Thursday, September 19, 2013

Focal length equivalent of digiscope

I posted a digiscoped picture of a Little Egret on Talk Photography and, amongst the comments, got asked "What was the focal length more or less (35mm equiv.)". I only had a fairly vague idea based, on the fact that the magnification is clearly more than I get from my Sigma 150-500mm lens. After a lot of searching, I found which offers a downloadable Excel spreadsheet to calculate a mass of information about a digiscoping setup.

It is pretty easy to use, you fill in the parameters of your setup in the yellow area at the top and it delivers a table of results - including the effective focal length and f-stop of the "lens".

My main problem was filling in all the required details. The diameter of the objective lens of the telescope was easy - its called "TSN 823" - and the 82 part is the diameter of the objective in mm. Anyway, if I was stuck I could always get out a ruler and measure it! What stumped for some time me was the focal length of the telescope's objective. This particular model is no longer made by Kowa, so there is nothing on their web site. However, after a lot of searching, I did manage to find specifications which gave me an answer: 450mm. The telescope has a 15-60x zoom eyepiece and I rarely use anything other than the minimum setting for digiscoping - so the magnification in this case was set at 15x.

The camera side was easier, The dimensions of the resulting image in pixels is easy (just check the properties of the image file) and I use an old Olympus Zuiko 50mm f1.8 standard lens on the camera (see this post for details of mounting the camera on the 'scope). Its a prime lens so the max zoom must also be 50mm and, since the Camera (Canon EOS 60D) has an APS-C sensor with a 1.6x crop factor, the 35mm equivalent focal length is 50 x 1.6 = 80mm.

The distance to the object (the Little Egret) I am just guessing, but it does not make any difference to the calculation I am interested in, the focal length of the setup.

So we have our answer: its effectively a 750mm F9 lens (or 1229mm 35mm equivalent). If I bump the magnification up to 20x, then it is 1000mm F12 lens or (1638 35mm equivalent). At 60x is would be a 3000mm F37 lens (5000mm 35mm equivalent) but, in my experience, the higher magnifications are unusable for digiscoping. The light getting through is just too dim (F37!) and the quality of the image is frankly dreadful!