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05-19-2015, 01:08 PM   #16
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QuoteOriginally posted by stevebrot Quote
The minimum aperture restriction with the K10D is f/8 for the center points and somewhat wider for the others. This is a hard boundary and is based on the aperture size (actually, exit pupil diameter) and not the amount of light.
Thanks, this is very interesting :-)

QuoteOriginally posted by stevebrot Quote
f/5.4 with the 1.4x convertor is not f/10
I know, this were two different cases. Hence the "and". Wy the way, I think the f/10 in question was also a refractor lens, but I'm not sure.

QuoteOriginally posted by Stone G. Quote
I would suspect the lens (: catadioptric mirror design). In contrast to refractor type lenses, light rays forming the image in a catadioptric design come from the perimeter of the lens, arrive at the sensor at a rather shallow angle and thus, precise focusing is VERY critical (if at all possible).
Hmm... I'm not sure about this. (or not sure to understand) If I follow your assumption, a wider lens (lower f-stop) would be even more problematic? As a wider lens would also lead to shallower angles? And does phase detection AF not rely on "different light beams"? It would be interesting to obstruct a normal (refractor) lens to see if it has an impact on AF :-)
I'm not sure that the catadioptric design make focusing more critical than on other lenses. Catadioptric designs are used in telescopes (SC, Dall Kirham, Ritchey–Chrétien, Maksutov, ...) and can be focused perfectly ;-)

QuoteOriginally posted by Stone G. Quote
when I put a 1.4X TC (f/8) or a 2X TC (f/11) CIF and/or focus confirmation work very well
On a catadioptric lenses?

QuoteOriginally posted by Stone G. Quote
It may also simply be, that your (and my) lens needs some collimation. Again, mirror lenses are much more sensitive to proper collimation than are refractor lenses.
This is true! Collimation is really important. I've done it several times on telescopes, and it's rather difficult to do xD

05-19-2015, 01:39 PM   #17
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QuoteOriginally posted by Azergoth Quote
Hmm... I'm not sure about this. (or not sure to understand) If I follow your assumption, a wider lens (lower f-stop) would be even more problematic?
I should say "Yes". With catadioptrics you get proportionally increasingly larger central obstructions the faster your design is, (and I believe that f/5.6 or f/6.3 is pretty fast compared to normal astronomical SCs and the like, which are usually around f/10?). Here is what adaptall-2.org has to say: "Accurate focusing is critical since the secondary mirror obstruction diameter is around 50%." (Link:Tamron SP Adaptall-2 350mm F/5.6 Model 06B ).

QuoteOriginally posted by Azergoth Quote
On a catadioptric lenses?
Yes, with my Kenko Pz-AF 1.5X Teleplus SHQ and my Tamron SP F System 2X Teleconverter, I do get CIF and focus confirmation with that same 350mm f/5.6 mirror lens! (And I don't need excessively bright scenes for that to work).
05-19-2015, 02:47 PM   #18
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QuoteOriginally posted by Stone G. Quote
With catadioptrics you get proportionally increasingly larger central obstructions the faster your design is, (and I believe that f/5.6 or f/6.3 is pretty fast compared to normal astronomical SCs and the like, which are usually around f/10?).
I believe the central obstruction is dependent on the optical design, and is frequently expressed as a percentage (up tu 50% for telescope for planets, for example). And indeed, a lot of SCs have apertures of f10 (good examples are f=2000mm and D=200mm (8")) but have less obstruction. Other design have much wider apertures, and are a hell to focus and to collimate (really? xD).
But in astronomy, focus is not done by phase detection (or I have never heard of) but by contrast (FWHM of a star, or how "wide" a star appears).

QuoteOriginally posted by Stone G. Quote
Yes, with my Kenko Pz-AF 1.5X Teleplus SHQ and my Tamron SP F System 2X Teleconverter, I do get CIF and focus confirmation with that same 350mm f/5.6 mirror lens! (And I don't need excessively bright scenes for that to work).
This is very interesting. Maybe I'll buy a teleconverter to try :-)
05-19-2015, 03:11 PM   #19
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Are you using a lens hood with your mirror lens? I think some people mentioned a dramatic improvement in image quality once they added their custom lens hood, which consisted of a dark matte paper tube (or was it plastic?). It was a really long lens hood, practically as long as the lens itself.

