I wrote a blog post about attempting to calculate the focal length of my digiscoping setup some time ago (
here) for the DSLR setup I described in
this blog post. I came to the conclusion that, with the telescope's eyepiece on its minimum magnification of 20x, it was acting as about a 1,000mm F12 lens (and since the camera - a Canon 60D - has a 1.6 crop factor, the 35mm equivalent focal length would therefore be 1,600mm).
Back in February, I was preparing a talk on digiscoping for a local camera club and decided to tackle the same question a different way. I went to the Rothschild's Mere hide at Woodwalton Fen and spent a couple of hours taking pictures of the same subjects with various digiscoping setups and also with my Canon 300mm F4 prime lens which I could then use as a standard for comparison.
Here are some Cormorants sat on the tern raft in front of the hide taken using the Canon 60D with the 300mm F4 lens.
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| Scaled down image from full frame: 300mm, F8, 1/1000s, 400 ISO |
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| 100% crop showing the measurement I made from the beak tip to the back of the head on a line projected through the point of the yellow triangle on the face |
Whilst viewing the image at 100%, I measured the width of the head of the adult Cormorant on the right, from the tip of the bill to the back of its head along a line projected through the point of the yellow triangle below its eye. This line was 263 pixels long in this case.
To check that this worked reasonably well, I put my 1.4x converter on the lens and took, as near as possible the same shot (bearing in mind that this all takes time and birds move!).
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| Scaled down image from full frame: 300mm with 1.4x converter, F8, 1/1000s, 800 ISO |
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| 100% crop. Same measurement - 374 pixels |
Note I had to bump the ISO up by a stop (from 400 to 800) to get the same exposure as you would expect. As near as possible the same measurement of the same bird (but of course there is some error because it may well have its head in a slightly different position) was now 374 pixels. 374/263 = 1.42, so pretty close to the 1.4x I would expect from the converter.
Here is a digiscoped picture of the same bird with my telescope's eyepiece on its minimum magnification of 20x.
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| Scaled down image from full frame: Digiscoped image, Kowa 823 with 20-60x eyepiece on minimum, Canon 60D with an Olympus 50mm F1.4 lens wide open at 1/320s, 800 ISO mounted on the eyepiece. |
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| 100% crop, Same measurement: 925 pixels | |
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The whole head no longer fits in my 100% crop shown here, but the same measurement on the original image now comes out at 925 pixels. 925/263=3.509 suggesting the focal length equivalent of 1,053mm (300 x 3.509). This is pretty close to the 1,000mm I calculated in my previous blog post. Note also I have gone from an exposure of 1/1000s at F8 to 1/320s with the Olympus 50mm F1.4 wide open to maintain the same ISO setting.
With the telescope's eyepiece bumped up to its maximum magnification of 60x here is what I got.
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| Scaled down image from full frame: Digiscope at 60x, 1/60s, 800 ISO |
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| 100% crop, same measurement 3052 pixels |
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Now we can only get the eye in on a 100% crop, but the same measurement across the head comes out at 3052 pixels. 3052/263=11.583 suggesting a focal length of 3,375mm. The exposure had to go from 1/320s to 1/60s (which is probably one reason why it is not very sharp and shows signs of camera shake!) - another 2.5 stops. Given that this is a 1.6x crop factor camera, the 35mm focal length equivalent is a whopping 5,560mm!
