Using a resolution chart I took a series of images from 20' of the 18.5 mm f/1.8 lens on the 20-60X Zoom eyepiece. At 20X zoom the center is sharply resolved, however, resolution drops off quickly to either side. This is also evident at 30X zoom. At 40X zoom the effect is not horrible, but still somewhat noticeable. Images acquired at 50X and 60X zoom appear to be sharp across the entire image, but magnifications are a bit high. Also, shutter speeds drop to a level where hand-held digiscoping is extremely difficult.
Similar results were observed when I tried the 10-30mm f/3.5-5. lens on the 20-60X zoom. Zooming the lens to ~18.9mm f/4.5 produced similar 'soft' edges when I zoomed the eyepiece from 20 - 40X. Unfortunately I don't have a suitable adaptor for this combination of lens / eyepiece, so resolution testing couldn't be performed.
Conversely, both the 10-30mm f/3.5-.6 and 18.5 f/1.8 are sharp across the entire field of view when coupled with the 40X W eyepiece of the Diascope. So I decided to modify the adaptor for the 18.5 mm lens so that it would fit the smaller-diameter 40X W eyepiece.
I then performed resolution comparisons between the 18.5 mm f/1.8 and 10-30 mm f/3.5-5.6 lenses on the 40X W eyepiece using the ISO 12233 resolution chart set up 20 feet away. I used the 10 sec. self timer on the camera, since the shutter speeds were only on the order of 1/5 to 1/15 sec. at ISO 100. I set the 10-30 mm lens at 18.9 mm and f/4.5 to compare w/ the 18.5 mm at f/1.8.
This modified adapter appears to be stable and provides sharp, full frame views w/o corner shadowing. Image quality is quite good, and by setting Autofocus to AF-C I tend to get more accurate focal lengths. But also note that fine-tuning focus w/ the EVF will improve keeper rate - just make sure the diopter is set to give the sharpest view through the EVF!