Anyone else tinkering with a mirror lens?

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Anyone else tinkering with a mirror lens?

Postposted on Wed Jan 12, 2011 12:50 am

Who else has played, or is playing, with a mirror lens? (Didn't get many hits in a quick search of the forum.) Just picked up a Vivitar 800mm f/8 off eBay about a week ago, Canon EF T-mount adapter included. Not the greatest quality option, but for $92 shipped, it roughly matches the depth of my interest in astronomy.

Image

That's a reduced, but uncropped, frame from my Canon 40D, shot at 1/40 second and ISO 200. Not the sharpest thing in the world, but all else equal, way better than I could get with my Tokina 80-200mm f/2.8 and Tamron-F 1.4x teleconverter. And that combination generally required much higher ISO and slower shutter speeds.
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Re: Anyone else tinkering with a mirror lens?

Postposted on Wed Jan 12, 2011 1:56 am

Haven't tinkered with one, but have always kinda thought about it...

Pic looks great - you have a rock-solid tripod for that thing?
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Re: Anyone else tinkering with a mirror lens?

Postposted on Wed Jan 12, 2011 2:59 am

ludi wrote: Not the sharpest thing in the world, but all else equal, way better than I could get with my Tokina 80-200mm f/2.8 and Tamron-F 1.4x teleconverter. And that combination generally required much higher ISO and slower shutter speeds.


The 70-200/2.8 with 1.4x converter should come out to f/4, which is 2 stops faster than the f/8 mirror lens, so in theory should require lower ISO and faster shutter speeds than the mirror lens.

Agree that it is not the sharpest sample I've seen from a cat lens, the good ones I've seen have been razor sharp. Not sure if that is due to atmospheric effects, the slow shutter speed (allowing movement of the moon and air current distortions), the lens, or the user (sorry, have to put this in to be safe) though.

Shooting the moon at its zenith will probably allow you to shoot through the thinnest cross section of atmosphere, you can find online calculators that will tell you what time that happens at your location. It may also be worth focusing in Live View to get the best focus - you might even consider hooking up your body to a laptop and outputting the Live View image to screen as a focusing aid if your body supports it. Use a cable release if you have one or set the timer if you don't. Also use mirror lockup as you are very sensitive to vibrations at 1280mm effective focal length. Shoot RAW, underexpose slightly and pull shadow detail/recover highlight detail to expand your (effective) DR. Lastly, I'd try to get the shutter speed as fast as you can to minimize effects of moon transit, airmass and tripod/ground vibration. Oh, and where you're shooting from also makes a difference. If you're shooting across an urban area, you can get stray light and you'll also have more air distortion due to rising heat waves.

Hope this helps!
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Re: Anyone else tinkering with a mirror lens?

Postposted on Wed Jan 12, 2011 4:40 am

Great shot. Try to get some shots of stars. It's tricky and you pretty much have to hook up your camera to a computer and focus electronically (using Canon software) from the computer (AF won't work) to focus on the stars just right, but you should be able to get some amazing shots with that lens.
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Re: Anyone else tinkering with a mirror lens?

Postposted on Wed Jan 12, 2011 1:44 pm

swampfox wrote:Pic looks great - you have a rock-solid tripod for that thing?

Nope, just cheap one. I had to align the shot and step back for a few second to make sure the lens wasn't shaking, then trigger remotely. Tripod's a future upgrade, maybe in February or so. Have some routine car maintenance to deal with this month.

Voldenuit wrote:Agree that it is not the sharpest sample I've seen from a cat lens, the good ones I've seen have been razor sharp. Not sure if that is due to atmospheric effects, the slow shutter speed (allowing movement of the moon and air current distortions), the lens, or the user (sorry, have to put this in to be safe) though.

All of the above. It was about 2F (-17C) last night when I shot this, so I only stepped out to the porch for a few minutes to get an idea what it could do.

More experimentation needed once things warm up a bit. Should have a full moon around this time next week, will maybe go up into the foothills and try again from there.
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Re: Anyone else tinkering with a mirror lens?

Postposted on Wed Jan 12, 2011 10:50 pm

ludi wrote:All of the above. It was about 2F (-17C) last night when I shot this, so I only stepped out to the porch for a few minutes to get an idea what it could do.

More experimentation needed once things warm up a bit. Should have a full moon around this time next week, will maybe go up into the foothills and try again from there.


Ah, make sure you haven't got any condensation on the front element when you bring it out of a warm environment to shoot, as that can soften the image and rob you of contrast. You might want to bag the lens (with a little silica gel inside the bag) or leave it out for a few minutes (preferably in a dry state so you don't get frost buildup) to equilibrate before shooting. Also the bright side of the moon is overexposed. I'd recommend underexposing by 2/3 to 1 stop (or more) - you can always add brightness in post, but once you clip a channel, you can't get back information. On the plus side, this will give you a faster shutter speed. Oh, and you might want to center the moon in the viewfinder, because the sharpest part of your lens will be in the center - you can adjust composition by cropping later.
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Re: Anyone else tinkering with a mirror lens?

Postposted on Thu Jan 13, 2011 1:00 am

I wouldn't mind one if they were more powerful. 800mm at f/8 is just too lame. They really could be longer at that aperture and still be cheap. Mirrors are very cheap to make large.

Manual focus wouldn't be a deal breaker for me if it had a hard, factory infinity stop.
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Re: Anyone else tinkering with a mirror lens?

Postposted on Thu Jan 13, 2011 1:47 am

Well, I am in the sense that I just purchased a T-adapter for my 8 inch Celestron Schmidt-Cassegrains. Its so darned cold I have not yet taken it outside to give it a try.
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Re: Anyone else tinkering with a mirror lens?

