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01-29-2015, 05:57 PM   #16
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QuoteOriginally posted by johnmflores Quote
AH. ok. THx.
Crewl1 and his DA 300mm. Awfully sorry. (can't Edit)

01-29-2015, 11:42 PM   #17
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QuoteOriginally posted by Phenix jc Quote
Yes, I'm following you from the beginning ( ? well, more or less I guess). TY for your threads !
I should have come here earlier, but I lack time.
+1

QuoteQuote:
So, is the telescope better ? More powerfull for sure (500mm), and probably better, yes. But I don't know for sure, perhaps not THAT much.
Bernall convinced me : Less is more. 3 lenses APO, a crop sensor, Q, Nikon 1, m4/3, that's it.
+1

In addition I have translated the scheme I used to explain my point of vue to convince JC.

First, the principle of digiscopying, with many lenses, often misaligned, and many resulting aberrations.
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Last edited by bernall; 02-07-2015 at 04:34 AM.
01-29-2015, 11:48 PM   #18
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and the equivalent setup with the combo PentaxQ-APO-ED-Triplet telescope (see the attached picture below).

The small number of lenses and the small size sensor limit the risk of aberration, and give an equivalent focal lenght as long as digiscopying.

Classical zoom lens or fixed focal lenght are good, but have many lens too. With APS-C or FF, it's OK. Not always with small photosites of the Q system.

---------- Post added 01-30-15 at 08:00 AM ----------

In fact, pictures are good proofs

Situation (~80m) :



Q7/ED80T-CF (reduce to 33%) :



Not the same target and not the same conditions (120m), but digiscoping is less sharp and shows more CA :



(more details here : http://www.chassimages.com/forum/index.php/topic,168569.msg4513404.html#msg4513404)
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Last edited by bernall; 01-30-2015 at 01:25 PM.
01-30-2015, 02:49 PM   #19
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This is cristal clear., many thnaks for the explanation. I never did the distinguish between both systems.
Now i m just wondering if i need to add such gear to my Q,
i like birding and the long range offered by K lens on Q - let say with a 300mm - is already fantastic and provides a good reach . However here we've clearly reach another quality level

01-31-2015, 04:19 AM   #20
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QuoteOriginally posted by rafa75 Quote
This is cristal clear, many thanks for the explanation.l
Hi Rafa !

Thanks for your comment.


QuoteQuote:
Now i m just wondering if i need to add such gear to my Q, (...). However here we've clearly reach another quality level
Sure, but it's a special case of use : longer is the focal length, higher is the difficulty and narrower is the field of view. Shooting is obviously more complicated due to the vibrations ...

So DA300mm keeps a lot of advantages indeed.
06-02-2015, 01:45 PM - 1 Like   #21
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More pictures :

Pentax Q :







Pentax Q7 :







My set up on the right ;
On the left, an Orion 80 ED Doublet with a 4/3 reflex (= 1200mm 24x36. Nice too)




You can have a comparison with a Nikon 500/8 mirror here :
(same distance only)
https://www.pentaxforums.com/forums/136-pentax-q/288562-nikon-500-8-mirror-pentax-q-q7.html
06-02-2015, 03:35 PM   #22
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QuoteOriginally posted by rafa75 Quote
This is cristal clear., many thnaks for the explanation. I never did the distinguish between both systems.
Now i m just wondering if i need to add such gear to my Q,
i like birding and the long range offered by K lens on Q - let say with a 300mm - is already fantastic and provides a good reach . However here we've clearly reach another quality level
QuoteOriginally posted by bernall Quote
Sure, but it's a special case of use : longer is the focal length, higher is the difficulty and narrower is the field of view. Shooting is obviously more complicated due to the vibrations ...
So DA300mm keeps a lot of advantages indeed.
One of the issues is where are you going to do your shooting. Sometimes I work in our backyard, but sometimes I'm out hiking/birdwatching with my wife. In the latter case, the long lens and tripod would be a real hindrance. On US Memorial Day weekend (10 days ago), my wife and I were at Pt. Pelee National Park in Canada. We saw people with complicated setups, including tripod, but they went to just one site, and sat there, while we were hiking all over the place. It was late in their migration season, so I'm not sure that any of us actually saw anything worth photographing (although, if I'd been even with my wife instead of trailing her from trying to take a picture that didn't work out I could have grabbed a picture of a pair of courting turkeys).
06-03-2015, 09:32 AM   #23
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QuoteOriginally posted by reh321 Quote
One of the issues is where are you going to do your shooting.




