How to understand digiscoping magnification (2 Viewers)

[looksharp65]

This might not prove helpful, possibly rather irrelevant, but I still feel the urge to post it.

Suppose you're using a 30x75 scope and a P&S camera with a 6 mm f/3 lens. The eyepiece is of the wide-angle type and there's no vignetting.
The illumination within the exit pupil is compromised by the scope's transmission rate.
The exit pupil of the scope delivers a beam pencil that is 2.5 mm wide.
The maximum entrance pupil of the P&S camera is 2 mm.
This means that the EV will be the same through the scope as outside of it, apart from the minor light loss by the scope's transmission percentage.

If the 6 mm lens is equivalent to a 30 mm lens in full format and the magnification is 30x, that will yield the same magnification as a 900 mm lens. (30x50)*(30:50) = 900

But since the beam is wider than the camera's aperture, the latter will be the limiting factor. Thus, we have got a 900 mm f/3 equivalent!

//L

 

[RJM]

Yes, and that is beauty of digiscoping with a small sensor compact. You can get relatively fast shutter speeds compared to larger sensor cameras, BUT the smaller sensor produces more noise at an equivalent ISO.

If you can shoot the digicam at or within a stop or two of its base ISO, your IQ can be indistinguishable from a dslr when digiscoping.

 

[Neil]

It might be a bit late for me to butt in here but I have both scopes and eyepieces.
The Kowa PZ adapter 680-1000 mm is not the same type of device as the new TLS APO for the STX/ATX scopes. It's similar to the old TLS for previous models of Swarovski scopes.
The new TLS APO uses the Swarovski 25-50 eyepiece ( the previous TLS replaced the eyepiece), which becomes 30-70x on the STX 95.
So to compare Kowa and the new Swarovskis use the Kowa 20-60 zoom eyepiece or the new 25-60 zoom eyepiece. Using a Kowa DA adapter you will get similar magnifications
as with the STX/ATX scopes.
The advantage of the Kowa might be that you can use the 25x LER (long eye relief ) eyepiece for specialised lenses eg Canon G series cameras, long zoom digicams 24-500 mm zooms and Videocams with 10x zooms, or the fixed 30x eyepiece which will suit more zooming range on digicams eg Sony RX 100/100 M2 from 35 - 100 mm due to wider Field of View.
Using a DSLR/Micro 4/3rds camera without zoom lens makes the framing fixed and so less flexible eg from Great Egret to duck at same distance.
If you want more magnification and the ability to quickly switch from DSLR body only or with lens and for viewing then the Swarovski TLS APO approach is the way to go. A very flexible solution.
That being said I would love to have both options with me out in the field and a DSLR + 300/4 around my neck for the flight shots and closeups.
Hope this helps and doesn't confuse.
Neil.