0.5x and 0.7x TCs - good idea?

Thinking of ways to go wider on an EOS mount had me thinking - what about a wide converter (the opposite of a teleconverter) designed to work with Canon's tilt-shift lenses, which have large image circles and would therefore be able to support it? Essentially, condensing the 66mm-or-whatever wide image circle into a smaller circle, thereby producing a wider image on the sensor without the need to photostitch. The same could also be used to make EF lenses wider for 1.3x and 1.6x crop sensors. Viable?

Though I don't really know a lot about the dynamics and mechanics of lenses I'm pretty sure this would be impossible, unless you put the converter on the front of the lens.

Through say a 100mm, your angle of coverage is 24º, this doesn't change from format to format. What comes through that lens is a coverage of 24º. With the converter between the lens and body it wouldn't be effective, as a wider angle cannot be derived as the light you change change the dynamics of the lens to a wider dimension after the fact.

Whether or not that makes sense to anyone else is beyond me.

They do have wide angle converters that will attach to the front of the lens, though I couldn't tell you what the quality is like.

Except that what comes through a TS-E 17mm lens is actually a 130 degree angle of coverage, but spread out over a larger image circle than is covered by a full-frame sensor. A full-frame sensor is only wide enough to cover 103 degrees of this at a time.

By attaching a wide converter to the rear of the lens, you'd be preserving the 130 degree coverage, but, instead of projecting it as a 60mm-wide circle, you'd be projecting it as a 44mm wide circle (for argument's sake), allowing a full-frame sensor to cover the full 130 degrees of view of the lens.

there are very low quality wide angle converters available for EF lens, and they go in at the front of the lens. do a search on ebay for 7D, and you'll see these 7D +5 lenses or 7D +7 lenses bundle deal, those usually include wide angle converters.

AFAIK making "telecompressors" is possible and could actually make lenses sharper. I bet a lot of crop users with EF lenses would love them

RaZe42 wrote in post #10594303
AFAIK making "telecompressors" is possible and could actually make lenses sharper. I bet a lot of crop users with EF lenses would love them

I think this is a very interesting idea for cameras with APS-C sensor-size. But for full frame camera it will cause vignetting, maybe even cutting in the corner.
For Canon cameras I would appreciate a 0.625x converter for my 50D.
Converting EF 28mm to "EF-S 17.5mm" or EF 24mm to "EF-S 15mm".
It is some wide and normal-prime lenses I would like to have this converter. I don't think I'll use it with tele-lens, unless I'll gain light, like you lose with the teleconverter.

thelleha wrote in post #11657145
I think this is a very interesting idea for cameras with APS-C sensor-size. But for full frame camera it will cause vignetting, maybe even cutting in the corner.
For Canon cameras I would appreciate a 0.625x converter for my 50D.
Converting EF 28mm to "EF-S 17.5mm" or EF 24mm to "EF-S 15mm".
It is some wide and normal-prime lenses I would like to have this converter. I don't think I'll use it with tele-lens, unless I'll gain light, like you lose with the teleconverter.

Use it with a tilt-shift lens or a medium-format lens and you won't get vignetting, and have the possibility of super-wide, super-sharp lenses.

I looked into this a while ago. Turns out such a thing exists, it's called a telecompressor. In theory you could put a one on the back of say a 50mm F/1.8 lens and not only would you get that FF field of view on a APS-C sensor, but you would also gain about a stop of light IIRC. The few results google gave me indicated that it really wasn't practical due to, I believe, focusing issues. I would've expected Canon would've made one long ago if this wasn't the case.

I've always wondered about this, too--seems like one could be made for APS-C cams that would enable full frame lenses to be used at their normal field of view....

woos wrote in post #11706860
I've always wondered about this, too--seems like one could be made for APS-C cams that would enable full frame lenses to be used at their normal field of view....

This is what Olympus did with their 12-35mm f/2 and 35-100mm f/2. The lenses are actually their old full frame f/2,8 lenses with a 0,5x "telecompressor" built into the design. This should have given a 2 stop advantage (f/1,4 zooms!), but there was some physical/optical limitation to f/2(I don't know what, all this info is just second hand). If Olympus would have based the new lenses on f/4 designs they could actually have had f/2 lenses smaller than the f/2,8 full frame equivalents.

The problem with a "telecompressor" with a SLR is that it would reduce the backfocus distance and not leave enough mirror clearance. They work ok with telescopes as there is plenty of excess backfocus distance available. To get around the mirror clearance issue, a relay system would then be needed. Now you are talking about a lot of glass to make this work and probably image degradation unless it gets expensive too. I would think it would be cheaper to just buy wider lenses.

macroimage wrote in post #11760946
The problem with a "telecompressor" with a SLR is that it would reduce the backfocus distance and not leave enough mirror clearance. They work ok with telescopes as there is plenty of excess backfocus distance available. To get around the mirror clearance issue, a relay system would then be needed. Now you are talking about a lot of glass to make this work and probably image degradation unless it gets expensive too. I would think it would be cheaper to just buy wider lenses.

A telecompressor should still leave plenty of mirror clearance for a crop body - you wouldn't use them on full-frame, except for with TS-E lenses.

Also, I'd like to see an optical de-fishing element, to convert fisheyes into ultra-ultra wide lenses...