With the Digidapter now customized for the 40X W eyepiece I was able to mount the RX100 III so that the lens of the camera at 50mm butts up against the eyepiece for a sharp focal point. Note that the lens barrel extends at 24 - 50mm range, so the Digidapter needs to be slid back so as not to cause a lens error on the camera. I make it a point to turn on the camera and zoom it to 50mm and leave it there. If I decide to zoom out to 30mm I just slide the Digidapter back a bit. The lens on the RX100 III gives a sharp vignette circle at all focal lengths when in contact w/ the scope eyepiece. With an adapter ready to go I was able to test image resolution of the three cameras.
My assistant Beaker was placed under a fluorescent drafting lamp and the scope was set up about 25' away. I set each camera up at ISO 200 and Aperture Priority, and used a 10 second self-timer to take the exposures. I let the camera choose shutter speeds. Note that in all cases the shutter speeds were the same for all three cameras! I took exposures at f1.8 (when possible), f2.8, f3.5, f5.6, f8 and f11 so that I could compare w/ the 10-30mm f/3.5-5.6 Nikon lens. All images were taken in RAW format (NEF for Nikon and AWR for Sony).
The above image was taken at 27mm equiv. for the V1 and V3 and 24mm equiv. for the RX100 III. The RX100 III produced a consistently cooler image than the V1 and V3, which gave warmer colors. I think the Sony produced a more realistic color. The V3 tended to give a slightly darker exposure (~-0.3 EV). In terms of sharpness all three appeared equal with similar noise and resolution.
At 50mm equiv. (middle zoom for all three cameras) results were similar. Noise and resolution were very similar with only the slight color shift noted between the three cameras. As expected for RAW the images have a softness to them that sharpens easily in Photoshop.
At the highest magnifications for each camera the V1 and V3 provide 81 mm equiv. magnification while the Sony provides 70mm. Again, all three cameras produced very similar resolution and noise properties. I didn't test any other ISO ranges. Check out the studio comparison tool at DPreview.com to view the RX100 III against similar Nikon and other brand cameras.
I did compare the RX100 III at each Aperture to see if there was a resolution difference between f1.8 to f11. In general most spotting scopes give an equivalent aperture of around f8 so adjusting camera apertures between f1.8 - f8 really does nothing to improve depth of field or resolution. I found this to be true in this case, as well. The only difference between exposures was a faster shutter speed. When you get to f11 you'll find that the cameras slow down, and tend to overexpose the image - this was more evident w/ the Nikon V1 and V3 relative to the Sony. Bottom line: shoot at the widest aperture possible to get the fastest shutter speed. You won't be sacrificing resolution or depth of field. The only observable differences are due to focus accuracy.
At this point I feel very comfortable that the Sony RX100 III will prove to be a very capable digiscoping camera for my Zeiss 85T*Fl and 40X W eyepiece. With my Digidapter™ modification I have a very stable rig that will give me an effective focal length (EFL) of 960 - 2800 mm for digiscoping. The 21Mpx images that this camera will provide should give me a push of resolution over the Nikon 1 V1 camera (10 Mpx) with no added noise.
Some nits about the Sony RX100 III: The EVF is very nice, but smaller than the EVF of the V3, which I liked very much. The AWR (RAW) files produced by the Sony will not be read by the Nikon Transfer Software (expected) but will not be visible in NikonView either, so reviewing images is difficult. The Sony software requires that files be 'imported' into their viewer, ie., if you copy the files using Finder (on the Mac) to the Desktop, the software won't allow you to select that folder for viewing. Plus, in order to Delete photos, you have to go to the drop-down menu to select Delete, then verify, then wait while the camera deletes the file. I found it easier to grab a bunch of files and copy them into Adobe Raw and preview the images using their zoom tool - at least you can Delete images on the fly. Even though I have the latest Adobe Raw Converter software, the files from the V3 and RX100 III cannot be viewed as icons in Finder, so that's an additional limitation. But I'll get used to it.
Now to see if I can improve my keeper rate with this rig.