Stopping down a mirror lens

Karl Persson wrote:

Mirror lenses have fixed apertures, they can´t be stopped down.

Well, they actually can. I´ve seen examples where a section of the front has been covered.
But I have a better idea.

I made a 5mm wide ring (in plastic) to fit inside the filter thread and put it on my Samyang 300/6,3 mFT lens.
Not only did it stop the lens down by ~2/3 stop, it transformed the lens from unusable to nearly usable

So, if you have a better mirror lens, that is close to usable, you might be able to make it usable with the discribed method.

I've done similar things with black card/thick paper, but for projector lenses rather than a mirror lens.

I found my 300/6.3 to be quite good straight out of the box, I think I actually prefer it to the 100-300 I subsequently got!

It's probably worth the investment to try the trick on my other mirror lenses

Karl Persson wrote:

Mirror lenses have fixed apertures, they can´t be stopped down.

Well, they actually can. I´ve seen examples where a section of the front has been covered.
But I have a better idea.

I made a 5mm wide ring (in plastic) to fit inside the filter thread and put it on my Samyang 300/6,3 mFT lens.
Not only did it stop the lens down by ~2/3 stop, it transformed the lens from unusable to nearly usable

So, if you have a better mirror lens, that is close to usable, you might be able to make it usable with the discribed method.

That will give you even worse (thinner) doughnuts.

You can actually end the doughnuts entirely by making a front-mounted stop that has a hole that fits between the outer edge of the lens and the inner blocked circle. It doesn't matter that the hole isn't centered; it still works as an aperture. It will stop the lens down a lot -- typically about 4 stops minimum. That means your f/8 mirror becomes more like f/32, which implies resolution will be diffraction limited and any tiny spec of dust on your sensor will make a crisp shadow (i.e., you'll need to clean your sensor).