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Clear View 500mm Mirror lens with adjustable aperture
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PostPosted: Sun May 03, 2015 6:27 pm    Post subject: Clear View 500mm Mirror lens with adjustable aperture Reply with quote
Anyone have any idea who made this? It's the only mirror lens I've ever seen with an adjustable aperture, f/8, f/?, f/11 and f/16. BTW, the church building is 1.2 miles away; the biker around 1000'. The old jeep is about 250' away.






PostPosted: Sun May 03, 2015 6:55 pm    Post subject: Re: Clear View 500mm Mirror lens with adjustable aperture Reply with quote
tim48v wrote:
Anyone have any idea who made this? It's the only mirror lens I've ever seen with an adjustable aperture, f/8, f/?, f/11 and f/16. BTW, the church building is 1.2 miles away; the biker around 1000'. The old jeep is about 250' away.






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PostPosted: Sun May 03, 2015 8:38 pm    Post subject: Reply with quote
This is the second mirror lens with adjustable aperture I've seen - the first was an Ohnar brand (I think 300mm?). According to what I read the "aperture control" on that lens is only good for adjusting light transmission, and doesn't actually affect depth of field, which is physically impossible due to how mirror lenses work. Probably the same thing is true for this lens, though I don't know for sure.


PostPosted: Sun May 03, 2015 10:53 pm    Post subject: Reply with quote
glaebhoerl wrote:
This is the second mirror lens with adjustable aperture I've seen - the first was an Ohnar brand (I think 300mm?). According to what I read the "aperture control" on that lens is only good for adjusting light transmission, and doesn't actually affect depth of field, which is physically impossible due to how mirror lenses work. Probably the same thing is true for this lens, though I don't know for sure.


If it is 'physically impossible due to how mirror lenses work' then it MUST be true for this one too.
I'm not sure that it is physically impossible for all mirror lenses, but adjusting DOF with aperture certainly isn't practical for the normal design of mirror lenses. I've not seen any designs for reflecting telescopes that don't use a central secondary mirror but I have a reflecting 360° adapter (Kaidan 360) that could be considered to be a mirror lens. Aperture certainly works with that, and perhaps could with other oddball mirror systems.

Neither of the Catadioptric lenses I've had came with any alternative filters (many had 4 when new) but I'm working on some adapters for my 25mm technical filters to try my current 600mm on my full spectrum camera. Wink


PostPosted: Sun May 03, 2015 11:38 pm    Post subject: Reply with quote
I never gave DOF a thought when using a mirror lens. An adjustable aperture would be great if you could just stop it down enough to get rid of the softness of full aperture. I bet you can't do that.

How does an aperture work on a mirror lens anyway. And where is it?


PostPosted: Mon May 04, 2015 9:26 am    Post subject: Reply with quote
DConvert wrote:
If it is 'physically impossible due to how mirror lenses work' then it MUST be true for this one too.


Of course. But, again, I only *read* this somewhere - I don't know if it's true.


PostPosted: Mon May 04, 2015 10:07 am    Post subject: Reply with quote
My experience with mirror lenses, even a good one like the Tamron AD2 500mm f8, is that they are pretty dark for an f8 already, so it is rare that you would want to go lower. Mind you on a bright day, a bit of extra DOF would be welcome, as nailing focus is a bit of a nightmare.


PostPosted: Mon May 04, 2015 11:02 am    Post subject: Reply with quote
Basilisk wrote:
My experience with mirror lenses, even a good one like the Tamron AD2 500mm f8, is that they are pretty dark for an f8 already, so it is rare that you would want to go lower. Mind you on a bright day, a bit of extra DOF would be welcome, as nailing focus is a bit of a nightmare.


There are 300m 'f/5.6' versions though the 500mm 'f8' is much more common.
I too find them a little slow under any but bright light especially as the quoted apertures are generally very optimistic. I think apertures are quoted based on objective diameter & focal length & just ignore the obstruction of the secondary mirror at the center or the objective, which will typically reduce the light be nearly one stop from what I've read.

More DOF would certainly be useful, but I suspect I'd find camera shake even more of a problem. Using a 600mm on a �4/3 body for 1200mm EFL makes a sturdy support rather helpful. On my DSLR with IBIS hand held shots have proved practical, but shooting motorbike racing I found as soon as there was any action it moved out of the normal line where I could prefocus. None of the shots I got of riders sliding into the gravel are in focus
Embarassed

Does masking the outside of the objective increase the DOF with a mirror lens?

