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10-18-2011, 09:38 PM   #46
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Just try flipping the entire group. That's the easy way to start and who knows, it might even work! Of course, if it was that easy, then it would be a product, and we'd probably know about it. So it's probably not easy. Hmmm...

10-18-2011, 10:10 PM   #47
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I'm a bit confused on how a teleconverter even works. I don't think they work like an afocal teleconverter does (like a front mount teleconverter or a magnifying glass). If you look through a rear mount teleconverter (not on a camera), it actually has a wide angle effect. On a camera with a lens, it has a telephoto effect. Flip the theleconverter around with a lens in front of it (hand held), it still has a telephoto effect.

If you move a lens away from the camera, it increases magnification. I couldn't find my macro tube so I tried hand held, and if you have the camera close up so you are starting out of focus, and go into focus as the lens is moved away from the camera, it is in focus long enough you can see the magnification increases a lot as you move the lens away. So what if the magnification is being provided by moving the lens away, and the teleconverter is just adjusting the focus so that it is no longer macro. That would explain shy a 1.4x teleconverter is thinner than a 2x and a 3x is much longer.

Of course I have no idea what I am talking about, it is just a possibility on how it works based on observation. I also don't know how it would play in trying to convert one either.
10-19-2011, 02:12 AM   #48
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Ok, I found the thread where I asked about this optical structure: https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/154274-focal-r...ographers.html. When I gargle for FOCAL REDUCER LENS or search eBay for FOCAL REDUCER I find various products, and this rather old page on Focal Reducers (Field reducers)and Focal Magnifiers (Barlow lens) which I am still trying to digest. Something in there gave me the idea that flipping a TC would effect focal reduction.
10-19-2011, 02:51 AM   #49
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QuoteOriginally posted by RioRico Quote
Really, it's not a wild-ass fantasy.
Yes, it is.

10-19-2011, 06:57 AM   #50
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QuoteOriginally posted by thoughton Quote
Yes, it is.
Would you care to share why not?
10-19-2011, 08:21 AM   #51
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Let's say for example your lens has a field of view of 60 degrees. So the front element lets in light covering a 60 degree field. A teleconverter simply magnifies the central section of that field of view, and makes objects appear closer. However the reverse is not possible. No matter what sort of imaginary "WC" you add to the rear of the lens, you cannot magically make the front element take in a wider field of view. To do that you'd need another lens in front of the front element, like one of those crappy Digital King 0.7x wide angle adapters that screw onto the front of your lens.

Last edited by thoughton; 10-19-2011 at 08:27 AM.
10-19-2011, 09:05 AM   #52
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QuoteOriginally posted by thoughton Quote
Let's say for example your lens has a field of view of 60 degrees. So the front element lets in light covering a 60 degree field. A teleconverter simply magnifies the central section of that field of view, and makes objects appear closer. However the reverse is not possible. No matter what sort of imaginary "WC" you add to the rear of the lens, you cannot magically make the front element take in a wider field of view. To do that you'd need another lens in front of the front element, like one of those crappy Digital King 0.7x wide angle adapters that screw onto the front of your lens.
I hadn't really though about that but it makes me wonder something else. What if it is a full frame lens like an old film lens on an aps-c sensor camera? Wouldn't you then have a wider field of view coming through the lens, with the camera cropping out a center portion? Couldn't you optically compress the light to put more of that wasted space onto the sensor? I'm not saying there would be an easy way to make this happen, but its theoretically possible, right?

Since you seen to have a clue about how optics work, would you by chance know if my observations are correct? Does a rear mount teleconverter gain its magnification from moving the lens, and the teleconverter just corrects focus?
10-19-2011, 09:28 AM   #53
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@RioRico, as best as I can tell from your links, thoughton is correct but My last statement is correct too (only glanced through the info so I could have easily missed a lot). You can not turn a 24mm 2.0 into a 12mm 1.0 (not on a dslr anyway, but I bet you could on a 4/3 sensor camera). Your limit seems to be the crop factor. A full frame 50mm lens has an effective field of view comparable to 75mm on our 1.5x crop sensor cameras. You could convert it back to the equivalent field of view of a 50mm lens on a full frame camera. As you are compressing the light you would get a faster lens. In therory, you could go further with a medium or large format lens. That is what they seem to be doing with the astro photography thing. Taking the full image from a full frame lens and compressing the entire image down onto a much smaller sensor.

@thoughton, does that sound about right?

10-19-2011, 10:30 AM   #54
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I'm not any sort of expert in optics, but I suppose if the lens is designed for a larger format then what you describe might be possible, I suppose you'd need a teleconverter designed for the larger format as well, and you'll probably have CA issues. However ... the mere fact that no one has attempted to market such a device to all the Pentax and Nikon users using Full-frame lenses on crop bodies makes me think there are other problems I'm unaware of. Something to do with registration distance, maybe. In any case it certainly won't work with the 18-55 lens as described earlier in this thread.

