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08-15-2013, 09:45 PM - 1 Like   #1
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The TC Special!

A while ago, I built a lens using the front element of an old JCPenney zoom, scotch tape, and toilet paper tubes. You can find it here: https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/229186-tp-special.html Since then, I've done a little more experimenting with minimalist optics.

So, teleconverters have lenses in them, right? Why not use one of them? Of course, just putting a teleconverter on the camera won't get you an image. You gotta take some stuff out first.

I just took all of the elements out of an old Vivitar 2X except for a single converging element. I'd estimate the focal length of is was about 40mm, give or take a few. Just to make sure I could get an image out of it, I hand-held it out in front of the naked sensor.


Super swirlies! Take that, Helios 40!




I put the one element back in its teleconverter tube, and stuck it on the camera. Instant lens! A real cheap one, too. Unfortunately, corner sharpness leaves something to be desired...




It has a simple focusing mechanism, too. It involves tilting the camera and shaking it until the element moves. The lens doesn't seem well-optimized for close focus, or, quite honestly, any type of focus at all.




At this point, it occurred to me that the teleconverter, with its glass removed, could act as a nice round aperture for the JCPenney element, and some bellows would make a much more robust focusing mechanism than, well, two toilet paper tubes. Consider this the TP Special 2.0, improved mainly by removing the TP. I'll just call it the "TC Special" to avoid confusion.




The bellows meant no light leaks, and the smaller aperture meant less fringing. Apart from using smaller apertures, this is about the best that front element is going to do all by itself.




I also decided to add one more element to the lens. Well, two, technically. In the JCPenney lens, the front assembly featured the front element (duh) and a cemented pair behind it. Now I put the full front assembly on the end of the bellows and gutted TC, giving me a three-element lens: three elements in two groups. It's starting to sound like a real lens now.






The pictures kind of look like they were taken by something you'd buy, too. Kinda boring actually. The fringing is gone, replaced by this weird glowy sensation. Maybe I put the cemented pair in backwards?




Let's see what the dog thinks.



Skippy is... mildly interested? Can it be!?




No? Okay, I'll put it away now...

08-15-2013, 10:14 PM   #2
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Very interested to see how your project has evolved. The foliage shot with three elements is surprisingly sharp, though not terribly interesting without the fringing, as you noted.

Out of curiosity, what kind of minimum/maximum focusing distances can you achieve with this set up? I'm guessing you can't get infinity focus.
08-15-2013, 10:41 PM   #3
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QuoteOriginally posted by LowVoltage Quote
Very interested to see how your project has evolved. The foliage shot with three elements is surprisingly sharp, though not terribly interesting without the fringing, as you noted.
Thanks for your interest! What you say is what's so funny about this. The most interesting picture here, IMO, is the first one, which was just holding a piece of glass between my fingers in the open air in front of the camera sensor. When it gets more complicated, it gets closer to normal, and it's not that interesting anymore. It's just a crappy telephoto again.

QuoteOriginally posted by LowVoltage Quote
Out of curiosity, what kind of minimum/maximum focusing distances can you achieve with this set up? I'm guessing you can't get infinity focus.
A little background first: the reason I can guess the focal lengths of these lenses is because they're simple optics. I think the JCPenney element is about 200mm because, when the element is about 200mm from the sensor, it's at infinity focus. That's the definition of focal length. Thanks to the bellows, putting the lens 200mm from the sensor isn't hard to do, like in this shot across the parking lot:




The TC element has to be about 40mm from the sensor for infinity focus, which would be inside the camera. All wide lenses have a retrofocal group that allows the lens to sit much further from the sensor than its focal length would normally dictate. Since I don't have that luxury, the TC element can't go to infinity.

As for close focus, the TC element could get closer than I'd ever want to go, thanks to the bellows. That close-up of my dog's nose was still within the teleconverter's wiggle room; I didn't even need the bellows for that. The JCPenney element, attached to the bellows, reaches its limit maybe a foot or two from the subject. A lens with a long focal length like that has to move out much further to focus closer.
08-16-2013, 01:55 AM   #4
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I'm most def following this. I just need to find $300 and I'll have a three-element...
.
.
.
200mm f/1.25 (take that Nikon )

And then I just need an AF module... Frankenstein mode /on

08-16-2013, 02:05 AM   #5
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Scratchpaddy: amateur lens engineer extraordinaire
08-16-2013, 06:27 AM   #6
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QuoteOriginally posted by geomez Quote
Scratchpaddy: amateur lens engineer extraordinaire
+1

Hmm... What should I do with these?



Broken Jupiter 50mm f/2

I50U 50/3.5 enlarger lens

2 pairs of convex elements
08-16-2013, 07:27 AM   #7
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QuoteOriginally posted by geomez Quote
Scratchpaddy: amateur lens engineer extraordinaire
I'm submitting my resumé to Zeiss as we speak. I hope you don't mind me adding you as a reference.

QuoteOriginally posted by Giklab Quote
Hmm... What should I do with these?
Get some cheap bellows (or maybe just toilet paper) and give those puppies a ride!

I'd start with that Jupiter. It's a rangefinder lens, right? If you take out the retrofocal group (all that stuff behind the aperture), you should still get infinity focus if you put it right in front of the camera mount. Fun!
08-16-2013, 07:52 AM   #8
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Very interesting stuff Scratch! I especially like the effect in this one. It gives a feeling of movement.



And the purple (and red and blue!) fringing in the other shot is very unique.

May I offer a suggestion? Get some gaffer's tape to use instead of scotch tape.

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