Re: [Leica] 6.8 Telyts

[Richard Ogden <rjo@provider.co.uk>]

At 22:33 05/05/98 -0700, you wrote:
>Henning J. Wulff wrote:
>> The explanation given to me was that some glasses (glass being a fluid) can
>> indeed flow enough in a normal span of years to cause this. Leica has used
>> a number of exotic glasses, and I was told that this might have been the
>> cause. Separation of the two elements had not taken place. 

Forgive for butting into this discussion. I have a booklet produced by
Leitz in Autumn 1982 entitled 'Leica R-Lenses'. On page 32, under the
heading 'The follow-focus lenses', is the following:

'The 400mm TELYT-R f6.8 and 560mm TELYT-R f6.8 are achromats i.e. each
consists of two cemented elements. They occupy a special place among the
LEICA lenses because these highly corrected achromats produce extremely
brilliant pictures. Resolving power and colour correction are unusually
good owing to the use of special glasses produced by Leitz. It is a
well-known fact that at full aperture achromats suffer from a slight
curvature of field. This shifts the sharpness in the marginal zones to
slightly closer objects. In practice this fact is usually taken advantage
of, because normally it affects the reproduction of the foreground: a
slight gain in sharpness in the nearer foreground will almost always be
welcome. With plane objects such as walls of houses, focusing should be a
compromise, i.e. it should be slightly outside the centre of the picture,
and the lens must be sufficiently stopped down if optimum sharpness is
required also in the marginal zones.'

Incidentally, the preface to the booklet consists of two pages pointing out
the inadequacy of merely using MTF results to assess the performance of a
lens. They argue:

'Many lens manufacturers endeavour to keep astigmatism and curvature of
field at a minimum. This must often be bought with the loss of maximum
possible contrast, although it is the contrast performance of a lens that
decides the visual impression of sharpness in the picture. Leitz are
therefore less keen on a flat image field and minimum possible astigmatism,
if this is at the expense of contrast performance.'

and later:

'Leitz obviously endeavour to compute lenses that produce a completely flat
field. Even so, the practical requirements are never neglected. And if a
compromise has to be found, which is often necessary, it will only be in
favour of applied, i.e. practical photography.'

Which is exactly the point made in the recent LUG discussion.

I hope this is of interest.



**************
Richard Ogden
rjo@provider.co.uk
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