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05-07-2010, 01:46 PM   #16
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Here are some more photos; note the softness:

https://www.pentaxforums.com/forums/post-your-photos/3636-doughnuts.html#post30681


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05-07-2010, 02:04 PM   #17
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QuoteOriginally posted by Adam Quote
There is very little DOF, and you can't stop down. Thus, the slightest hand motion (assuming you're not on a trippod) is an immediate killer to the focal point.

AF-C would theoretically solve that problem.
Okay--now I understand the concept. But you can still hit the shutter (on a k-x anyway) in AF-C mode even if the focus isn't locked in.

And wouldn't catch focus accomplish the same thing anyway?
05-07-2010, 02:09 PM   #18
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QuoteOriginally posted by Ira Quote
Okay--now I understand the concept. But you can still hit the shutter (on a k-x anyway) in AF-C mode even if the focus isn't locked in.

And wouldn't catch focus accomplish the same thing anyway?
Theoretically, yes it would, but there is so little DOF that even the slightest delay in the capture would throw the focus off from tack-sharp.

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05-07-2010, 02:26 PM   #19
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I don't know, I find DOF at 500mm and f/8 more than deep enough to withstand the millimeter or two the cmaera might move as a result of pushing the shutter buttton. The mirror lenses I have used (Sigma 600/8 and Samyang-made Vivitar 500/8) are no harder to manually focus than any other MF lens I've used, except that they do indeed make for a pretty dim viewfinder image.

But given that there *are* no AF mirror lenses for Pentax (only a Minolta/Sony one as far as I know), and that there are also no other lenses in this focal length range for even remotely close to the same price range, the issue is kind of moot - it's an MF mirror lens or nothing for most of us.

OK, there are also fixed-aperture simple refractive lenses - ridiculously long hollow tubes with glass on either end like a cheap telescope - that are no more expensive. But they suffer the same disadvantages, and don't really perform as well overall - plus they're just a lot bigger.

Any, do check out the Mirror Lens Club thread as another poster mentioned - tons of sample pictures and discussion there.

05-07-2010, 03:03 PM   #20
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QuoteOriginally posted by Rense Quote
You're not seeing it???? It's all over the place, especially in de reds!
There is a mirror lens club, with many photos, you have to look there! I will find the link for you.
Now I see it. I was looking for 1 GIANT doughnut ring in the picture. I didn't even pay attention to the bokeh in the photo. Thats not too bad, I can live with that.

Thanks for the mirror club link too. There are actually some very nice shots on that club forum. I think I will actually buy one now.
05-07-2010, 08:38 PM   #21
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QuoteOriginally posted by Ira Quote
Okay--now I understand the concept. But you can still hit the shutter (on a k-x anyway) in AF-C mode even if the focus isn't locked in.

And wouldn't catch focus accomplish the same thing anyway?
Catch focus does work fairly well on my Opteka (Samyang)500/8. The biggest difficulty is keeping the camera steady. 500mm is a lot and even though the lens is very light, it's difficult to keep steady without a tripod and the least movement seems to show up. Maybe I drink too much coffee..
05-08-2010, 12:04 AM   #22
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QuoteOriginally posted by Adam Quote
There is very little DOF, and you can't stop down.
Some allow you to stop down, but given that they start with a small aperture, they can be hard to use. With some bricolage you could stop any of them down. The idea would be to have a cap with a cutout in the middle - I can't remember which mirror model also had this thing - you could remove the middle of the lens cap to stop it down.
05-08-2010, 12:07 AM   #23
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And the doughnut bokeh can be fun:



Or is it just me?

