Macro pic for checking clear aperture

I have just received one sample, reportedly from last batch, of the Chinese NORINCO binoviewers.
The opening at the bottom of the EP end seemed somehow small to my eyes so I took a macro photo of that end, blew it up on the monitor and measured with a ruler: I am getting just 16mm, less than I expected.
Do I make a logical mistake somewhere or is it so ?

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David,

I tried to duplicate your image with my unit. I have placed the two images side by side with mine on the left. I have repeatedly measured the opening of the bottom of the eyepiece holder on my Burgess and it is 20mm.

If you have a compass or something similar, try carefully inserting the compass into the eyepiece holder and setting it to the width of the opening, Then remove the compass and measure the distance you have it set to.

The true clear aperture of these units is measured at the bottom of the unit and they are roughly 22mm. There is a little bit of edge clipping by the prism that makes me say roughly.

Don Durbin

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David Simper said:
I am getting just 16mm, less than I expected.
Do I make a logical mistake somewhere or is it so ?

Place a scale right against the entrance aperture and take a picture through the eyepiece end (or look through) to see how much of the scale you can see. This will give the true entrance aperture and will account for any internal vignetting.

Roland Christen