May 5, 2025
SVBONY SA401 APO 25-75X100mm Prime Photography with Full Frame Camera
This article demonstrates one way to adapt the SVBONY SA401 APO 100mmm spotting scope for modified prime focus
photography with a full frame camera. The configuration shown in the LEFT column above is specific to the
Canon EOS RP camera. SVBONY provide an SA407 photo-adapter kit shown in the RIGHT configuration above for eyepiece projection photography using
the zoom eyepiece included with this spotting scope (21.6mm to 7.2mm for 25 to 75X magnification) and this
approach allows photography over the range of magnification. Because the adapter is designed to work only with
the zoom eyepiece, the adapter/camera combination is quite long as shown at RIGHT and heavier full frame cameras
might present a weight support issue. Of course afocal photo systems, using smartphones are used frequently but
this tends to introduce more issues with optical aberrations since more optics is introduced between the scope objective
and the camera photo sensor. Some examples of afocal photo usage with a smartphone
and the SA401 are shown elsewhere.
I was interested in trying to configure a simpler "almost" prime focus adapter for my Canon EOS RP. The main
obstacle is of course the back focus distance which for this type of spotting scope, is built into the design.
The focal plane for infinity of the main SA401 objective (focal length 556mm) is almost exactly at the mouth of the spotting scope
opening.
For a Canon R mount camera, the focal (sensor) plane is 20mm (back focus distance) from the camera lens flange
so obviously true prime focus photography with a typical full frame camera sensor is not possible with the SA401 scope.
However, a basic focal plane extender such as a Barlow lens (thanks to Bruno Guarino for suggesting this)
might enable the focal point to be extended out sufficiently to overlap the focal plane of the camera sensor. One of
the problems initially was finding a T-ring for a Canon R mount (almost all are for Canon EF mount). Of course
an adapter for EF to EOS R is available (as used in the configuration at RIGHT for the zoom lens approach)
but this type of mount adapter, intended for lens compatibility, adds too much distance. Luckily, I found the Baader T-ring for Canon R mount shown at left.
So a simple calculation using focal distance changes suggested it might be in the range to achieve infinity focus. The configuration at above left
involves using the SVBONY 2X Barlow lens (after removing the top section), and inserting this tube into the SA407
eyepiece adapter. This was done by first carefully screwing in the SA407 adapter fully into the spotting scope, and then carefully
sliding the Barlow until it touches the ring just above the prism in the scope. Then the 3 SA407 screws were tightened firmly.
(NOTE: securing the Barlow is critical as the Barlow lens is not blocked at the camera size and could potentially slide into the camera sensor!).
The photo at above left shows the Barlow just peeking out of the SA407 adapter after the adjustment above.
Then the Baader T-ring inner 42*0.75 thread was screwed onto the top matching thread of the SA-407 tightly. Then the assembly was attached to the
EOS RP body. The camera orientation now is of course in a random orientation but luckily the threaded insert in
the Baader T-ring can be adjusted with 3 set screws and voila .. perfect camera orientation with respect to scope.
I was quite anxious to test for infinity focus and voila .. it just reaches infinity focus and just slightly beyond!
The infinity focal plane in this configuration is moved to about 34mm from the SA401 tube opening as shown in the details image at right.
(NOTE: Canon EF mounts have a much longer back focus distance of 44mm compared to 20mm for R mounts so the highly custom method used here would NOT work for a Canon EF camera!)
The closest object distance for focus is 22'.
With this custom setup, the FOV on the full frame sensor was measured to be about (2.0 x 1.4 °), roughly that
expected for a focal length of 2X the main objective or 1112 mm. (True prime focus for the objective lens would achieve a FOV about double this).The full frame FOV obtained with the zoom lens approach at RIGHT above at 25X magnification is (1.6 x 1.1 °).
Preliminary images taken with this "almost" prime focus setup at left show sharper images as is expected from
a simpler optics train from main objective to camera sensor.
The 3 photos below at a distance of 330' (click each for full screen high resolution views)
compare the image quality for 3 different camera configurations with the SA401 100mm APO scope.
For each photo, the focus was carefully adjusted to the large whitish power-line insulator in the centre.
Left: Prime Focus with X2 Barlow with Canon EOS RP (inverted image rotated 180 °)
Center: With default zoom eyepiece at X25 and Canon EOS RP projection
Right: With default zoom eyepiece at X25 and Samsung S23 Ultra X1 Camera (f_eq=23fmm) afocal
Left and Center: ISO-200 1/125 sec Right: ISO-64 1/1000 sec
The result at left with the Prime Focus/Barlow is distinctly better wrt chromatic aberation and the more natural color as expected, particularly noticeable in the wires at the top of the photo. The afocal result with the S23 smartphone at right shows distinct edge
aberations but the center section looks fairly good.
Some sample images for the Prime Focus & 2X Barlow, rotated by 180 ° are shown below. Click for enlarded views:
Moon: May 3, 2025 8:57pm EDT ISO-100 1/30sec
Sunspots: Apr 23, 2025 9:48am EDT ISO-400 1/50sec
Jupiter & 4 moons: Apr 22, 2025 8:58pm EDT ISO-20000 1/13sec
Tree Top: Apr 23, 2025 6:28am EDT ISO-200 1/100sec
