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2X teleconvertors with telescopes,how is image quality? (1 Viewer)

B

bogert

Well-known member
Hello~
I was wondering how teleconvertors work with telescopes? I have an Astro-tech 72ED refractor. I have read that TC don't work well with zoom lenses. They work better with prime lenses I hear. The telescope is essentially a prime lens right (still kinda new at this)? So my telescope, which has excellent image quality, should work pretty good with a teleconvertor right?
If anyone has examples of work they have done with the same scope and a teleconvertor, or a similar scope, that would be great to show what it can do.

Also, if they work good. Which 2X is best, and second best..(probably can't afford the best one). I was looking at the Canon one... kinda expensive.. the Sigma one, and the Kenko one. Again, anyone with examples of birds shot with a similar setup and one of those 2X, I'd love to see them.
Any advice would be greatly appreciated.
Thanks
Chris
 
Perhaps you may find these OT's helpful.

http://www.birdforum.net/showthread.php?t=170183

http://www.birdforum.net/showthread.php?t=166895

http://www.birdforum.net/showthread.php?t=166219

http://www.birdforum.net/showthread.php?t=165477

http://www.birdforum.net/showthread.php?t=134826

http://www.birdforum.net/showthread.php?t=136881

http://www.birdforum.net/showthread.php?t=134074

There are other topics regarding the use of Barlows, modified telenegatives verses teleconverter etc., at this forum. Any part you are not familiar, you can always activate those older threads or ask here. It'll be fun going through them.
 
If you want to go the teleconverter route as opposed to making one then the kenko Pro 1.4X is very good and more versatile than a 2X. With a 1.4X you can place a macro tube between the camera and teleconverter and this will boost its magnification to 2X with no noticeable effect on image quality. From memory I think it's a 60mm macro tube that will boost the 1.4X up to 2X.

I find a 1.4X suits me most of the time and it's rare I need to use 2X. Plus I find just cropping the photo to 2X equivalent produces just as good a result.

On the whole, using a teleconverter on the scope will have very little effect on image quality and photos are better than those of even a very expensive prime camera lens using the same teleconverter. There's was a side by side comparison done on a website somewhere which showed this to be true but not sure where it is now.

Paul.
 
I think I'll give that a try. I already have the extension tubes, I just have to get the Kenko 1.4X.
I read through all those threads too and that setup seems the best.
Thanks
 
bogert, Paul did numerous testing using TN from cheap lens and they preform just as good as the higher quality TC if not better. They come so cheap that just for the experiment, they are worth it. I no longer use TC but only TN.
 
There's a long thread on various telenegatives that we've tried. This link goes to the page with info and photos from the Sunagor 100-200mm lens that I found to be my best telenegative. Mag is around 1.6X, sharpness is excellent and slightly sharper than the Kenko Pro 1.4X. If you don't mind a bit of diy to take the lens apart and get the telenegative mounted then it can be a very cost effective way to get some extra magnification. Zoom lenses in the 100-200mm format seem to be best. Other zoom sizes work and produce varying amounts of magnification but they don't produce a flat image.

http://www.birdforum.net/showthread.php?t=182146&page=9

Paul.
 
Paul Corfield said:
If you want to go the teleconverter route as opposed to making one then the kenko Pro 1.4X is very good and more versatile than a 2X. With a 1.4X you can place a macro tube between the camera and teleconverter and this will boost its magnification to 2X with no noticeable effect on image quality. From memory I think it's a 60mm macro tube that will boost the 1.4X up to 2X.

...
Paul.
Click to expand...
Hi Paul,

I have been practicing using TCs, both 1.4 and 2.0 (both Zuiko) and while the 1.4 is capable of delivering sharp images on my Skywatcher ED80, using the 2.0 is a real challenge. At F/15 the viewfinder becomes dark to the extent that, unless the subject has a distinctive silhouette/contrast and light conditions are sunny, my (premature?) conclusion is that the likelihood for sharp picture is really low, 1:10 or even less. Maybe I am doing something wrong. Maybe the 2.0 TC has some defect that is revealed when using it on the scope.

Anyway, while browsing across threads where teleconverters are discussed I found this post. I have limited insight into optical system design and how it comes that reversing the order of the TC and tube would result in added magnification but I have no reason to question it. Since macro extension tubes are relatively cheap compared to optics it sounds to this could be a way forward to extend the application areas for my scope.

