Metabones adapter, reduces focal length and increases F-Stop for mirrorless cameras.

Same size hole equals...same aperture value?

F-stop is a ratio of the focal length relative to the lens aperture.

Consider an 18-55 mm f/3.5-5.6 kit lens.

At 18 mm and f/3.5 the diameter of the lens aperture is - 18/3.5 = 5.14 mm wide
At 55 mm and f/5.6 the diameter of the lens aperture is - 55/5.6 = 9.82 mm wide.
With the lens set to f/3.5 at 18 mm, the f-stop is not changed, and the lens is zoomed from 18 mm to 55 mm, the lens aperture actually gets larger.

This can be confusing, possibly because of the lack of distinction between the physical lens opening and the entrance pupil. The f-number (which is a defined term) is calculated from the diameter of the entrance pupil, not the diameter of the physical opening. The entrance pupil is the image of the physical opening as seen from the front of the lens. Because it is an image, its magnification can change as the front lens elements move during zooming, thus the size of the entrance pupil can change even though the physical aperture remains the same. This effect happens with both variable aperture and constant aperture zooms. It may be combined with some change in the physical aperture.

The Angenieux adapters were for video/film zoom lenses - ie things that cost tens of thousands of dollars. Even though I had a few Angenieux zooms I decided they weren't for me, and I don't think they were widely used.

The Metabones website does mention other lens mounts and other camera mounts. The m4/3 version has different optics from the E-mount version. Brian Caldwell has optimized the optics for the different formats.
 
I wonder if they'll make one for Nikons F mount. It would be great for hobbyist trying to work a DX camera indoors with tight surroundings.

Was the E series body a regular F mount design as far as converter-to-body? Or was the converter a non removeable piece of the camera body?

The "Reduction Optics" were integral to the body, behind the mirror. This was a regular F-Mount camera, basically a Nikon F4 in a new housing.
 
The Nikon E appears to have used a relay lens system - ie it relayed the normal image to a new image plane well behind the old one. The optics lay at and behind the original image plane, as far as I can tell / understand the principle. This is optically quite different from the Metabones reducer which sits right behind the main lens, in front of the original image plane.
 
The Nikon E series used both relay and reduction. The full-frame image circle was reduced to a 2/3" CCD. The increase in intensity was expressed as ISO, which was set at "800" for a base for the light meter. The actual sensitivity of the CCD was closer to ISO 50. The relay lens was at the image plane. As stated, "it did something similar" in that in reduced the image circle of the full-frame lens to a detector that was much smaller, and had a big increase in effective ISO as a result.
 

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