Negative factor lens converters

I just read about Hasselblad's 0.8 converter for the X Camera used with HC/HCD lenses. What interested me is the converter, having a less than 1 factor, actually makes the attached lens faster (and wider). I wonder why this has not been done for other cameras/lenses...it sounds like a neat idea.

It has been done!
I cannot tell you which specific brand with what lenses, but years ago this was done.

Read about Metabones. A category of product referred to as 'speed boosters'.
https://zebrazone.tv/what-is-a-speedbooster/

The focal length of an optic is reduced, while the max aperture diameter remains the same, so the f/stop becomes faster

• If we start with 100mm f/4 lens, its aperture is 25mm diameter
• If we reduce 100mm to 50mm, and its aperture is still 25mm diameter, the f/stop is f/2.

It trades coverage for speed.

ic-racer said:

It trades coverage for speed.

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Sounds like it gives both more coverage and more speed.

At the film/sensor side, there is less "coverage". It helps to think of coverage in the way that large format photographers do.

MattKing said:

At the film/sensor side, there is less "coverage". It helps to think of coverage in the way that large format photographers do.

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So, a lens designed for a larger format would work then.

Pieter12 said:

So, a lens designed for a larger format would work then.

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The problem with that comes when the 'larger' format lens starts with lower resolution (line-pairs per millimeter)...the result may not be as good as a native FL for the smaller format. Remember, the extra optical elements of a teleconvertor hurt the resolution of the primary lens; same principle at play when the multiplier is fractional rather than >1, but we start with a handicap (even without the convertor) of lower resolution for larger format.

Effectively, a focal reducer is like the opposite of a teleconverter. https://en.wikipedia.org/wiki/Telecompressor A 2x teleconverter takes the central part of the image, such as 12x18mm on 35mm and magnifies it to cover the full 24x36 frame. As you know, the teleconverter increases the focal length and causes an effective speed loss. (In practice, the teleconverter intercepts the light rays before they reach the focal plane and rearranges them; this is a subtlety but is one of the reasons a teleconverter will work better with some lenses than others).

A focal reducer or telecompressor (or "speed booster") does the opposite, it takes the full frame and demagnifies it, scaling it down a little, like 0.8x, to fit on a smaller format. It shortens the focal length and so the effective lens speed increases. An 0.7x focal reducer will just about get full-frame 35mm onto an APS-C sensor, so these are/were used with small-sensor cameras when few people could afford a full-frame sensor, but there were a lot of 35mm lenses,. (I note that this is primarily a digital issue.)

Obviously, you can't demagnify lens coverage that wasn't there to begin with. I think the Hasselblad H is 6x4.5 (56x44mm) and X is 44x33cm, so it takes a larger lens and scales the image down. You couldn't take a 6x4.5 lens and demagnify its image onto a 6x4.5 piece of film and get more coverage for free; it would vignette, or the image would be terrible. Additionally, the combination has shorter back focus, which is why they tend to be used on mirrorless cameras.

"Speed boosters" usually seem to go to about 0.7x, I'm not sure there are common 0.5x demagnifiers corresponding to the ubiquitous 2x teleconverter. My guess is that poor optical performance and/or mechanical issues limit them to about 0.7x.

The obvious question is what happens if you stack a telecompressor and a teleconverter. I am not sure this has been tried. It will probably vignette a lot.

The main purpose of these converters is that they allow someone to squeeze the result from a lens designed for a larger format into a frame/sensor that is smaller.
That allows you to use the wide angle lens you like so much with full frame 135 on a "crop sensor" camera.
The apparent increase in speed is just a byproduct.

Pieter12 said:

I just read about Hasselblad's 0.8 converter for the X Camera used with HC/HCD lenses. What interested me is the converter, having a less than 1 factor, actually makes the attached lens faster (and wider). I wonder why this has not been done for other cameras/lenses...it sounds like a neat idea.

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'm sure, this idea is begging for disappointment in form of optical performance.

RalphLambrecht said:

'm sure, this idea is begging for disappointment in form of optical performance.

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and BY THE WAY,, I never had the plesure to work with a decent teleconverter either; they are all crap.

RalphLambrecht said:

'm sure, this idea is begging for disappointment in form of optical performance.

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There is no free lunch. There will be losses.

RalphLambrecht said:

and BY THE WAY,, I never had the plesure to work with a decent teleconverter either; they are all crap.

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Then you never used the Hasselblad 2XE? You would be pleased with it, especially if you used it with the 250mm lens instead of the 500mm lens. [lighter, not need for a tripod, ...]

RalphLambrecht said:

and BY THE WAY,, I never had the plesure to work with a decent teleconverter either; they are all crap.

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DPReview tested Canon 70-200mm lenses, with and without IS, and then retested with the Canon 1.4X teleconvertor. Measured loss in detail resolution amounted to about -10%, and this was repeated with later generations of 70-200mm Canon zooms. That is hardly 'crap' performance, IMHO.

Admittedly, those same teleconvertors did turn the 100-400mm (v.1) to crap, to the dismay of many owners of the 100-400mm. Ergo the primary lens was more responsible than the teleconvertor for the results going to hell.

Even Zeiss Oberkochen, after being reluctant, offered a teleconverter.

Pentacon, Zeiss Jena and KMZ never offered such... but likely for other reasons.