05-08-2010, 12:13 AM   #24
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No LC, not just you.
I like the effect too:
05-08-2010, 05:00 AM   #25
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QuoteOriginally posted by Laurentiu Cristofor Quote
Some allow you to stop down, but given that they start with a small aperture, they can be hard to use. With some bricolage you could stop any of them down. The idea would be to have a cap with a cutout in the middle - I can't remember which mirror model also had this thing - you could remove the middle of the lens cap to stop it down.
Mirror lenses with a variable aperture er near and far between and I do not know of any current modell with a aperture setting. The old ones suffered from visible astigmatism due their construction (folded mirror lenses) and the absence of the needed correction lens (which would have made these lenses way too expensive). So these variable aperture mirror lenses are not really a feasible alternative.

One could instead add a mask in front of the mirror to step down. My old Sigma 500/4 has a mask in the lens cap, which gives you f/11 or so, for added DOF.

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05-08-2010, 05:02 AM   #26
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QuoteOriginally posted by Laurentiu Cristofor Quote
And the doughnut bokeh can be fun:



Or is it just me?
Very nice image - but your lens has severe coma and/or spherical aberration.

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05-08-2010, 05:03 AM   #27
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QuoteOriginally posted by Marc Sabatella Quote
I don't know, I find DOF at 500mm and f/8 more than deep enough to withstand the millimeter or two the cmaera might move as a result of pushing the shutter buttton. The mirror lenses I have used (Sigma 600/8 and Samyang-made Vivitar 500/8) are no harder to manually focus than any other MF lens I've used, except that they do indeed make for a pretty dim viewfinder image.
The dim viewfinder image is due to the central obstruction of the catadiotric lenses, which takes away app. 1 more f-stop from the relative aperture. So you get the resolution of a 500/8, but only light gathering of a 500/11.

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05-08-2010, 12:57 PM   #28
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QuoteOriginally posted by Ben_Edict Quote
Very nice image
Thanks.

QuoteOriginally posted by Ben_Edict Quote
but your lens has severe coma and/or spherical aberration.
I assume you're talking about the variable thickness of doughnuts towards the edges of the frame, right? It was my impression that all mirror lenses give this - the outer side of the doughnut gets thinner the closer it is to the edge. It's probably more obvious in my shot because it only includes doughnuts and those lights were a mile away.

QuoteOriginally posted by Ben_Edict Quote
One could instead add a mask in front of the mirror to step down. My old Sigma 500/4 has a mask in the lens cap, which gives you f/11 or so, for added DOF.
That's what I was trying to explain. So the Sigma had this then? I've seen so many mirror lenses on ebay that I forgot which had which feature.
05-08-2010, 03:34 PM   #29
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QuoteOriginally posted by Laurentiu Cristofor Quote
I assume you're talking about the variable thickness of doughnuts towards the edges of the frame, right? It was my impression that all mirror lenses give this - the outer side of the doughnut gets thinner the closer it is to the edge. It's probably more obvious in my shot because it only includes doughnuts and those lights were a mile away.
Yes, your shot makes that very onbvious. But not all mirros lenses need to show that. It is probably a mixture of different aberrations. I think, the smaller the size of the lens, the harder it is to correct these. I see, you have used the Tokina, which is a nice lens for most things (I have one myself...). It could also be simple vignetting by oblique rays, which cause
1. the offset central dark spot and
2. the "cut off" on the outer rim of the disk.

Catadioptric lenses have internal baffles around the central hole in the main mirror and ofcourse the standard barrel components, which may cause vignetting. With catadioptrics it is always hard to decide, whether to use more effective baffles, to increase contrast, or to use minimum sized baffles, to reduce vignetting. You can't have high contrast and low vignetting at the same time - and it is harder to achieve, the smaller the lens.


QuoteOriginally posted by Laurentiu Cristofor Quote
That's what I was trying to explain. So the Sigma had this then? I've seen so many mirror lenses on ebay that I forgot which had which feature.
You could cut masks for any mirror lens out of simple cardboard. The Sigma is quite a rare lens and I have seen only very few on ebay over time. My own copy needs a good internal clean, which I have to postpone, until we have moved into our new house late summer...

Ben
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