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Stopping down a mirror lens

Started Jan 14, 2021 | Discussions thread
ProfHankD
ProfHankD Veteran Member • Posts: 9,147
Apertures for mirror lenses
1

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).

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