Image Size

Calculate the image size of any astronomical object by multiplying the size of the object as measured in seconds of arc by the focal length of the camera divided by 200,000. The result will be in the same units of measure as the focal length.

The moon is roughly 1860 seconds of arc in size (31 minutes or just over half a degree). When you divide 1860 into 200,000 the result is 0.0093 which is multiplied by the focal length to achieve the image size.

Out in the field you can approximate this same result by just dividing the focal length of the lens by 100 to achieve the image size.

The trick is to match the image size with your film or image plane size. For the eclipsing moon the best results will be about 50 to 70 percent of the image plane size.

For example when using 35mm film the size is about 24mm by 36mm. A good image size will be between 12 and 18 mm. The lens recommendation would be somewhere between 1200mm to 1800mm (image size multiplied by 100 for approximation). When using a digital camera with an image size 1.6 times smaller, divide those values by 1.6 to get a recommended value between 750 to 1125 mm.

Exposure calculation...