Why mirror lenses come without aperture blades?

[wolan]

Thanks.

[willy35]

[Gerald]

Essentially because the central rays are blocked by the secondary mirror. It is possible to use a diaphragm, but its action would be only on the marginal rays, so as to have practically no effect on the depth of field, for example. Besides, a diaphragm would further worsen the intrinsically bad bokeh of a mirror lens.

[dnas]

The aperture blades when stopped down in a regular lens, only allows light to pass from the central part of the lens.
A mirror lens has the secondary mirror in the central portion of the lens, so aperture blades can't work in the same way

[calvin83]

The Ohnar 300/5.6 Mirror comes with aperture blades. For other lens, it is more easy by adding a ND filter in the rear to reduce light.