Tele-inverters?

[Ishkabibble]

I'm curious, using the standard SLR lenses on dSLRs like my DS wastes a lot of the lens. The smaller sensor captures what is in the centre of the image and leaves the outer edges unused, though there is image there.

It would seem to me that there should be a device that could be used along with our standard SLR lenses that would function as the opposite of a tele-converter, focusing the light to a smaller area and thereby increasing the speed of the lens... a sort of poor-man's faster lens. It would be useless for SLR users, but for us DSLR users, it would mean dramatically increased options. It would also correct the settings for the old lenses, so lens focal points could be accurate again.

I've searched, but can find no such device. Are the physics wrong for this? Is there such an item?

[astro]

The only way to make the F-stop faster is to increase the front diameter of the lens. Larger opening means more light gathering power. So to make it faster, it would have to be a lens adapter on the front. I don't think they can make something that works well for all lens.

[ktixx]

I understand your point, however the only downside of the crop factor on DSLR's is the reduction on the wide end. The benefits of the crop factor are longer telephoto without add-ons and very little edge distortion becaues it is cropped out for you. Most people will just purchase lenses to fit their situation ie: instead of getting a 24mm lens on 35mm film, they will just purchase an 18mm lens (28.8 on 20D & 350D -- 27mm on D70 & D50). To the best of my knowledge such a wide angle converter does not exist.

[jeisner]

The crop also makes many full frame lenses appear better, as you see less of the soft edges or vignetting... And to fill in the wide end, pentax has a number of great new DA ultra wide lenses like the 16-45/4, 12-24/4 and 14/2.8

edit: ohh and the upcoming 10-17 fisheye zoom