I have a couple of questions related to this:

  • I presume that the loss of light would be same as if using equivalent TC?
  • Would this also be applicable for telezoom lenses as well, or is it because of the optical design of the scope?
  • What about normal lenses e.g. 50mm?
  • Would focusing on infinity be supported, or is this a tradeoff you need to accept?
  • Do you happen to remember the formula?

Thanks,
Tord
 
Imagine the image leaving the teleconverter as a cone which is ever increasing. If you move it further from the camera sensor then the cone gets bigger and so the image landing on the camera sensor is also bigger.

I did a quick test this morning in the rain and I'd say the kenko 1.4X with extension is somewhere in between standard 1.4X and my 2.5X with regards to the viewfinder darkness. I had to use a 2.5X to do the test as I don't have a 2X. The difference between 1.4X and 1.4x with 60mm extension didn't pose any problems at all for me to focus, mind you even at 2.5X I found it fine. It's a cheap enough experiment for you to try out with some macro tubes.

When you put an extension tube between camera and TC then you will need less extension in the actual scope but you can still reach infinity.

You can't use this method on a camera lens because you can only focus just a few inches once you put extension between camera and TC.

Paul.
 
Paul,

Thank you for all the time you are putting into support newbies.

I tried the method, mounting EC14 and adding 25 mm extension tube closest to camera (25 mm is the only I have at hand) and assessed the resulting magnification to somewhere between 1.5 and 1.6. However the result looked strange with unsharp picture except from the centre of the picture, looked almost "distorted".

I unfortunately did not save the pictures, will re-do the test at later occasion (it's raining now) if you are interested in having a look.

/Tord
 
That's interesting. Maybe the Olympus TC is a different optical design to the majority of other TC's. I've never owned one yet that couldn't be mounted far from the camera, for example, I've gone up to 180mm away from the camera and the photo was still perfect.

Paul.
 
I'm not sure, but since four third's lenses/converters are specifically designed for the sensor - telecentricity (or what it's called) so when moving it out from the mount, you will get vignetting etc. OM converters and other converters for film, or bigger sensors are better suitable - I guess - for this kind of use (in front of extension tubes, that is)
 
Hi,

You may be right regarding the design optimized for smaller sensors.

The phenomenon however does not look like vignetting as such, it rather looks like taking a picture through a lens badly suffering from lack of sharpness except from the center. Also when comparing with and without extension tube I have a feeling that the picture is distorted except from the center. Maybe I should take a picture of a subject having clear geometric features.

I will upload some samples in a few days' time, when I have access to all bits and pieces (EC14 and EX25 currently on loan).

/Tord
 
I believe the light will fall onto the sensor wrong and distort the image, except for the center as you witnessed.
Since it's not working (ec14&20) for you, you should give those away to charity (me).
;-)
 
The old Nikon 14mm and 25mm extension tubes arrived, and it was a snap mounting the Canon TN inside the 14mm. I stripped out the guts of the tubes, trimmed down the TN mount a little on my mini lathe, and screwed it in. Works fine and is much closer to the body than just stuffing it inside the T2 tube. It gives 1.55x, and 2x with the 25mm tube in between.
3T212187_resize.jpg
3T212189_resize.jpg

PS
I wanted to add this to the thread on TNs, but the thread is too old, so I put it here.
 
DanC.Licks said:
The old Nikon 14mm and 25mm extension tubes arrived, and it was a snap mounting the Canon TN inside the 14mm. I stripped out the guts of the tubes, trimmed down the TN mount a little on my mini lathe, and screwed it in. Works fine and is much closer to the body than just stuffing it inside the T2 tube. It gives 1.55x, and 2x with the 25mm tube in between.

PS
I wanted to add this to the thread on TNs, but the thread is too old, so I put it here.
Click to expand...

Looks really nice. A mini lathe, now that's something I could do with. :)

Replying to an old thread would have been fine, it would have moved that thread back to the top of the list.

Paul.
 
I tried to but I was told "This Thread is more than 267 days old, you can't reply to it.".
My lathe is an EMCO Unimat 3, about 35 years old. Great little tool. I used it a LOT in my musical instrument repair work.
 
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