Why not a 0.7X teleconverter?

[spudgunr]

I was just thinking about this, a 1.4x teleconverter loses 1 stop of light and zooms in at 1.4x. What about a .7X (1/1.4) teleconverter that would zoom OUT and gain you a stop of light? This would ONLY work using a film lens on a crop body as it would focus the full image circle smaller until it was the size of a DX lens image circle, but making it smaller would make it brighter. It would also give the same field of view then as they did on film.

Just think, your common $500 80-200mm f/2.8 would be like a 57-142mm F/2.0! An 85mm 1.8 would be like a 60mm 1.2. A 50mm 1.4 would be a 35mm 1.0!

Is this feasible? Why doesn't anybody make it? It would turn any mediocre film lens into a wider angle FAST lens. I know I for sure would love one if I could get it for $200 or so.

 

[Formatted]

Although the idea sounds good in theory. I don't think this is possible.

 

[spudgunr]

Why, am I missing something on why it wouldn't work? Obviously you cannot get something for nothing which is why it wouldn't work on a DX lens on a DX body (it'd shrink the image circle smaller making it brighter, but then you'd have serious vignetting), but I don't see why it wouldn't work on an FX lens on DX body shrinking it to the DX size. Shrinking it would then intensify the light, giving a better F value. Or am I missing something?

 

[Dwig]

...Or am I missing something?

At the theory level, you are missing at least one, although this one problem might be surmountable in few cases. Even so, the odds of being able to create a device to insert between the body and lens that would deliver usable image quality at any practical price is virtually nil.

A 2x extender does two things. One is to increase the lens' focal length by the addition of optics that have an overall negative focal length. The second is that its barrel acts as an extension tube to move the lens forward to restore proper infinity focus.

An extender with a less than 1x "magnification" would have to have a positive focal length. Without some serious optical gymnastics, the barrel would have to have a negative length (a bit difficult in our space-time continuum). The only hope of creating such a thing in any practical way would be for it to also be a mount converter where the lens attached was in a mount with a long register distance (e.g. Nikon, Canon, ...) and the body mount was one with a short register distance (e.g. Micro 4/3). Even there, some optical gymnastics would be required as there is only room to shave off about 10mm of off the lens to film distance.

BTW, closeup lenses work on this principle. They effectively shorten the focal length of the lens to which they are attached. That lens remains at its old position and is thus extended from the proper infinity position for its new shortened focal length. The aperture is still the same size relative to its position from the sensor (same effective f/stop) balancing the increase in speed from the shortened focal length with the loss of speed from the bellows factor.

 

[spudgunr]

Whoops, never even considered the ability to focus. Well, I suppose that may be a good reason ;-)

 

[epp_b]

This thread makes me

 

[Grafiq]

Using the Canon WD-H58 0.7 wide-angle screw-on converter (

as a Nikonian I must say it is a beautiful piece of glass, €300,-) on a Canon Legria camcorder I just wanted to know if this would fit my Nikkor 50mm 1.4G (58mm filter). And gues what? Not only it fits, it works

! AF, light metering (no stop lost) etc. making this a cheap 1.4G 35mm

(€ 1.550,-). Maybe a perfect combi for low-light photography? First results on a D300s DX show some softness. I wil post some pics and shoot it with a FX D3 later (seems photo upload is not working..!?)...

 

[Trever1t]

Uh, no but if you remove the lens altogether you will have f.0