Archived posting to the Leica Users Group, 1998/06/08

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Subject: Re: [Leica] Re: New 50 R Summilux
From: Chris Bitmead <>
Date: Mon, 08 Jun 1998 12:11:57 +0000

Pascal wrote:
> On 06-06-1998 16:32 Chris Bitmead wrote:
> >It is a fact of life that no 50mm f1.4 lens will measure up
> >to an 80 f1.4 lens.
> How is that? Could you explain this, Chris?

Well the main thing I would say is, don't worry about why, but
just look at the performance of lenses from different
manufacturers. There seems to always be quite an increase in
quality from 50 to 85mm.

But if you want to think about it, when a lens is being shot wide
open, each part of the image is coming through every part of the
front element. Like the nose in a portrait isn't just entering
the centre of the front element, but the entire area.

Therefore if you picture a wide angle lens, the light has to be
defracted at very severe angles between the front element and the
film in order for all those points of light from one side of the
lens to the other, to all meet in the same point. The defraction
in a long lens is much more mild. The longer the lens, the milder
the defraction.

There are other issues the wider a lens gets. Like with the rear
element tending to be closer to the film plane, the centre tends
to be more illuminated. Film plane flatness tends to be more of a
problem. And there is more distortion with the designers
wrestling to keep a larger angle of view square and rectalinear.