Cyril wrote:
> Some lens definition tests can reduce the subjectivity factor. One of them
> consists in evaluating the ratio between 2 measurements taken by the same
> instrument : the same instrument measures the contrast of a source image and
> the one of a the same image through the lens on test. This method is often
> called MTF (**).
Yes, but this test doesn't test the optical quality of a lens but its MTF. They are
not measuring optical quality but MTF. Repeat 100 times. MTF not optical quality. This
kind of mistake is done repeatedly and its mostly done by people without scientific
training and who believes anything expressed in numbers are scientific and hence the
truth. I'm sure these numbers CAN be useful for someone who can read them but like
they usually are presented its like testing how something sound by measuring frequency
range. I remember kids that chose on amplifier over another because its frequency
range was specified at over 100 000 Hz. I'm sure these result has some relations to
final optical quality but its certainly not straightforeward. How else to explain the
wildly different results of the same lens from different laboratories? Sample to
sample variations are not that large.
BTW theres is hardly any subjective factor. If you put ten people at a light table and
ask them to evaluate the difference between two lenses based on tests shots, they
either don't see any or much difference in which case differences in real life is
negligible and little consequence, or they see a clear difference in which case there
will be no discussion. How important this difference is is another matter.
>CdI guys are not
>attached to a make and have to produce objective comments on what they
>test...
CDI is definitely attached to their test results and certainly not objective about
them. I don't hink anyone has questioned their numbers. If CDI say that this or that
lens gives damned fine numbers in the laboratory but we don't give a rats ass on how
it performs in making pictures on a film, then we agree.
Eg. the A* 135/1.8 is sharper than the FA* 85/1.4 at wide apertures and no person on
the planet have been able to prove otherwise. The numbers these lenses scores at those
apertures in CDI's laboratory is totally without interest for this fact.
BTW I once had a Leica lens brochure where they used four pages explaining in
panstaking details with examples why MTF test should be completely discarded when
choosing a lens.
Pål
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