Possible alternative to barlows/teleconverters (1 Viewer)

Paul Corfield · Sep 28, 2010

I've got some old lenses laying around and thought I'd try this after seeing something on the internet. This is basically the front lens assembly from an old Pentax zoom lens. Not sure how many bits of glass are in it but it seems pretty well corrected for CA. It's giving around 2X equivalent when mounted right next to the camera and magnification increases quickly if you mount it further away.

It was getting dark when I tried this out so I need to see if CA will become apparent in brighter conditions. Seems nice and sharp though, certainly comparable to my very sharp Antares barlow. To give an idea how dark it was this is only 1/5 sec so please forgive any soft feathers in the image.

All info is on the photos. The fence is a little soft in the bottom left corner of the Robin photo and I'm pretty certain this is because I was at a slight angle to the fence. I photographed a brick wall with the scope square on to the wall and the photo was sharp to the corners.

Paul.

thornlv · Sep 28, 2010

Once again Prof Paul delivers some more mouth watering projects for us to try. I can see the potential in this with so many old lenses available on Oboy

Something I have been wondering about, available light (for optimum shutter speed)

I did some tests with my TCs and ext tubes taking shots at the same target, same lighting, same ISO and same distance and came up with some interesting results.

eg: Shooting prime (600mm) = 1/1600
+1.4 TC (840mm) = 1/320
+1 No 3 tube (23mm) then 1.4 TC (aprox 1200mm) = 1/400 ? (faster than a 2X TC)

I got similar results with a 2X TC as well

So what would be interesting would be a comparison of this new method against TCs, Barlows in an above test. I would hazard a guess that this method may give faster shutter speeds as against equivalent mag with a TC or Barlow as there is more surface area of glass.

FernandoBatista · Sep 28, 2010

That's a very nice find, Paul. It looks very promising.
I usually use the front elements from old lens as macro closeups, but never thought about using one as a TC. Would love to see a comparison with your other TCs

FernandoBatista · Sep 28, 2010

thornlv said:
eg: Shooting prime (600mm) = 1/1600
+1.4 TC (840mm) = 1/320

That can't be right, if you're shooting the scope alone at 1/1600, with the 1.4x TC you should be at 1/800. Are you sure you're light isn't varying? Some clouds in the sky maybe?

thornlv · Sep 29, 2010

Yes , I thought that was odd, was expecting 1 stop loss, ie 1/800. Light was exactly the same throughout. dull and overcast, no breaks in the cloud. I may run the test again indoors where the light woul be more controlled.

Paul Corfield · Sep 29, 2010

Couple of images I took just now. The light is poor with a little drizzle but tomorrow should be sunny. With today's overcast weather the CA is very well controlled which is good and sharpness is on par with my 2" Antares barlow. Magnification is in the region of 2.4X with the lens close to the camera. Second photo is with a 45mm macro tube and there's a good increase in magnification, more than 3X at a guess but will need to check. The Pentax front elements I'm using are from the F version which just has basic lens coatings. Pentax made a better SMC (Super Multi Coating) version which might perform even better.

Paul.

thornlv · Sep 29, 2010

Have redone the test under studio lighting, no outside influence, 1st one must have been flawed. Shutter speed decrease was as expected, 1 stop for the 1.4TC, 2 stops for the 2XTC. However, using a 23mm extension in front of the 1.4XTC gave same mag as the 2XTC but with 1 1/2 stop decrease - which is better than using the 2XTC.

My question is, Paul, what light loss over prime does the pentax lens at 2X mag give over a 2XTC ?

Vic

alphan · Sep 30, 2010

thornlv said:
Have redone the test under studio lighting, no outside influence, 1st one must have been flawed. Shutter speed decrease was as expected, 1 stop for the 1.4TC, 2 stops for the 2XTC. However, using a 23mm extension in front of the 1.4XTC gave same mag as the 2XTC but with 1 1/2 stop decrease - which is better than using the 2XTC.

My question is, Paul, what light loss over prime does the pentax lens at 2X mag give over a 2XTC ?

