Autofocus with a telescope (1 Viewer)

[kurakura]

Ever so often I get down to abit of scope surfing. I have fallen abit in love with the Borg 77EDII scope. The new Olympus body looks very promising from what I have seen here lately, but then today I found out that Pentax make a 1.7x AF TC what can work with a scope - this cool setup on a Japanese site - The link is to the googletranslate version on the article

http://translate.googleusercontent....3.html&usg=ALkJrhgnLg7pWl9nYQQ_cSsmguEnYsvidQ

I am not sure how fast the AF is, but seems that it renders some pretty good results

 

[cango]

tried to read the translation, but could not understand how the af could work in that scope. the aperture control seems very nice though

 

[kurakura]

There is another one showing how to convert a Nikon AF TC to be used with telescopes aswell I cannot seem to find it now, will look again tomorrow.
However, I did find this one where they seem to use a Nikon 55-200mm lens for the same purpose. I agree that the translations do not make much sence - we need these Japanese guys to get in here and show us

http://translate.googleusercontent....LkJrhiP_BCYHYxpDht-yIJU6uBnUV7FmQ#comment3026

 

[cango]

ok I see now. the lens is stripped of all glass inside, and and a sort of "extension tube" is left but with and af-motor. I wonder if this can be adapted to - theoretical - the 80ed/600mm tube many of us have. Focuser must be getting rid of, and travel distance with extension tube replaced somehow - right?
Please help me here ;-) I'm out of my depth here

 

[Paul Corfield]

There is another one showing how to convert a Nikon AF TC to be used with telescopes aswell I cannot seem to find it now, will look again tomorrow.
However, I did find this one where they seem to use a Nikon 55-200mm lens for the same purpose. I agree that the translations do not make much sence - we need these Japanese guys to get in here and show us

http://translate.googleusercontent....LkJrhiP_BCYHYxpDht-yIJU6uBnUV7FmQ#comment3026

In this second link there is terrible vignetting from the small hole at the back of the modified Nikon lens. At least 50% of the image is partially shadowed. Not really sure what the first link is describing in the translation.

Paul.

 

[JvdM]

In this second link there is terrible vignetting from the small hole at the back of the modified Nikon lens. At least 50% of the image is partially shadowed. Not really sure what the first link is describing in the translation.

Paul.

He should have removed the aperture control (iris) as well, as this is what is restricting the light - don't know that this will be sufficient though.

 

[FernandoBatista]

I've tried that in the past with a D70 at the time. I used a 28-80mm.

There were two problems, one was the vignetting that Paul already pointed out, and is not so easy to get the clear aperture needed to don't block the light hitting the sensor. At least using that particular lens.

The other is back/front focus, the AF chip on the lens is programed to focus with the glass of that lens, when you put something else on it, you'll get a lot of focus variations. I supose that's not an issue with a D7000, but with the D70 I had at the time it was useless.

 

[kurakura]

In theory there should be no vignette using an autofocus tc right?

It seems that the use of telescopes is big (bigger) in Japan - am going to surf further

 

[Paul Corfield]

In theory there should be no vignette using an autofocus tc right?

That's correct.

Paul.

 

[kurakura]

There is so much cool stuff on that guys website... comprisons scope vs scope - scope vs Canon and nikon lenses. Tests of various camera bodies and makes.

take a look at this post (2pages). The Predator (Maybe Oriental Honey Buzzard) about 1/4 way down on page one is nice. I can only recommend, checking out all the different links on the left side on the site...

http://translate.googleusercontent....7.html&usg=ALkJrhhM7p76_CKW_Kem4dlHp-8HUruIDA

 

[DanC.Licks]

I get no vignetting at all with the iris stuck in the end of the first extension tube, even stopped all the way down, (which I wouldn't do due to refraction). I get it in a big way if it is in the end of the T2 adapter.
I have a chipped adapter that is all but useless unless the lens is wide open, in my case f7.5. It needs at least f8 to work, and as soon as I either stop down or install the 1.6x TN, it is useless. Back focuses like crazy or doesn't register at all. There may be better chips, though.
The Pentax K-5 has that feature built in, maybe because Pentax has made a point to incorporate their old lenses into their digital system. You do have to dial in the focal length. Don't know how well it works with the scope though. Does Nikon also have this feature? Don't know.

 

[FernandoBatista]

Vignetting depends on the position of the iris, it wont vignette if placed correctly. I've an iris on my scope and I get little vignetting, and if it were 2cm in front of where it is now, I would have no vignetting at all.

But the vignette I mention before was not caused by the iris, I removed the iris from that lens. But the internal diameter of the lens was still too small and very close to the mount, and that caused vignetting. If it wasn't so close to the sensor that I'd get no vignetting, but I'd still have light loss.
It wasn't very easy to increase that diameter and still keep the AF mechanism, at least on the 28-80, on other lenses it may be much easier. But as the AF wasn't hitting the right point anyway I never pursued the issue further.

