk, which will cover the entire height of the image. Focal length from 500-2000 mm is ideal.

With a digital camera it’s somewhat harder, since the size of the image sensor varies in different cameras. Some cameras have an image sensor the size of 2/3 of an inch, which equals 2/3*24,5 mm = 18,3 mm diagonally. The image size of a 35 mm film is 49 mm diagonally.

When using a digital camera you would usually need a shorter focal length to make the Sun fill the whole image. Most digital cameras don’t have interchangeable lenses, but you can try mounting the camera to an ordinary telephoto lens around 200-500 mm. Make sure no light is allowed between the lens and the camera.