Vic

Are your 23mm extension between the 1.4 TC and camera?

Paul, you are doing very well. I am sure you will find something to improve the Astroscope into a better option with DSLR than Digiscoping. That came from a zoom lens, would any part of a prime lens be better?

Since you mention about having a pile of old lenses in your garage, you might as well turn them into something..LOL. It would be the grownup's version of small kids in lego land. Now, the 1/5 sec shot, was it tripod mount or handheld?

My U 80ED gave a spec of maximum 196 X useful magnification. Wonder if that means can be seen or the maximum magnification before the quality drops. If that 196 X convert into SLR format, would give a 9800mm FL!!! We might possibly see heaven with that LOL. I still belief there ways to increase the FL and brightness substantially without loosing the quality.

thornlv · Sep 30, 2010

Alphan, My mounting was 'Camera:23mmEXT,1.4TC,EXT,Scope'. This set up but with the 2X TC gave similar larger mag, ie: 3X.

Paul Corfield · Sep 30, 2010

thornlv said:
My question is, Paul, what light loss over prime does the pentax lens at 2X mag give over a 2XTC ?

Vic

The Pentax lens and the 2X TC need the same shutter speed so there's no gain in that department unfortunately.

Paul.

Paul Corfield · Sep 30, 2010

alphan said:
Paul, you are doing very well. I am sure you will find something to improve the Astroscope into a better option with DSLR than Digiscoping. That came from a zoom lens, would any part of a prime lens be better?

Since you mention about having a pile of old lenses in your garage, you might as well turn them into something..LOL. It would be the grownup's version of small kids in lego land. Now, the 1/5 sec shot, was it tripod mount or handheld?

My U 80ED gave a spec of maximum 196 X useful magnification. Wonder if that means can be seen or the maximum magnification before the quality drops. If that 196 X convert into SLR format, would give a 9800mm FL!!! We might possibly see heaven with that LOL. I still belief there ways to increase the FL and brightness substantially without loosing the quality.

I tried the front lens group from my 50mm Pentax prime and the image is awful so the shape of the glass is all wrong in that particular case. Yesterday I took out the middle group from an old Vivitar zoom and that worked quite nicely. It was about as good as the Pentax for sharpness but a tiny bit of CA creeping in on the edges in high contrast images. It only looks like 2 lenses with nice coatings which is probably why there's some CA.

The 1/5 sec photo was tripod with 2sec mirror lock up. Luckily Robin's sit still for a while as they generally interested in what you are doing.

Usable magnification is around 50X - 70X per inch of aperture. So with our 80mm scopes that's 3" of aperture or around 150X to 210X. It's generally accepted though that for real life applications about 100X or below is usable in most cases. I have a 6mm eyepiece which gives 100X on my scope and it's good for the moon or planets like Saturn and Jupiter.

100X for prime focus would be in the region of 5000mm as 50mm is generally accepted as being 1X.

Paul.

thornlv · Sep 30, 2010

Paul Corfield said:
The Pentax lens and the 2X TC need the same shutter speed so there's no gain in that department unfortunately.

Paul.

Thats good to know, as at least there is no loss in that method.

Vic

Paul Corfield · Sep 30, 2010

Here's some images taken with the middle group of elements from a Vivitar 70-210mm 1:4.5-5.6 Macro zoom lens. When removed from the lens the elements are in a plastic housing which is just right for gluing into an old teleconverter housing or similar. When mounted straight to the camera it gives 2X but for these images I was boosting it with macro tubes between camera and Vivitar lens.

First image is with the scope at 600mm just to show how it looks at 30m (98 feet).

Second image is with a 45mm macro tube between camera and Vivitar lens which gives 3.8X and 1/125sec at ISO800.

Third image was with a 75mm macro tube between camera and Vivitar lens which gave 4.6X and 1/80sec at SO800.

Note - all images are uncropped.

