This is only experimental in the sense of an experiment which went astray!
The Kodak Duaflex II shown below takes 620 film which is no longer available.
So I respooled some 120 onto the 620 spool - I've done it many times. However,
I put IE400 T-Max on in error - for a camera which has one speed (1/30) and
only three apertures. I should have spooled IE100, or even IE50, film.
To avoid the overexposure disaster which awaited, I decided to rate the film
at IE200 for development, but also to use a neutral density 'filter' to also
cut the speed. What to do? I cut and taped a piece of clear negative film base
over the lens, and did the same for my exposure meter. This image is an
un-retouched scan, and the exposure is good enough.

I see here, that at f8 (max aperture), the vignetting is quite fierce. Very Lomo.
Other than that, at least I know that the camera
works, and I will do it all again with 50 asa film. :-)