We were discussing the possibilities of stopping down the 110 70mm lens with a disc on the front in another thread.
https://www.pentaxforums.com/forums/136-pentax-q/294184-pentax-auto-110-lenses-q-2.html
So I thought I would have a go.
The disc is black paper cut to fit in the 49mm filter thread of the lens and held in place by the lens hood.
I used a cheap panel steel circle cutter in my drillpress.
I removed one of the blades and sharpened up the other.
And clamped the black paper on top of a cutting board onto the base of the drill.
Cut the centre first and cut the outer ring without moving the paper.
I suspected that even wide open the lens used only the centre 25mm of the front lens.
So I cut one disc with a 25 mm aperture and the other as small as the cutter would go which was about 19 mm and should be about one stop slower than 25mm.
Test image is a ruler placed on mattboards-- one light grey -- one dark grey.
the camera is at about 45 degrees to the subject to display DOF on the ruler.
The 2 tones of grey was to create a 2 spiked histogram to compare exposure.
The results are screenshots with histogram included and turned on their side to better fit the format here.
The result show no difference in exposure between the 25mm disc and the lens wide open, confirming my suspicions the lens doesn't use the outer glass.
The 19mm disc is indeed one stop slower but only slightly sharper.
I will leave the 25mm disc permanently there to keep light from scattering off the unneccessary extra glass.
The last image is there to remind us that the lens can not make a decent image unless it is attached to a camera and used!