The issue with the TN backfocusing isn't related with the quality of the chip, is just how it works. I have all my tcs chipped and fine tuned for focus because of that.

 

[DanC.Licks]

The problem with my chip is that it can't be fine tuned, and it does not let the body's own fine tuning feature function. Do your chips work if your scope is stopped down?

 

[FernandoBatista]

The AF sensor work fine until around F/9, and only in good light at around F/12..13. By fine I mean, catching the focus point consistently. There's a backfocus problem as I stop down the aperture, at F/8 I have already very noticeable back focus, and by F12 it's completely off. In reality I can only use the AF confirmation until F/7.5 or so, after that the backfocus makes it useless.
It's the same as with the TCs, I'd have to get the chips AF tuning corrected for each aperture.

The aperture control is actually one of the main advantages of the m4/3 system for me. With the D90 as I stop down I have a much darker and harder to focus OVF, with the G5 I've exactly the same brightness even at F/22...and the EVF is easier to focus too.

 

[DanC.Licks]

"The aperture control is actually one of the main advantages of the m4/3 system for me. With the D90 as I stop down I have a much darker and harder to focus OVF, with the G5 I've exactly the same brightness even at F/22...and the EVF is easier to focus too."
I think that is a very good point, and something that Carlos has also pointed out. It might not look "natural" but it gets the job done. I wish I knew someone with an OM-D. I would really like to give one a try on the scope. I found it hard on my eye and otherwise irritating with a normal lens on it when I looked at one in a shop. On the scope, it could be very different! I have to admit, it is NOT easy focusing the scope with a 1.6x TN on it and the iris closed down to boot.

 

[cruedag]

Oh my god, until I've seen how does the Pentax 1.7x AF TC work I didn't understand anything.

It is very interesting, but I don't know why that japanese guy is using such a small diameter objective that produces so much vignetting. Well, in fact it seems that he uses it because of the long displacement introduced by the zoom is converted in a kind of helical focuser, maybe more comfortable to use that the typical astronomycal one. I don't know if the Pentax 1.7x AF TC works without a Pentax objective, but in that case will better use any fixed fast prime objective for having no trouble with vignetting, and pre-focusing with the standard focuser.

I've also seem the very interesting Nikon AF TC TC16A, with a very similar behaviour to the Pentax one... I want one of them! Fernando did you used this one? If it uses their bus to know about the focal length of the manual objective mounted, maybe you could use a AF confirm chip programmed with the real focal length of the scope, because it could make the TC16A work better.

What a pitty not having an AF TC for Canon!

 

[kurakura]

The pentax 1.7x should work with the telescope, however as I understand from reading through alot of the Japanese sites it is not optimal. The Nikon TC16A, should be far superior. One guy said that the AF speed is on par with canon 500/4, something I need to see before I can believe it. It should be noted that it is only semi AF - the BORG telescope use a 2way focussing system where you first use a drag/pull tube and then fine adjust with the helical focusser, so you do the drag and pull part, and the TC should take care of the rest. BORG recommend the use of the NIKON TC. Furthermore they say they developed the Black Borg to compete with the bigger canon and nikon lenses, only with superior optical qualities
Interestingly the new Pentax 560mm lens has been developed in cooperation with BORG, and the lens look alot like a scope.

 

[FernandoBatista]

I never used the TC16, but I did thought about it many times. Mainly I never used it because the mod it needs:
http://www.foolography.com/free-stuff/modify-tc-16a/

Not that the modification is very hard to do. But that and the fact that it is a 1.6x TC making the ED80 a F/12 lens made me forget it at the time. No way the body would focus reliably enough at F/12.

More recently, I actually though about replacing the glass for something less powerful to use it with my TLAPO, but even then it would be F/8 at best, and I'm not sure if it would work well enough to be worth the trouble. Honestly I got along fine with dandelions and trapfocus so I just forgot all of that.

One other thing, I don't think the TC16 would do any comunication with the lens, (it was made for MF lenses that weren't chiped) so having the scope chipped wouldn't give any advantage in focus correction.

 

[DanC.Licks]

Fernando,
Which dandelions have you had the best luck with?

 

[cruedag]

It seems that it is usual to find the Nikon TC16A already modified in ebay (there are some of them just now).

Pentax and Nikon use a teleconverter layout to achieve autofocus whith those products, moving internally the lenses to focus the projected plane at different distances. So the 1.6x or 1.7x seems to be unavoidable to that purpose, I think that it could be the payoff to obtain a compact size.

I've also seen in some of the japanese pages people using a reducer and flattener before the TC16A, it could work.