I'm assuming the lens focal length is indeed 80 mm, not 81.6 or something like that (which it usually is - never the exactt nominal focal length).
Originally Posted by Joe O'Brien
With the lens set to infinity, 18 mm of extension added, the scale is about 1:4.4, i.e. the smallest field of view will be about 25 cm square.
To calculate the scale and field of view with the lens set to the cose focus limit, i would need to know the minimum focussing distance. Which i don't.
So i'll assume (as i have done earlier) that it is about 90 cm/3 ft, i.e. an extension of just short of 9 mm.
Then, with the extra 18 mm, the scale will be about 1:3, the minimum field of view about 17 cm square.
By the way: assuming that minimum focussing distance of about 90 cm, the scale will be about 1:9 , the minimum field of view about 51.4 cm square.
So there is a leap from a field of view of about 51 cm to one of about 25 cm when adding the 18 mm tube. A gap you could bridge by using a tube of 9 mm length (if there is such a thing for the SQ-A).