Anyway, I did a quick search about mirror lenses and CiF. This came up:
https://www.pentaxforums.com/forums/6-pentax-dslr-discussion/168432-catch-foc...rror-lens.html

And one of the reviews mentions CiF:
Samyang 500mm F6.3 DX Lens Reviews - Samyang Lenses - Pentax Lens Review Database

Seems to be a common thing. You will notice mirror lenses are quite special - bokeh, CA, fringing, sharpness, contrast.. then again, I haven't heard much good about the ultra-tele Samyang telescope-style lenses either. When it comes to 300mm and above, you should expect to pay a lot if you want decent quality (unfortunately). Some say the DA 55-300mm is the best affordable way to get to 300mm. Anything with better IQ that reaches 300mm will cost significantly more

05-20-2015, 12:22 AM   #20
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QuoteOriginally posted by Na Horuk Quote
Are you using a lens hood with your mirror lens? I think some people mentioned a dramatic improvement in image quality once they added their custom lens hood, which consisted of a dark matte paper tube (or was it plastic?). It was a really long lens hood, practically as long as the lens itself.
Yes, I use (not always, but I try to) the "official" Samyang hood, a nice metal hood. I managed to buy it for €5 of about $5.6

I'll read your links and do a research about it ;-)

QuoteOriginally posted by Na Horuk Quote
You will notice mirror lenses are quite special - bokeh, CA, fringing, sharpness, contrast.. then again, I haven't heard much good about the ultra-tele Samyang telescope-style lenses either. When it comes to 300mm and above, you should expect to pay a lot if you want decent quality (unfortunately). Some say the DA 55-300mm is the best affordable way to get to 300mm. Anything with better IQ that reaches 300mm will cost significantly more
Yes, I know... The bokeh is "special", but I can live with it. CA should be relatively limited thanks to the use of mirrors. But it's obvious, it's not a high quality lens.
The big advantage of this Samyang was it's price: I payed €50 or $55. With the lens hood, I payed $60. I can live with it

I managed to take some correct photos with it, but I had to reject A LOT of images. (But I also have a back focus problem... This is another story )

QuoteOriginally posted by Na Horuk Quote
Are you using a lens hood with your mirror lens? I think some people mentioned a dramatic improvement in image quality once they added their custom lens hood, which consisted of a dark matte paper tube (or was it plastic?). It was a really long lens hood, practically as long as the lens itself.
Yes, I use (not always, but I try to) the "official" Samyang hood, a nice metal hood. I managed to buy it for €5 of about $5.6

I'll read your links and do a research about it ;-)

QuoteOriginally posted by Na Horuk Quote
You will notice mirror lenses are quite special - bokeh, CA, fringing, sharpness, contrast.. then again, I haven't heard much good about the ultra-tele Samyang telescope-style lenses either. When it comes to 300mm and above, you should expect to pay a lot if you want decent quality (unfortunately). Some say the DA 55-300mm is the best affordable way to get to 300mm. Anything with better IQ that reaches 300mm will cost significantly more
Yes, I know... The bokeh is "special", but I can live with it. CA should be relatively limited thanks to the use of mirrors. But it's obvious, it's not a high quality lens.
The big advantage of this Samyang was it's price: I payed €50 or $55. With the lens hood, I payed $60. I can live with it

I managed to take some correct photos with it, but I had to reject A LOT of images. (But I also have a back focus problem... This is another story )

QuoteOriginally posted by Na Horuk Quote
Are you using a lens hood with your mirror lens? I think some people mentioned a dramatic improvement in image quality once they added their custom lens hood, which consisted of a dark matte paper tube (or was it plastic?). It was a really long lens hood, practically as long as the lens itself.
Yes, I use (not always, but I try to) the "official" Samyang hood, a nice metal hood. I managed to buy it for €5 of about $5.6

I'll read your links and do a research about it ;-)

QuoteOriginally posted by Na Horuk Quote
You will notice mirror lenses are quite special - bokeh, CA, fringing, sharpness, contrast.. then again, I haven't heard much good about the ultra-tele Samyang telescope-style lenses either. When it comes to 300mm and above, you should expect to pay a lot if you want decent quality (unfortunately). Some say the DA 55-300mm is the best affordable way to get to 300mm. Anything with better IQ that reaches 300mm will cost significantly more
Yes, I know... The bokeh is "special", but I can live with it. CA should be relatively limited thanks to the use of mirrors. But it's obvious, it's not a high quality lens.
The big advantage of this Samyang was it's price: I payed €50 or $55. With the lens hood, I payed $60. I can live with it

I managed to take some correct photos with it, but I had to reject A LOT of images. (But I also have a back focus problem... This is another story )









05-23-2015, 12:47 AM   #21
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Ow! Interesting: I found information in this patent:

https://www.google.com/patents/US4951078

Apparently, it would be a related to the obstruction of the exit pupil.
05-23-2015, 04:47 AM   #22
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QuoteOriginally posted by Azergoth Quote
I managed to take some correct photos with it, but I had to reject A LOT of images. (But I also have a back focus problem... This is another story )
Great photos! Yes, but don't be discouraged by a high rejection rate. With practice, experience it will go down. And all photographers have that! The trick is to show the best work, instead of the worst work, or drowning the audience in all of it
05-23-2015, 06:11 AM   #23
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QuoteOriginally posted by Na Horuk Quote
Great photos! Yes, but don't be discouraged by a high rejection rate. With practice, experience it will go down. And all photographers have that! The trick is to show the best work, instead of the worst work, or drowning the audience in all of it
Yes
The (additional) problem is: I think the mirror of my K10D is a little tilted, making thins a little harder but I like it

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