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02-04-2018, 04:01 PM   #1
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Adding diaphragm to a mirror lens?

Hi all;

Just thinking out loud here.

My understanding is that many mirror lenses focus beyond infinity and that adding a short extension tube (which, depending on the lens/camera combination, might be necessary to clear the front of the camera's flash housing) does not prevent infinity focus. Most mirror lenses produce an image circle that covers a full frame film or sensor. These two observations lead me to ask several questions:

1) Could these two characteristics be used to introduce an iris diaphragm mechanism between the lens and the camera, permitting a bit of stopping down before vignetting happens?

2) Would this result in any improvement of image quality? Certainly there will be a penalty to pay in viewfinder brightness, but I wonder if you might also get a slight increase in sharpness. I suppose you're also running into diffraction effects, though as well.

I have an old Russian 1000mm f10 lens with M42 mount. There are iris diaphragm mechanisms that have M42 cases. I was thinking about using something like that to incorporate an aperture of sorts, assuming that a) it would work and b) not degrade image quality.

Any thoughts?

02-04-2018, 05:02 PM   #2
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I'm probably over-thinking this and making it more complex than it needs to be. Not the first time I've ever done that.... Adding a piece of black card into the rear opening (where many mirror lenses allow the addition of ND filters) would do the same thing, presumably. Using a fixed opening (as opposed to a variable aperture, much of the range of which would be unusable because of vignetting) would be a lot more straightforward and much cheaper.

I've come across conflicting information about adding a cap with a circular opening over the front of the lens, with the opening placed between the central obstruction of the secondary mirror and the edge of the lens. Some say this really only reduces the amount of light entering the system, while others claim this arrangement increases DOF or at least contrast https://www.diyphotography.net/improving-samyang-800mm-tuning-key-and-some-cardboard/. I've heard of astrophotographers using a series of step down rings to improve image quality by using the rings as as an external aperture control on a normal refracting lens, thereby obviating the need to close down the lens's built in aperture, which, if the opening is no longer circular due to the configuration of the diaphragm blades, could result in diffraction spikes.Step down rings - DSLR & Digital Camera Astro Imaging & Processing - Cloudy Nights

Any thoughts on any of this? I guess I could break out some cardboard and scissors...
02-04-2018, 05:23 PM   #3
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I use an 8" Dobsonian telescope and sometimes put a cardboard template over the thing when I want to look at the moon. This works to darken the image but obviously there is no way to tell what the dof would be since it's all at infinity.
02-04-2018, 06:05 PM   #4
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QuoteOriginally posted by Thagomizer Quote
Hi all;

Just thinking out loud here. ... Any thoughts?
Google Ohnar 300mm mirror. Sometimes crops up on ebay. This had a diaphragm at the front.

Ohnar 300mm F5.6-16 Reflex Mirror ??? @ Leon????? :: ??? Xuite??


Last edited by kh1234567890; 02-04-2018 at 06:11 PM.
02-04-2018, 07:06 PM   #5
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I have never tried it, but if you cut out a circle on a larger piece of cardboard (diameter equal to front of lens), and the opening is tangent to the inner and outer edges of the glass area in the lens front, it will act as a smaller aperture and (thus) should give greater depth of field.

Adding a short tube should make it focus closer, and likely lose infinity focus, and an aperture there is likely much to far back to work.
02-05-2018, 02:56 AM   #6
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QuoteOriginally posted by Thagomizer Quote
Hi all;

Just thinking out loud here.

My understanding is that many mirror lenses focus beyond infinity and that adding a short extension tube (which, depending on the lens/camera combination, might be necessary to clear the front of the camera's flash housing) does not prevent infinity focus. Most mirror lenses produce an image circle that covers a full frame film or sensor. These two observations lead me to ask several questions:

1) Could these two characteristics be used to introduce an iris diaphragm mechanism between the lens and the camera, permitting a bit of stopping down before vignetting happens?

2) Would this result in any improvement of image quality? Certainly there will be a penalty to pay in viewfinder brightness, but I wonder if you might also get a slight increase in sharpness. I suppose you're also running into diffraction effects, though as well.

I have an old Russian 1000mm f10 lens with M42 mount. There are iris diaphragm mechanisms that have M42 cases. I was thinking about using something like that to incorporate an aperture of sorts, assuming that a) it would work and b) not degrade image quality.

Any thoughts?
I suspect it may not work too well with an aperture mechanism like this, behind the lens.
It is far from the ideal position of the aperture and may lead to optical problems.
But it would be interesting to see how it works and how the images looks like.

Mirror lenses probably have fixed aperture as the ideal position for the aperture would be close to the main mirror, but it does not work to put the aperture mechanism there as it would block the reflected light. So the best compromise is to have fixed aperture. (the whole concept of mirror lens is a compromise to get weight and cost down)
02-05-2018, 08:59 AM   #7
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Unless the mirror lens has auxiliary optics near the eyetube, there's no way to introduce an iris there. Placing an iris at that location otherwise will just introduce a shadow of the iris. Placing an entrance stop in front of the objective is really the only way to "stop down" one of these mirror lenses, and if a donut (with a central hole) is used, it will increase depth of field. Some entrance stops are just an offset hole which reduces the light but doesn't affect DOF that much. You could conceivably make a large "iris" out of cardboard or plastic pieces to create a variable iris that would fit on the front, but I imagine it would be an awkward, hard to manage contraption (you can use two pieces with offset holes which rotate one on top of the other to make a variable iris but DOF is fixed). It's easier to just use a single piece of cardboard to obtain one reduced stop value. Another thing that's going to affect is the bokeh which is unique to mirror lenses anyway. Stopping them down with a donut will exaggerate that bokeh effect. The central donut hole can't be any smaller than the central obstruction or it will block all light from the lens.
02-05-2018, 05:13 PM   #8
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Most mirror lenses are of the catadioptric design. There is a curved mirror at the mount end of the tube that directs light back to the front where there is a secondary mirror that bounces the light down through a hole in the primary mirror to focus it on the film or sensor behind. Some designs use lenses placed in the light path to correct for spherical aberrations inherent in the design. A traditional iris at the front of the lens would quickly be obstructed by the center button that covers the secondary mirror. I may be mistaken but I think diffraction would degrade the image more than a smaller aperture would help it. I might be technically possible to design such a system but I believe it would be completely impractical.

02-05-2018, 07:03 PM   #9
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QuoteOriginally posted by dakight Quote
I may be mistaken but I think diffraction would degrade the image more than a smaller aperture would help it. I might be technically possible to design such a system but I believe it would be completely impractical.
I figured that might be the case; I'll experiment with cardboard at various parts of the light path and see what happens. I'll post my results here, but no guarentee how soon that will happen. Thanks everyone for the input!.
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