Overall I think this is very good for what is a very thin lens, the lens housing is only about 1cm thick. It's a useful amount of magnification all in one lens and little in the way of image degradation. I bet it would go over 5X with a slightly longer macro tube.

Paul.

alphan · Oct 1, 2010

thornlv, Paul did adviced earlier that moving the TC away from the camera actually increase the magnification. Basically, that's what happen between the 2X and 3X TC. Stacking more TC together to get the higher magnification gives a poorer result than moving the TC away from camera. Less glass also mean less light loss. But the limit of such magnification still need further investigation. What Paul is doing in post #13 above is the same, but instead of TC, he's using Zoom lens elements.

Paul, the acceptable 100X magnification you mentioned, are they visual viewing only or good for satisfactory photography. If that is acceptable for photography, that will be 5000mm which would be good enough for any birding. But we still need to work on the light. Otherwise we'll have an F 62 LOL.

Your second and third pic doesn't show any quality drop so that is very good. What are the light drop in comparison to using GSO Barlow or DOI TC?

Paul Corfield · Oct 1, 2010

alphan said:
Paul, the acceptable 100X magnification you mentioned, are they visual viewing only or good for satisfactory photography. If that is acceptable for photography, that will be 5000mm which would be good enough for any birding. But we still need to work on the light. Otherwise we'll have an F 62 LOL.

Your second and third pic doesn't show any quality drop so that is very good. What are the light drop in comparison to using GSO Barlow or DOI TC?

If we add in the Canon 1.6X crop factor for the posted images then the second photo would be 3648mm and the third photo would be 4416mm which is about 88X equivalent. I'd say then that the ideal 100X would apply here as well. And if these were on a 2X crop factor camera like an Olympus dslr or Panasonic G1 then the third photo would be 5520mm or 110X equivalent.

The Vivitar glass elements beat anything I've used up till now.

Here's a couple more images from yesterday. The first image is of the same TV aerial wire that I photographed for the orthoscopic eyepiece thread. This photo easily beats what I could get out of the P&S digicam. Also when viewed at 100% there's not a single bit of CA detectable which is amazing for something spaced 75mm away from the camera.

1st image ISO800 1/125sec and 2nd image ISO800 1/80sec.

Also added photos showing front and rear of the Vivitar lens unit glued into an empty teleconverter housing.

It's 9am and looks like heavy rain has set in for the day here so no testing today. :C

Paul.

thornlv · Oct 1, 2010

Paul

I could add it would also be simple to mount a glass element inside an ext tube barrel, then the unit could be screwed into any position.

Looking on ebay there are lots of vivitar lenses available so should be able to pick one up at a daft price.

On all your pictures we mere minions have to take into accoount your excellent photography skills.

Paul Corfield · Oct 1, 2010

A couple of images to compare the Vivitar lens group to the DOI 2X TC.

These were both taken with a 45mm macro tube between camera and lens and both were taken at the same shutter speed from 7m range. The Vivitar produces about a 32% bigger image than the DOI 2X. When they are both mounted straight on the camera the Vivitar gives 2.2X compared to the DOI 2X.

My lens is the same as the one in the photo below, just in case they differ internally it's best to look for the same one.

Paul.

thornlv · Oct 1, 2010

As there are absolutely loads of lenses on 'Oboy' at low prices (under a fiver), what should we be looking for - this could make a very cheap but effective means to achieve greater mags.

Paul Corfield · Oct 1, 2010

All I would say that it has to be a zoom because they are most likely to have the teleconverter style group inside them. They wont necessarily all work so don't spend too much. On ebay I shall buy some cheap ones too of various zoom lengths, just to see how they compare and whether they offer varying degrees of magnification.

Probably best we don't bid against each other so post anything you are definitely going to bid on here. :t:

Paul.

thornlv · Oct 1, 2010

Paul Corfield said:
Probably best we don't bid against each other so post anything you are definitely going to bid on here. :t:

Paul.

Good idea, I may bid for a few as well, i have quite a few ext tubes spare that could be housings.

Possible alternative to barlows/teleconverters (1 Viewer)

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