I have very little information about this Lens. But the big thing is: you can set the aperture between 5.6 and 16 in full steps. The iris diaphragm has by the way 15 blades. See also the link below to the photo of the diaphragm.
It seems to me that the diaphragm really doesn't have any or just little effect. Use the link "Ohnar 300mm Test Page with 4 photos" to see for yourself.
With Babelfish i was able to squeeze these tech data out of the Japanese website below. Ohnar also made a 300mm mirror lens with a fix aperture. I guess this is the customized version - must be rare.
|Minimum focus distance||250 cm|
|Minimum length||66.0 mm|
|Front Filter diameter||67|
|Rear Filter diameter||-|