I've spent some time going over Alan Wood's stuff again. The field number quoted for a 2.5x photo eyepiece with a full frame image sensor is 17.3. Further down the page, the field number quoted for the 2.5x photo eyepiece (not relative to any particular sensor size) is 21.6. Since the 'real' field number for the 2.5x photo eyepiece is 21.6, one needs to multiply this number by 2.5 to get the real image diagonal of 54mm. A full frame image sensor has a diagonal of 43.3mm, which comfortably fits inside this beam of light. With a relative field number of 17.3 [(1/2.5x) * 43.3mm], one knows they're missing some of what can be seen through the [non-photo] eyepiece, but the field-of-view is pretty closely matched. In return, there shouldn't be any vignetting. Now let us consider what would happen if we attached a U-PMTVC with its 0.3x reduction lens. It takes the circular image with a 54mm diagonal from the previous stage and multiplies it by 0.3, which should give us a circular image with a 16.2mm diagonal. This matches Alan Wood's number, and the 15.9mm video camera image diagonal he mentions fits inside it.
Now that we've seen how the math worked with the 2.5x photo eyepiece, let's see how it should apply to the 3.3x photo eyepiece. Please correct me if my facts or math is wrong. The real field number quoted for the 3.3x photo eyepiece (not relative to any particular sensor size) is 16.7. When we multiply 16.7 by 3.3, we get a real diagonal of 55mm. Multiplying that by 0.3 gives us an image diagonal of 16.5mm, which is way smaller than the micro four thirds image sensor; however, as I previously noted, the Panasonic Lumix GH4 has a UHD 4K diagonal of 15.63mm, which definitely fits. This wouldn't be true by default on the GH5 though.
If I'm understanding the math correctly, it doesn't seem like the U-PMTVC affects the field number.
Tom Jones wrote: ↑Sat Dec 26, 2020 3:33 pm
The image circle matches the sensor, and the eyepiece FOV very well.
Since you have all of the photo eyepieces, would you be so kind as to grab a UHD 4K frame with each photo eyepiece. I imagine centering that U-PMTVC is kind of a pain, so I'll be patient. You could resize the resulting frames down to something reasonable for posting here. If all of the frames were shot using the same objective and looking at a calibration slide and you told us the objective magnification, we should be able to know how much wider the field-of-view would have been had we been looking through your [non-photo] eyepieces.