Flange diameter etc

Started Aug 25, 2018 | Discussions thread
bobn2 Forum Pro • Posts: 69,811
Re: Flange diameter etc

rondom wrote:

The new Nikon Z makes me wonder: How come Leica is able to retain their legacy mount and lenses and make them work with their digital cameras? I am aware of sensors with microlenses etc, and I'm sure there is some image engine manipulation but is there any other secrets to it?

Looking at the new "full frame mirrorless" out there, it looks like what Leica is achieving is unique: a digital M body has the same thickness as a film M except for a small additional thickness around the flange.

Meanwhile Nikon is introducing this considerably larger mount. I doubt that Leica has some secret technology that Sony or Nikon don't have access to.

This is rather puzzling to me. So what is it?


p.s. maybe sensor needs more light for AF? and also I assume the ability to introduce very fast lenses?

Below is a diagram that might help explain this

Looking at the top diagram, the black line to the right represents the frame diagonal. The two L shaped lines are the lens flange, for the Nikon Z at a diameter of 55mm, 16mm from the image plane. The two dotted black lines from the centre of the frame are the light cone from an f/0.95 lens to the centre of the frame. The lens' exit pupil could be anywhere along the lens' axis, and wherever it is, must be large enough to project this light cone (that's what makes it an f/0.95 lens). I've placed exit pupils at 28mm (green), 50mm (red) and 100mm (blue). You can see that the 28mm one can easily project to the corner of the frame, as can the 50mm one. The 100mm one just makes it (in fact, so close that I'm wondering whether an f/0.95 lens with a 100mm distant exit pupil was a design goal).

The lower diagram represents the Leica mount, with a 39mm clear mount at 27.8mm. Here you see that the 28mm exit pupil can easily project to the corner of the frame but that the 50mm and 100mm ones can't. We can fit an f/0.95 lens to the Leica, so long as it has an exit pupil close to the image plane.

Digital image sensors with microlenses work better with distant exit pupils, and most 'designed for digital' lenses have an exit pupil in the range of 100mm or so. Thus the new Nikon mount can take a 'designed for digital' f/0.95 lens, the Leica mount can't. The Leica lens will work with a digital camera, but there is liable to be a lot of vignetting in the corners, because the microlenses can't work well with the light as oblique at the corners as the close exit pupil makes it.

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Ride easy, William.

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