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Would it be feasible to reverse a 1.4x teleconverter's optics into a cross-system lens mount adapter & have it function as a focal reducer?

For example, like this:

Camera    hacked adapter       Lens with 
 body  ←      w/           →    longer 
         reverse TC optics   registration

This would entail cannibalizing a 1.4x TC & mounting its optics backwards into a mount adapter.

Could it also sharpen the center of the field a scosche, by forcing optical concentration of the field, as well as gain a partial stop by intensifying the field?

If this is feasible, it might also be feasible for Tx/T2/T 135/35mm mount lenses, assuming they were engineered with extra field to accommodate Nikon's long lens registration, when adapting to far shorter registration length/flange distance systems (basically anyone shorter than Leica R or Nikon F).

Plasticine / JB-Weld not included.

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Addendum:

"Poor Man's Focal Reducer" at mflenses.com (a "manual-focus" lense afficionado site):

Entails some trial & error:

http://forum.mflenses.com/viewtopic.php?t=57639

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    Or you could just buy a metabones speedbooster - metabones.com – dpollitt Dec 13 '13 at 16:26
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    Granted, but Metabones' stuff comes at a premium. And, Metabones doesn't make those for every imaginable cross-system scenario. – L. Rodgers Dec 13 '13 at 16:31
  • You might discover the resulting image circle doesn't completely cover the sensor, depending on the sensor size. – Michael C Dec 13 '13 at 17:18
  • Real black-ring vignetting isn't necessarily a problem w/ a 24MP sensor, provided it doesn't realize too large a black ring around the image... As you point out, it's more likely with a larger sensor, but with a crop sensor (APS-C or Four-thirds) I might wager a better than even chance of either no, or normal, vignetting. By gaining sharpness & 1 stop (assuming a 1.4x TC doublet) the crop option is surely opened up some, esp. considering mediocre lenses might actually be forced to resolve down below the sensor's circle of confusion. – L. Rodgers Dec 13 '13 at 17:26
  • Hmmm. 50mm f/1.8, 2 x TC , ... :-) – Russell McMahon Dec 14 '13 at 6:36
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Yes you could put reversed TC optics into a tube and use it as a focal reducer for shorter registration distance systems, in the process sharpening and intensifying the field. You could even give it a silly name like "lens turbo" or "speed booster" and sell loads of them!

The quality obtained by re-purposing an existing teleconverter for the task would be highly questionable as modern TCs are a long way from general purpose optics and are specifically designed to be paired with certain telephoto lenses.

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    I disagree, "lens turbo" sounds awesome. So awesome I am looking into it on Google right now. – dpollitt Dec 13 '13 at 16:26
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    Indeed. Or how 'bout "Optical Laxative." Faster, narrower, less grainy ... – L. Rodgers Dec 13 '13 at 16:32
  • I don't doubt there might prove some side-effects in image quality, especially at the corners. First find the proper TC match for a given lens & try that first. After that, then try various TC's fitting the body. Past that, perhaps onto Franken-lensing ... introducing field flatteners, coma reducers .. these are used in telescopy all the time, of varying magnitude to rectify simple defects in a telescope's main elements. – L. Rodgers Dec 13 '13 at 23:06
  • Wild conjecturing from hereon: The element diameter on the TC is close to the optics for 2" telescope eyepieces. Might work... :O – L. Rodgers Dec 14 '13 at 14:41
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    This answer seems to have been written in jest, but it's wrong. Flipping a teleconverter around won't turn it into a focal reducer. Here are sample images to demonstrate what happens. – xiota Mar 10 '19 at 6:18
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Can I build my own focal reducer by reversing a teleconverter?

... mounting its optics backwards into a mount adapter ...

Focal reducers and teleconverters are specialized lenses, and flipping lenses around doesn't change their optical properties the way you want. It would be nice if optics worked like that, but then someone would already be selling convertible tele-focal-converter-reducers.

Flipping lenses around will change the distance to the focal points on either side of the page (when drawing diagrams). However, the "power" stays the same. Teleconverters and focal reducers have opposite powers (negative vs positive).

To demonstrate, here is an image taken with a 35/2.8 lens on a crop sensor body:

Hansa

Here is an image taken with the same lens and a teleconverter:

Hansa + teleconverter

Here is an image taken with the same lens and "reversed" teleconverter. In this case, the field of view is about the same, but with added vignetting. The image circle did get smaller, but no focal reduction.

Hansa + "reverse" teleconverter

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