|Using a Hastings Triplet as a digiscoping macro eyepiece
Since image contrast is reduced in some measure by each air to glass surface, I ordered a 40mm focal length
Hastings cemented triplet lens to try as an eyepiece with only two air to glass surfaces. As a normal eyepiece it
works well for the center of the image, but the outer sides of the frame left and right show distortion at the edges. My
hypothesis is that my Swarovski AT-80 HD spotting scope has too short of a focal ratio, 5.75, for this type of
The nearest that extension can reduce the minimum focus distance to is about 8 feet with the 40mm Televue Plössl.
At this distance the entire movement of the focus wheel alters the minimum to maximum focus distance only about 4
to 6 inches At this extreme limit of the optical system using the 40mm Plössl, the outer edges of this system show
edge distortion similar to the Hastings triplet under normal conditions. .
Through experimentation at this distance, the Hastings triplet seems to counter the edge distortion noticeable in the
Plössl when two conditions are met:
1) Extension beyond normal focus is adjusted to between 75mm to 100mm
2) The telescope objective lens is stopped down to 20mm from 80mm by taking a scope cap and drilling a 20mm
circular hole in the center, and the lens hood is not extended. This helps extend the short depth of field that occurs
when taking photographs at such a close distance.
The above photograph is uncropped. Pickerel frogs are about three inches long, this was taken from
about 8 feet away.