BirdForum is the net's largest birding community dedicated to wild birds and birding, and is absolutely FREE!

Register for an account to take part in lively discussions in the forum, post your pictures in the gallery and more.

Reply to thread

Message: <blockquote data-quote="RJM" data-source="post: 2348094" data-attributes="member: 66340">Yes, the SYSTEM F ratio calculation is valid for any dslr that has a lens attached to or behind the scope eyepiece. What you may notice if you run the calculation across many camera types and sensor sizes is that as the sensor gets larger the system F ratio tends to get slower and slower.  Most suitable compact digicams deliver ~fl=1000mm-3000mm effective focal lengths @ ~F3-8 assuming an 80mm class scope and 30x eyepiece. On the same scope the Nikon V1 with its 10-30mm kit lens delivers ~fl=1000-2400mm @F5-11 while the Sony NEX5n with a 35mm F1.4 prime would be ~fl=1500mm @ F13!  So you see, any high ISO image quality advantage of the larger sensor tends to be negated by 2-4 stop slower shutter speeds they deliver over a compact at the same digiscoping focal lengths. Simple physics as you have a fixed amount of light leaving the exit pupil of the scope eyepiece. As the sensor gets larger, that light energy is spread out over an increasingly larger area. If you want to keep a system aperture of ~F8 @ ~fl=3000mm effective, the Fuji X10 is about the largest sensor camera we can theoretically use now. In practice however, a smaller compact type camera still rules for digiscoping at extreme focal lengths 1500mm-3000mm or more.</blockquote>