Experimenting today (1 Viewer)

This is the experiment.... I have a swaro 80 HD scope...20-60 zoom eyepiece and a Fuji F30.

Rather than going to full 3x on the Fuji that appears to be a conclusion of all (full camera zoom), why not back off and go half way with camera zoom but increase the scope zoom from 30 to 40, thus off-setting the less camera zoom. See what I mean?

So under normal conditions when I would take my Fuji out and zoom to camera 3x with scope zoom at 30 or less, I would be at F5. I want lower at times.

So by setting camera zoom at half or around 1.8 or so instead of 3x, and increasing scope zoom to 40, my F is now F3.2. Much better and no vignetting.

If I go to 1x or no zoom on camera, I get increase vignetting. If I increase eyepiece zoom beyond 40, that always is increased vignetting. So I am trying to find the optimum camera zoom setting plus scope zoom to give me the best overall sharpness, in the F3.2 or 3.4 range and high shutter speed. I might have to increase ISO to 800 instead of 400 depending on amount of sun/shade.

Anyhow....fun to experiment...

Jim,

Here is a primer to compute the true F-stop boundary range of your digiscoping system. You can also use the math to help you determine the optimum camera/eyepiece magnifications and where exit pupil vignetting (eyepiece AFoV and eyerelief also play a role) will likely occur though the result will probably not be suprising and it sheds some light on what/why of our gear choices.

First, we determine the max aperture range boundary of your camera/scope combo:

F30 true focal length wide open (8mm) X eyepiece magnification low (20x) ÷ scope aperture (80mm) = 160÷80= F2. BUT the camera's largest aperture is F2.8 wide open so the largest SYSTEM aperture is F2.8.

By having 2.8 as the result in the above equation and keeping the camera zoom WIDE and solving for eyepiece magnification you come up with 28x. This is the OPTIMUM eyepiece magnification for the brightest possible image and minimal vignetting with this combination. In fact, for most camera/scope combos ~25-32x is the optimum choice.

Now we determine camera tele zoom boundary.

F30 true focal length tele zoom (24mm) X eyepiece magnification low (20x) ÷ scope aperture (80mm) = 480÷80= F6. Because the camera's max aperture of F5 at tele is larger, the largest SYSTEM aperture at full camera zoom remains F6.

Thus, the max system aperture range boundary is F2.8-F6 with 20x scope magnification.

We can also compute the smallest aperture boundary range, using the max eyepiece magnification of 60x, to be F6-F18. The important thing to know is that for most compact digicams, diffraction adversely affects resolution at apertures smaller than F8 while autofocus performance becomes increasingly difficult. Most digicams suffer from the peculiar fact that their physical lens aperture diameter actually increases with focal length. This is opposite to what happens with most dSLR lenses. Thus F8 should be our upper boundary for best performance.

So again, using 8 as the result in the above equation and keeping the camera tele zoom at 24mm and solving for eyepiece magnification you come up with ~27x.

Now determine camera focal length boundary by using 8 as the result in the above equation and keeping the eyepiece magnification at 60x and solving for camera focal length you come up with 10.67mm. Covert that to the 35mm equivalent my multiplying by the sensor crop factor of 4.5 equals 48mm for trivia.

From all this you can calculate precisely the optimum values BUT camera zoom lens step limits and the inability to precisely dial in the eyepiece magnification exert their own limitations.

So the result of all this fun with numbers is that most digiscoping systems are optimized around a ~1500mm effective focal length, with camera lens zoomed to ~50mm eqv. X ~30x scope magnification, and a system aperture of ~F4 for a 80mm scope. The angle of view of a 50mm lens is also now within the AFoV of most eyepieces so vignetting usually disappears IF the eyepiece eyerelief can accomodate the physical distance of the lens from the sensor. For total effective focal lengths above 1500mm it is always better to zoom the eyepiece instead of the camera to avoid light falloff at the edges. The "soft" upper boundary is thus limited to ~3000mm @ ~F8 afterwhich IQ starts to THEORETICALLY deteriorate.

Finally, this makes it obvious why smaller aperture scopes will have greater depth of field as their system F ratios get larger.
Still having fun now!?8-P
Rick

Gulp..... ... I need to come back to this in the morning when my mind is thinking,....interesting stuff. I will look at the math.

For now....I played with it just on the sly and found out that visually looking at images taken of the same bird at scope zoom of 30X and F5 equals camera zoom of 35-40 and F 3.4--3.7. So, based on visual only, not sure if there is enough of a distinction.

Tomorrow, I will play with the math! thanks, real interesting....