Archived posting to the Leica Users Group, 1996/10/04
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In-Reply-To: <199610042059_MC1-A07-7797@compuserve.com>
> Responding to Marc Small:
>
> Uh, what's an MTF test? And, in plain English, is
> it possible to explain how intentional lens aberrations
> can be made to selectively fuzz parts of an image and
> not others?
>
> I am genuinely curious. I am thinking of sinking a lot
> of money into a camera system, and if you can prove your
> point, my decision will lean toward seasoned Leicas.
>
> George Zachar
MTF stands for Modulation Transfer Function. In the simplest terms it's
the 'frequency response' of the lens system from input to output.
The difficulty with MTF tests lies mostly with their interpretation.
Unlike an audio amplifier - for example - where the response can be shown
as one line on a graph from 20Hz to 30kHz, a lens MTF has to consider
centre and edge performance, sagittal and tangential performance,
performance at difference optical wavelengths, and the variation of all
of these with focus distance and aperture.
As a result you get a lot of curves, and it takes *all* of these to
accurately describe the performance of the lens. The difficulty arises
when someone attempts to simplify the results to give a 'figure of
merit'. This is a trap which most (all?) of the photographic magazine
reviewers frequently fall in to, and since the marketing dept at the lens
manufacturer is driven by the opinions of the reviewers, it's easy to see
why less than satisfactory lenses result.
dmorton@cix.compulink.co.uk |
david@cassandra.compulink.co.uk | "The loss of an old man
(+44) 181 450 5459 | is like the destruction
| of a library"
Kilburn, London, England |