Photographing Solar Eclipses - Zoom Lens
Author: Bill Kramer
Last update: Saturday, 18-Apr-2015 18:51:30 EDT
Using a Zoom Lens
Go out and look at the moon one night. Note how small it appears. Try to get a picture of the moon and you will see how small it is in an image. The moon is only a half a degree in apparent size (there are 360 degrees in a circle, 180 degrees in a half circle thus you could fit 360 moons side by side across the sky from West to East through the Zenith).
Chances are you are going to want to use a zoom or telephoto lens to get a closer look. The picture of the crescent moon at right was taken using a 300mm telephoto lens coupled to a standard film camera. A telephoto lens is like having binoculars or a small telescope for your camera. You can get a closer view and see more details.
There are two basic types of telephoto lens - fixed and zoom. A fixed lens has one focal length. A zoom telephoto lens has a range of focal lengths you can use.
Some cameras come with a zoom lens built into the camera. Film cameras of the SLR type (Single Lens Reflex) use interchangeable lenses with varied focal lengths, which may be fixed or zoom types. As you increase the focal length of the lens the magnification of the resulting image size increases as well. Thus a 200mm lens provides a closer view than a 50mm lens. The closer view comes at a cost. The 200mm lens will be four times larger than the 50mm lens and be heavier too. A mount or tripod to help support the camera and lens becomes essential. See Zoom accessories for more information about the types of things you need to bring along when using a Zoom lens of any type.
Basic digital cameras provide two kinds of zoom options, optical and digital. When shooting the eclipse event it is best to avoid digital zoom in the camera and strictly work with the optical zoom. In fact the best way to treat digital cameras is to turn off all the extra digital enhancements (with the exception of image stabilization) when obtaining the images. There will be plenty of time for image enhancement work after the eclipse using the computer.
Cameras that claim to have a 200x digital zoom are fooling you. A general rule of thumb in telescopes and astronomy is not to exceed 60x per inch of objective lens. For a 200x camera you'd want a lens 3 to 4 inches across and that is as big as some of the new cameras! Shut down the digital zoom and stick to the optical zoom. Even that is pushing the limits of the small optics involved - experiment on the moon to see what kind of results you can get. If the maximum zoom looks bad, try reducing the magnification factor until you get an image where the moon appears sharp.
Digital SLR cameras accept multiple lens configurations. In a digital SLR there is a chip instead of film. The chip used will vary in size with most being a bit smaller than the traditional 35mm film.
Digital SLR manufacturers provide a multiplier that can be applied when using a lens to achieve the effective focal length (a value that is handy in some computations such as field of view and estimating the image size). The multiplier indicates the relative size of the imaging chip.
For example: The Canon Rebel has a multiplier of 1.6x. That means that the chip is 1.6 times smaller than traditional 35mm film. This factor can be applied to the lens focal length to determine the effective focal length. The 1.6x factor is the result of the imaging chip being roughly 5/8ths of the size of 35mm film.
Great images can be obtained using lenses with a focal length of 100mm and greater.
The use of teleconverters or lens multipliers is not recommended most of the time. Unless you are using a high grade, low reflection, lens enhancement it is recommended that the extra lens not be inserted. The result of using such a lens enhancement is often extra reflections in the image showing up as blurs. Test the system by taking exposures of the young or very old moon (thin crescent phase) and over exposing the image to reveal the Earth shine. Look for blurry areas in the picture, those are the result of internal reflections in most cases.
Selecting a good lens - it should be fast (low f-ratio), support all the features of your camera (be 100% compatible), allow for manual focus, and possibly support image stabilizing.
This is the lens I have been using since 2008 for eclipse photography. When used in conjunction with a Canon DSLR the 400mm lens yields an image that is roughly the same as achieved using a 600mm lens and 35mm film.
When using this lens the image of the moon uses 24% of the shorter side and about 17% of the longer side. For eclipse photography this means the corona can be photographed out to 2.5 lunar diameters on each side of the moon.