QuoteQuote:
In the latter case, the long lens and tripod would be a real hindrance.


Not heavier than a bigma or so (3.814 kg without tripod)

But the ED80T-CF is not the worse ...




crop 50% (but not with PtxQ7) :



distance about 700m

06-03-2015, 10:52 AM   #24
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QuoteOriginally posted by bernall Quote
The small number of lenses and the small size sensor limit the risk of aberration
We must be careful saying something like this because it is not always true. When photography first started, lenses would only have one or two elements. The performance was not good and so designers had to figure out what was causing the unsharpness. Aberrations were identified and ways to solve them was sought. Each element surface was used to correct an aberration and different glass types were used for the same purpose. As designers realized the importance of each surface, the complexity and number of elements increased. With the advent of low dispersion glass, both telephoto and telescope designs improved. The two designs are strikingly similar in solving color aberrations. Telescope designs cannot use a diaphragm to reduce off axis aberrations like a telephoto lens can, so the telescope designs must be sharp wide open. Not an easy task. In general, the more lens surfaces, the more aberrations you can correct. But there is a point of diminishing return, where additional elements will not improve the image but soften it. The only way a 3 element telescope or 3 element telephoto design can perform well is with the use of exotic, modern glass. Nearly all ED telephoto lenses use more than 3 elements because they want to further the color correction with rear groups.
06-03-2015, 12:42 PM   #25
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QuoteOriginally posted by desertscape Quote
We must be careful saying something like this because it is not always true. When photography first started, lenses would only have one or two elements. The performance was not good and so designers had to figure out what was causing the unsharpness. Aberrations were identified and ways to solve them was sought. Each element surface was used to correct an aberration and different glass types were used for the same purpose. As designers realized the importance of each surface, the complexity and number of elements increased. With the advent of low dispersion glass, both telephoto and telescope designs improved. The two designs are strikingly similar in solving color aberrations. Telescope designs cannot use a diaphragm to reduce off axis aberrations like a telephoto lens can, so the telescope designs must be sharp wide open. Not an easy task.
Absolutely. But in case of "large" sensor only or large aperture (f/2.8 and more). For very small sensors, near the optical axis, very few aberations are to be corrected (f/6 is not wide open).

QuoteQuote:
But there is a point of diminishing return, where additional elements will not improve the image but soften it.
in addition of misaligned elements

Last edited by bernall; 06-03-2015 at 12:49 PM.
06-03-2015, 03:16 PM   #26
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QuoteOriginally posted by bernall Quote
Absolutely. But in case of "large" sensor only or large aperture (f/2.8 and more). For very small sensors, near the optical axis, very few aberations are to be corrected (f/6 is not wide open).
I'm not sure if I'm understanding you correctly. Are you saying that a lens used on a Q will show little aberation unless it is used mostly wide open??

When I first starting adapting old lenses to my Q-7, normally using the lens stopped down two or three steps to f/5.6 (I didn't want to step it down any further because of known diffraction issues), I encountered all kinds of issues, including lots of CA. I got pictures acceptable to me only when I bought a new Sigma lens.
06-03-2015, 07:29 PM   #27
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QuoteOriginally posted by reh321 Quote
I'm not sure if I'm understanding you correctly. Are you saying that a lens used on a Q will show little aberation unless it is used mostly wide open??
Sorry, my answer was very short and my english very poor ...

I wanted to say that many elements are necessary to build a lens with large aperture or used on large sensors.

Q are abble to use less (good ED) elements in case of long focal lens as I show with my images and on the following diagram (#18 in the present thread)



Last edited by bernall; 06-03-2015 at 10:34 PM.
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