Using a Zoom Lens (Page 2)
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.
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70mm |
300mm |
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. The above lens is rather expensive. Some other recommended optics that are less expensive yet very capable include the 70mm f6.2 APO refractors sold by William Optic and the Stellarvue Raptor. |
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