>>3269720>>3269721If you don't know what you are talking about, please, stop posting. An ND filter doesn't block light. It is transparent and does not affect DOF. Opaque objects block light. You change the aperture yourself (which changes the DOF) when using an ND filter. Aperture shapes such as those in
>>3269716 all affect DOF, but in different ways. They also generate different forms of aberrations of varying levels. This is one of the few reasons mirror lenses have terribly thin DOF and rarely anything higher than f/10. An normal aperture, such as the adjustable aperture on this mirror lens, will change the DOF, but not noticeably at all. It is really only for T-stop control to view bright objects. While a center aperture does give higher F-stop numbers, it is also barely noticed on a mirror lens. The most noticed thing will be the T-stop change.
These are all reasons why mirror lenses are only used as telescope for viewing celestial objects where DOF doesn't matter. It is also why they are not something big name brand companies are willing to do much with. Their very nature forces them into niche roles.