I don't know anything about telescopes, sorry. Although from what I understand the eyepiece is already some kind of teleconverter, so I guess a focal reducer goes in between the eyepiece and the main body of the telescope, or replaces the eyepiece altogether. NB This is a wild-assed guess

Last edited by thoughton; 10-19-2011 at 10:36 AM.
10-19-2011, 10:51 AM   #55
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As I said earlier, if it was easy, everyone would be doing it, and products would abound. So it ain't easy. Is it even possible, using camera lenses rather than telescopes? I wish I had space to setup an optical bench and find out. Hmmm, I wonder if easy-to-use lens design software exists that can model the effects of a TC in both normal and reversed positions?

But no, it's probably not going to result in a 9-28mm f/1.7-2.8 zoom. Oh bother.
10-19-2011, 11:23 AM   #56
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It still might be worth playing with just to give a full frame lens a full frame field of view on a crop sensor camera (not to mention the increase in speed). I kind of have my doubts about using a teleconverter but wonder if other kind of lenses might work. I guess maybe some more reading up on how they do it with telescopes would be in order. Of course I don't know that a teleconverter won't work either?
10-19-2011, 12:16 PM   #57
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I'm thinking marketing and demand have as much to do with what they make as how hard it is to do. Price is obviously an issue too. Pentax made a teleconverter (1.7x if I am not mistaken) that will convert a manual focus lens to auto focus. The focus mechanism is in the teleconverter and focusing is done in the teleconverter. I would think that would be a very sellable product but I guess not. If pentax can make one then it can be done yet it's not something you see often (I'm not sure if anyone else ever made one).

One other test I did was to take a front mount wide converter and hand hold it in front of the camera. I held a 50mm prime in front of that. Compared to just the prime, it went telephoto despite the wide converter. I am guessing again that pushing the lens forward is creating a telephoto effect. You are probably going to have to have a pretty strong lens to counter that if my thinking is right. It still might be worth playing with though (again, gonna have to read up more on how they do it with telescopes).

If that it the case, then my question is, exactly how much can you hack saw off the back of a lens before it hits the mirror (not kidding). Obviously it would depend on the lens. The other question is, would it have enough effect to make it worth it? If pushing a lens forward make it go macro, then pushing it in more is going to make it go towards the infinity end. If it went too far, something as simple as a front mount macro lens/filter could correct focus range some.

I'm just kind of thinking out loud here by the way.
10-19-2011, 02:10 PM   #58
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Sounds interesting, let us know what you find out

FYI Nikon also had an AF adapter that worked like Pentax's 1.7x one.

Regarding sawing off bits of a lens, there are numerous posts on this forum of people doing just that in order to change the registration distance when replacing some other brand lens mount with a K mount.
10-19-2011, 02:37 PM   #59
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QuoteOriginally posted by thoughton Quote
Sounds interesting, let us know what you find out

FYI Nikon also had an AF adapter that worked like Pentax's 1.7x one.

Regarding sawing off bits of a lens, there are numerous posts on this forum of people doing just that in order to change the registration distance when replacing some other brand lens mount with a K mount.
I have some telephoto zooms that I could guinea pig (just to see what happens) but I'm not so sure about wide primes. Of course if I don't have one I feel like sacrificing, they can go pretty cheap on ebay.

I personally have modified nikon, olympus, minolta and canon lenses but have never had to go so far as cutting back the actual lens (just custom mount and sometimes spacers). I have seen a post or two where they cut the actual lens back though. If I'm not mistaken, RioRico has modded a wider variety as well as a larger number of lenses than I have.

Sounds like we have some good ideas for some experiments after a little bit more research. I kind of wonder if you would be able to push a lens back far enough to make a big difference but it might be fun to try anyway.
10-19-2011, 03:05 PM   #60
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QuoteOriginally posted by ripit Quote
I personally have modified nikon, olympus, minolta and canon lenses but have never had to go so far as cutting back the actual lens (just custom mount and sometimes spacers). I have seen a post or two where they cut the actual lens back though. If I'm not mistaken, RioRico has modded a wider variety as well as a larger number of lenses than I have.
I've modified Nikon, Olympus OM, Yashica C/Y, and Petri. I've adapted Exakta, Argus-C3, and various non-mount (projector etc) lenses. I'm still trying to figure out a Pentacon Bayonet mount, still thinking about Minolta MD. I'll probably not bother with Canon, Konica, X-Fujinon, Leica R, etc. I haven't chopped any lens-bodies yet. My database says I've modded and adapted 25 lenses total. Whew!

Every experiment teaches us something. Often it teaches us FAILURE. Most experiments fail. If we knew the results ahead of time, they wouldn't be experiments. It's like learning judgment. We gain good judgment from experience. We gain experience by making bad judgments. That's just how reality works. Oh bother...
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