Last edited by DConvert on Thu Feb 24, 2022 1:57 pm; edited 1 time in total


PostPosted: Mon May 04, 2015 12:02 pm    Post subject: Reply with quote
philslizzy wrote:
I never gave DOF a thought when using a mirror lens. An adjustable aperture would be great if you could just stop it down enough to get rid of the softness of full aperture. I bet you can't do that.

How does an aperture work on a mirror lens anyway. And where is it?

The Makowsky Katoptaron was a mirror lens with a variable aperture and an original design featuring two mirrors and an "off-axis" folded optical path.

See the pictures on our fellow member kds315*'s website:

http://www.macrolenses.de/ml_detail_sl.php?ObjektiveNr=292

The LDM-1 was also based on the same optical construction:

http://www.macrolenses.de/bilder/LDM-1.pdf

Cheers!

Abbazz

[EDIT] Here's an article from the August 1980 issue of Popular Science on this lens:



PostPosted: Mon May 04, 2015 4:15 pm    Post subject: Reply with quote
I don't want to ignite a discussion on the nature of light and I admit it's been several decades since I took a physics class, but I would think that depth of field would be independent of how the light is being bent.

I've posted two more photos of a quick experiment: the houses are about 1200' away. The focus point was the blueish triangle area in the middle of the photo. You can see the shingles on the on house in front of it, appear to be a bit sharper at f/16. (It was easier to see before I reduced the files for uploading. Of course, it may just be my imagination or digital noise or whatever.) I've also posted two photos of an old hitch (50' away), you can see the bokeh is different between f/8 and f/16. It's hardly conclusive and I need to do more testing.

The issue may be that the depth of field is so shallow that it really doesn't matter. According to my calculations, the DOF at 1200' is only 180' in front of the focus point; at f/16, it's only 300'. And at 50', it's only around .7'.

Regarding how it works, this lens has a tube that extends towards the front mirror as you increase the f number, blocking the light from the main mirror.






PostPosted: Mon May 04, 2015 4:36 pm    Post subject: Reply with quote
Abbazz wrote:
philslizzy wrote:
I never gave DOF a thought when using a mirror lens. An adjustable aperture would be great if you could just stop it down enough to get rid of the softness of full aperture. I bet you can't do that.

How does an aperture work on a mirror lens anyway. And where is it?

The Makowsky Katoptaron was a mirror lens with a variable aperture and an original design featuring two mirrors and an "off-axis" folded optical path.

See the pictures on our fellow member kds315*'s website:

http://www.macrolenses.de/ml_detail_sl.php?ObjektiveNr=292

The LDM-1 was also based on the same optical construction:

http://www.macrolenses.de/bilder/LDM-1.pdf


Cheers!

Abbazz

[EDIT] Here's an article from the August 1980 issue of Popular Science on this lens:



I love it - it is bit of a monster, and of little practical benefit as far as I can see. It would be great to give it a go though.


PostPosted: Mon May 04, 2015 11:08 pm    Post subject: Reply with quote
tim48v wrote:
I don't want to ignite a discussion on the nature of light and I admit it's been several decades since I took a physics class, but I would think that depth of field would be independent of how the light is being bent.

I've posted two more photos of a quick experiment: the houses are about 1200' away. The focus point was the blueish triangle area in the middle of the photo. You can see the shingles on the on house in front of it, appear to be a bit sharper at f/16. (It was easier to see before I reduced the files for uploading. Of course, it may just be my imagination or digital noise or whatever.) I've also posted two photos of an old hitch (50' away), you can see the bokeh is different between f/8 and f/16. It's hardly conclusive and I need to do more testing.


Depth of field would indeed by independent of how the light is bent but having an off access system removes the need for a mirror obstructing the central portion of the objective. This makes normal style aperture control practical.

I wasn't convinced of any great difference between either of your sets of shots, perhaps 100% crops might show it better...


PostPosted: Wed May 06, 2015 5:27 am    Post subject: Reply with quote
Here's an interesting link: http://www.diyphotography.net/improving-samyang-800mm-tuning-key-and-some-cardboard/
He claims that increasing the f number improves DOF and contrast.

Again, I think that since the DOF is so shallow that it's hard to see the effect. I'll try and take some more photos later.


PostPosted: Wed May 06, 2015 7:16 am    Post subject: Reply with quote
tim48v wrote:
Here's an interesting link: http://www.diyphotography.net/improving-samyang-800mm-tuning-key-and-some-cardboard/
He claims that increasing the f number improves DOF and contrast.

Again, I think that since the DOF is so shallow that it's hard to see the effect. I'll try and take some more photos later.


Thanks, interesting link. If it boosts contrast it's got to e worth a try!
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