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 Post subject: Tele-un-converter?
PostPosted: Sun Nov 02, 2008 12:22 pm 
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A random thought I had during a quiet moment. I'm wondering has anyone ever made the opposite of a teleconverter before?

It would reduce the effective focal length of a lens and increase the effective aperture (lowers f-numbers).

When everyone used 35mm, there probably wasn't any point in doing this as all you would end up with was extreme vignetting. But now we have crop factor sensors, there is a lot of potential image area unused with full frame lenses.

In essence, such a device would negate the crop factor and give you more light density. If the strength is made to be the reciprocal of the crop factor, it would make a crop format camera effectively the same as full frame.

So to recap:
*essentially a teleconverter with a magnification of less than 1x
*reduces effective focal length and f-number
*would only be useful with a full frame lens on crop sensor body

Possible problems may include:
*like teleconverters, the extra glass may reduce image quality
*I don't know if those pesky laws of physics may get in the way of practically implementing this. In particular something to do with the distance between the mount point and sensor plane seems to be a common hindrance in lens design.

Examples: For a typical 1.5x crop factor camera, the maximum reduction will be 1/1.5x or 0.66x magnification. Assuming no losses, your 50mm f/1.4 could be effectively 33mm f/0.9. Ok, all of a sudden this device is sounding a bit "too good to be true". But regular teleconverters also don't like working with short focal length lenses, and only work on longer focal length lenses. Or the popular bright zoom 70-200 f/2.8 could become effectively 47-133 f/1.9. Given lenses like these don't exist, I can only assume that things are not as simple as they may first seem.

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 Post subject:
PostPosted: Wed Nov 19, 2008 10:28 pm 
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Wide-angle- or even fisheye-converters exist, popo! But not for DSLRs. They of the variety that is mounted in front of the lens - not between thelens and the body. You can buy those for vidoe cams and p&s cameras.
Whether it is physically infeasible to build one that mounts like a tele-converter? I don't know, but I think you can build a contraption that demagnifies the imge from the primary lens. This will naturally increase the brightness of the image. Unfortunately you will demagnify the image-circle of the primary lens too. So don't expect this to wrok well with DX-lenses. But with FF/FX-lenses no a DX/APS-C body you could easily shrink the focal length by 1.5x without getting in to problems with the image circle. Thus your 50/1.4 could be converted to a 33mm f/1.0 :idea: :shock: :!:

Btw.: Modern wide-angle designs (of the "inverted tele-photo" type) already demonstrate that you can have a 10mm focal length although the back of the lens is 30mm away from the sensor.

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