Does that mean you don’t have to fiddle about putting a camera on the telescope, then?
That was the question asked when I was showing off the capabilities of the Nikon P510 at the 35 mm camera equivalent of 1000mm. The question was also loaded in that it implied I would not need to buy a camera for digiscoping to replace the old Canon Ixus that fitted the scope awkwardly. The follow-up question — Surely that 1000 mm lens is as good as a scope isn’t it? — sent me in search of data. Evidence that useful as the 1000 mm is, it could not match the focal lengths of a telescope plus camera was needed. What is the focal length of a digiscope set-up in terms equivalent to a 35 mm full-frame camera?
To determine the equivalent focal length for a digital camera compared to a 35 mm camera, you need to carry out a few calculations. Each digital camera has a 35 mm equivalent figure. For the Nikon P-6000, for example, the 35 mm equivalent figure is 28–112 mm. Since the Nikon P-6000 has a digital focal length of 6–24 mm, this results in a factor of 4.66 (28 mm divided by 6 mm). This factor or the 35mm equivalent focal length is usually stated in the camera’s user manual.
In this case, the equivalent focal length is the adjusted focal length of the digital camera multiplied by the factor 4.66. Example: The Nikon P-6000 is set to a focal length of 14 mm. 14 mm x 4.66 = 65.2 mm focal length equivalent for a 35 mm camera.
To obtain the entire 35 mm equivalent focal length from your digiscoping equipment, you need to multiply that focal length by the magnification of your scope.
Example: Your telescope is set at 20x magnification. 65.2 mm x 20 = 1,304 mm total 35 mm equivalent focal length
To determine the total digital focal length of your digiscoping equipment, you simply need to multiply the focal length of your digital camera by the magnification settings of your telescope.
Example: Your digital camera is set to 14 mm focal length; the telescope is at 20x magnification: 14 mm x 20 = 280 mm total digital focal length