Postposted on Thu Jan 13, 2011 3:49 am

Mr Bill wrote:Well, I am in the sense that I just purchased a T-adapter for my 8 inch Celestron Schmidt-Cassegrains. Its so darned cold I have not yet taken it outside to give it a try.


Does the T-ring + T-adapter mount directly to the lens mount on your DSLR? I've been looking at their product pages, but i don't see any bayonet flanges. I'd probably have to get the adapter for Canon/Nikon then couple an EF or F to m43 adapter for my GF1.
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Re: Anyone else tinkering with a mirror lens?

Postposted on Thu Jan 13, 2011 11:30 am

Voldenuit wrote:Ah, make sure you haven't got any condensation on the front element when you bring it out of a warm environment to shoot, as that can soften the image and rob you of contrast. You might want to bag the lens (with a little silica gel inside the bag) or leave it out for a few minutes (preferably in a dry state so you don't get frost buildup) to equilibrate before shooting.

I thought that usually goes the other way -- when you bring it in from the cold, the condensation forms.

We usually get single-digit percentage humidity around here when the temperature gets that cold, so I doubt it had any moisture to pick up until I brought it back inside.
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Re: Anyone else tinkering with a mirror lens?

Postposted on Thu Jan 13, 2011 10:37 pm

ludi wrote:
Voldenuit wrote:Ah, make sure you haven't got any condensation on the front element when you bring it out of a warm environment to shoot, as that can soften the image and rob you of contrast. You might want to bag the lens (with a little silica gel inside the bag) or leave it out for a few minutes (preferably in a dry state so you don't get frost buildup) to equilibrate before shooting.

I thought that usually goes the other way -- when you bring it in from the cold, the condensation forms.

We usually get single-digit percentage humidity around here when the temperature gets that cold, so I doubt it had any moisture to pick up until I brought it back inside.


Oops, you're right, the cold air should be a lot drier than the warm air of your house/apt/car.
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Re: Anyone else tinkering with a mirror lens?

Postposted on Fri Jan 14, 2011 12:27 pm

Voldenuit wrote:
Mr Bill wrote:Well, I am in the sense that I just purchased a T-adapter for my 8 inch Celestron Schmidt-Cassegrains. Its so darned cold I have not yet taken it outside to give it a try.


Does the T-ring + T-adapter mount directly to the lens mount on your DSLR? I've been looking at their product pages, but i don't see any bayonet flanges. I'd probably have to get the adapter for Canon/Nikon then couple an EF or F to m43 adapter for my GF1.
Yes it does. You buy the T-adapter and then you need to also buy the actual ring that will fit your camera.
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I wonder, if you have to buy an additional adapter, if you will have problems getting a focus because of the extra distance offset?
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Re: Anyone else tinkering with a mirror lens?

Postposted on Fri Jan 14, 2011 1:25 pm

Mr Bill wrote:I wonder, if you have to buy an additional adapter, if you will have problems getting a focus because of the extra distance offset?

Could, the long end of the focus suffers first. I noticed my Opteka has focus travel at least five rotational degrees past the infinity mark, perhaps to compensate for the different adapters that might be required.
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Re: Anyone else tinkering with a mirror lens?

Postposted on Fri Jan 14, 2011 9:30 pm

Mr Bill wrote:I wonder, if you have to buy an additional adapter, if you will have problems getting a focus because of the extra distance offset?


It will depend on the equivalent flange back distance of the 'scope and the dimensions of the adapter combo. Ideally, the T adapter + lens mount adapter combo should be the right length to match the camera's sensor distance (remember that an optical scope has optics, so even if the physical length is shorter, the focal length might be longer than the physical length of the tube. Also, the eye-point adds effective length - your retina is futher away than the end of the tube).

If, however, the adapter + lens mount combo is the wrong length, you will lose some infinity focus if it's too long (as ludi says) or close-up focus if it's too short.

Many lenses (telescope or otherwise) focus past infinity for this and other reasons (such as temperature causing expansion of components).
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Re: Anyone else tinkering with a mirror lens?

Postposted on Thu Jan 20, 2011 2:09 am

Hmmm...I'm not sure my lens CAN get any sharper. I spent an hour or so Tuesday night shooting from a relatively dark plateau, tethered, at 11pm, and got nothing worth posting here. So then I went back to my house, laid flat on the front sidewalk with the camera at ISO 1600 and shutter of 1/8000, tweaked the focus as best I could, and got the five best images of the evening. None of them were any sharper than this:

Image

I'm thinking this particular lens just isn't that spectacular, although my manual focusing skills may not be too hot, either. Would help if the lens had some sort of ultra-fine screw focus mechanism, as the rotating barrel is a blunt instrument.
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Re: Anyone else tinkering with a mirror lens?

Postposted on Sat Feb 19, 2011 3:45 am

Conclusively, this mirror lens is not terribly sharp. I tried shooting the moon again Wednesday evening. The following have a bit of resizing, and some noise cleanup and brightness/contrast adjustment applied, but nothing else:

7:30PM, Tokina ATX 80-200mm f/2.8, Tamron-F 1.4x teleconverter, moon at about 60 degrees above the horizon:

Image

9:20PM, Opteka 800mm f/8 mirror, moon at about 80 degrees above the horizon:

Image

The mirror lens shot is slightly sharper but not by a lot. Granted, these were both handheld shots, but at 1/3200 second, it doesn't seem like vibration would be a hindrance. And the higher angle of inclination should have meant less atmospheric scatter.
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