CA-35 can someone explain? (1 Viewer)

The CA-35 camera adapter is well respected, but f12.5 for the PF80 and PF100-ED (f12 for the PF65 and PF65-ED II) scopes reduces it to bright light photography.

Can anyone explain why the CA-35 "pushes" (doubles) the focal length of the scopes? A camera adapter that matches the focal length of the scopes would be of great utility and in my mind should be optically superior.

I would like to see Pentax introduce a new camera adapter that approximates focal length of the scopes at f6 with pricing comparable to XF eyepieces.

--jeepnut

jeepnut said:

The CA-35 camera adapter is well respected, but f12.5 for the PF80 and PF100-ED (f12 for the PF65 and PF65-ED II) scopes reduces it to bright light photography.

Can anyone explain why the CA-35 "pushes" (doubles) the focal length of the scopes? A camera adapter that matches the focal length of the scopes would be of great utility and in my mind should be optically superior.

I would like to see Pentax introduce a new camera adapter that approximates focal length of the scopes at f6 with pricing comparable to XF eyepieces.

--jeepnut

Click to expand...

This is a post I made sometime back it will answer most of what you want to know:

http://www.birdforum.net/showthread.php?p=729783#post729783

Briefly though:

1. Light. Yes it is f12. But ANY lens at these magnifications would be pretty dim. It you bought a $6000 Canon 800mm prime and used tele extenders to give you the same equivalent focal lenth as my 100mm Pentax (1910mm) it would be as dim if not dimmer. Can you imagine how impractical, huge and heavy, not to mention costly, a 2000mm f5 prime long lens telephoto would be? Also for most daylight use I have found an ISO of 400-800 works out well.

2. The CA-35 does NOT change the actual physical focal length of the scope. But it can give you an EQUIVALENT magnification of a 1910mm prime telephoto (about 38x using a DSLR). The CA-35 is a afocal device just like any other eyepiece except it's really an "eyepiece" optimized for a Pentax SLR camera rather than the human eye.

3. "A camera adapter that matches the focal length of the scopes" - No problem. This is called prime focus and a simple camera adapter for this purpose would cost about 20 bucks. But remember that instead of 38x you would be getting about 18x on the 100mm Pentax at prime focus.

Again see the post I mentioned.

SF

CA-35 more questions

Sout Fork,

Thank you for your assistence. I have read the post you referenced a number of times, with great interest, over the last month or so. I guess I need some mentoring.

Refering to your numbers:
1) No problem, I understand. I also understand that a spotting scope is not a camera lens, the erecting prism introduces extra optics, hence, reduced light transmission, less resolution, more distortion, more CA, extra weight, etceteras.
2) What is "afocal", does that mean not having focus ring or knob? Does the CA-35 have optical glass elements or is it just a tube? What is "prime" telephoto vs. "prime" focus?
3) This is exactly what I want, the inherent focal length of the scope at it's inherent focal ratio (630mm f6.3 is not so bad), except, I want to focus to infinity, notwithstanding that, 300m would be great, 9m just won't do. My question is: am I asking for the optical impossible, or is it just not made, what if a focus ring were added to the device?

Hey, Pentax, how about introducing a CA-KD (630mm f6.3 on PF100-ED), or better yet, how about a CA-KD SMC zoom? And, let's ad an AF version to the lineup? Yea, now were talking!

Please advise.

Happy Trails, jeepnut

What is "afocal", does that mean not having focus ring or knob?
-------------------------------------------
SF: Technically afocal means-
A lens of zero convergent power, whose focal points are infinitely
distant. If you want to know more I leave that up to you.

Practically anytime you look through an eyepiece of a scope the image
formed on your eye is done by the scope and eyepiece afocally.
Substitute a camera for your eye and you are doing afocal projection
photography.

It has nothing to do with focus rings or knobs. The camera is a passive
device as far as focus is concerned. You look through the viewerfinder
on the camera and focus with the focus knob on the scope as you would
normally do using an eyepiece and your eyes
---------------------------------------------


Does the CA-35 have optical glass elements or is it just a tube?

SF: Yes it has five elements. At $400 bucks it better be a lot more than
a simple extension tube.
-----------------------------------------------


What is "prime" telephoto vs. "prime" focus?

SF: You are confusing the two terms.

Prime telephoto simply means a lens of a fixed focal length rather than
one with a variable focal length (a zoom).

Prime focus, practically, means nothing optically between between the
objective cluster on the front of the scope (leaving aside the issue of
prisms in a spotter) and the focal plane of the camera.
-------------------------------------------------


This is exactly what I want, the inherent focal length of the scope at
it's inherent focal ratio.

SF: That's what I do all the time. It does not require any special
attachment specially made by Pentax to do this. You can get it at any
astro supply store for 20 bucks.

Below you can see a picture taken with this setup and the setup it's
self. It's giving me about 15x at a the native focal length of the scope
of 500mm at f5.
----------------------------------------------------


My question is: am I asking for the optical impossible, or is it just
not made, what if a focus ring were added to the device?

SF: I really don't understand your concern about focus rings and knobs. All
focus is achieved wither prime or afocal using whatever focus mechanism
is built into the scope.
------------------------------------------------

Perhaps there are others on this forum that can explain all this better
than I?

SF

Prime focus question

Sout Fork,

Thank you for the response.

In a thread you referenced earlier you stated:
"Prime focus:
Tried this only on the 80mm. Focus from about 8 feet to 30 feet."

Have you managed to get prime focus to work to inifinity?

Happy Trails,
jeepnut

jeepnut said:

Sout Fork,

Thank you for the response.

In a thread you referenced earlier you stated:
"Prime focus:
Tried this only on the 80mm. Focus from about 8 feet to 30 feet."

Have you managed to get prime focus to work to inifinity?

Happy Trails,
jeepnut

Click to expand...

No. But I really havn't tried. The 80 is never used for photography. I always use the 100mm with the adaptor and not in prime focus because I need the magnification the adaptor gives.

However I always use the 100mm celestron in prime focus and that works very well for low light close ups. For prime focus use it has the advantage of not having a prism to complicate things.

Perhaps the spotters could be made to achieve infinity in prime focus but, I really have no need to use them this way. I do know that some people have reported that sometimes even with certain eps the Pentax scopes won't focus at infinity. I realy don't know why.

Henry- any ideas what the problem might be?-the prism perhaps?

SF