A large number of test
reports on cameras and lenses can be found in many camera magazines and on the
internet, and it seems that in many cases these are more confusing than
helpful. Owners of Leica equipment often
wonder why their supposedly superior equipment in some cases does not perform
to their high expectations. The reasons
are diverse and difficult to explain.
As we all know, a modern
photographic lens is an immensely complicated instrument, with a great number
of performance characteristics. For
years, photographers have tried to find a valid method to determine the overall
performance of a lens. Unfortunately, it
is almost as difficult to measure true lens performance as it is to design the
lens in the first place. The emphasis
here is placed on “true” performance.
Many readers of magazines
and the internet are misled to consider resolution as a valid measure in
determining the quality of a lens. The obvious argument is that a lens must be
good if it is capable of high resolution.
Unfortunately, resolution is not a very good measure, especially if it
is taken alone and at face value. This
is the very reason why some test reports also publish contrast figures. The contrast level of a lens is at least as
important for overall performance as the resolution. As a matter of fact, Leitz has at times
sacrificed overall resolution of lenses in order to gain a higher contrast
level. Contrast is a very difficult to
explain criterion. In simple terms, it
can be referred to as the ability of a lens to reproduce high resolution at a
recognizable level.
Virtually all resolution
tests are done with high contrast (black and white) targets which make it very
easy to differentiate between minute details.
The results are expressed by lines per millimeter, l/mm, where each
black or white line is counted. Actual
picture taking is different, however, because a lens is rarely subjected to the
ideal conditions of a test target.
Instead the lens is confronted with multicolored scenes or subjects, all
of which need to be distinguished.
Regardless of photographing in color or black and white, a photographic
lens must (in most cases) reproduce a colored subject or scene. A lesser lens might very well reproduce two almost
identical shades of the same color as one color or the same shade of gray, and
thus not be able to resolve this slight difference at all, while a better lens
will be capable of making that distinction.
On the other hand, in a common lens test, when confronted with high
contrast test targets, the lesser lens will inevitably show a higher resolution
than it is capable of under everyday conditions.
This is a perfect example
showing the importance of contrast over resolution. When viewed from a normal distance of
approximately two feet, the bottom picture will immediately look better. Upon closer inspection one will notice,
however, that the actual resolution of the top picture is substantially
better. This can be seen especially by
the small test targets in the center.
The actual resolution of the bottom picture is only 50% of that on the
top. Yet the substantially higher
contrast of the bottom picture immediately gives the impression of a better
image, even higher resolution. As I
pointed out, a pure black and white target will show relatively high
resolution, even with a low contrast lens.
Had the center target been separated only by different shades of the
same color, rather than a high contrast target, the differences would not have
been visible, in spite of the relatively high resolution as displayed by the
top image. This is a rather extreme
example. Differences from one lens to
another rarely are as great as in this example.
The gradual disappearance
of detail with low contrast subject
matter
For that and a number of
other reasons, lens manufacturers use a computer read out, the modulation
transfer function (MTF), to better determine lens performance. Rather than giving resolution figures, the
MTF is based on a fixed resolution value, giving the contrast level of the lens
over the entire image area at the various f/stops.
MTF functions also rely
on a test target. Like in the example above, the target shows
vertical and horizontal lines. This is
done because most lenses can distinguish fine detail better in one direction
than the opposite. This is the reason
why MTF function always show two curves, one for the sagittal test patterns and
another for the tangential or meridional one.
Either one shows the lens performance starting at the center of the
image and going out to the edge of the image.
This is the MTF function
of the Leica 50mm f/2 Apo-Summicron-M at f/2, f/2.8 and f/5.6
The four curve pairs show
the level of contrast at 10, 20, 40 and 80 lines per millimeter resolution.
The curve at f/5.6 is an optics designer's dream, virtually flat across the top of the chart,
a performance level unrivaled by other manufacturers.
A competitor's MTF function for a similar lens.
Please note: The function is only for 10 and 30 Lines/mm.
Higher resolution functions would be noticeably worse.
Most published MTF curves
are based on a rather low resolution of 10 and 30 lines per millimeter at the
most. The same “standard” is used by
most magazines for their resolution figures.
It is obvious that 30 l/mm is a criterion that can be met by virtually
any lens short of the bottom of a pop bottle.
So it is no wonder that the majority of the lenses tested do achieve
relatively good figures. Especially in view of the fact that a 100% contrast
rendering is unattainable. But even with
these relatively low criteria, the much more even performance level of the
Leitz lenses is still revealed. Even at
maximum aperture they usually perform to levels that many other lenses do not
achieve unless stopped down, if at all.
If the contrast figures were based on a resolution of 60 l/mm or even
higher, it might become apparent that there are substantially greater
differences among various brands of lenses than we are usually led to
believe. I have often maintained that
photographic magazines and websites which get the majority of their revenue
from advertising, have to conduct themselves in a manner that is not
detrimental to their advertisers.
Although these test are generally very good and unbiased, they are also
conducted and presented via criteria which work to the advantage of the
publication's subsidizers.
I think it is (or should
be) obvious that optical companies other than Leica are all very much capable
of making first rate equipment. But
their merchandise is also in most cases heavily mass produced. The bench-made process, as used by Leitz,
gives the Leica a good competitive edge.
But the superiority of Leica lenses especially shows itself in extreme
situations. This includes lens
performance at maximum apertures and performance under adverse conditions. Unfortunately, published tests do not always
consider this and subsequently Leica lenses often seem to less of an advantage
than generally expected.
One might also notice in
the written evaluations of Leica lenses that the writers often seem to be a bit
strained to avoid the overuse of superlatives.
But we have seen comments like “...the lens against which all others
have to be measured (400mm Telyt)...one of the best lenses ever tested (50mm
Summicron)...the closest to our test standards ever (40mm Focotar).”
So far we have talked much
about optical performance, the sole criterion used in most lens tests. The mechanical quality of a lens, however, is
just as important. Poor mechanical
design and quality will eventually degrade even the best lens to mediocre
performance. Automobile magazines
routinely run 50,000 or even 100,000 mile performance tests on automobiles and
I have often hoped that something like this would be done with cameras and
lenses as well. The performance of a
lens, when brand new, is one thing, especially if it is delivered for tests by
the manufacturer after careful tweaking and adjusting to assure the best
performance possible. But what about its
capabilities after it has gone through several months of use, after it has been
knocked around in gadget bags, in cars and airplanes for an extended period of
time? Heavy use and the rigors of
professional use take a great toll and only the best will remain on top. This is where mechanical quality translates
into optical performance and shortcuts in quality will show lower grade results
immediately.
Price is the final
criterion I should mention. Nobody in
the optical industry today is able to perform any miracles. Although mass production does have a
beneficial influence on the price of lenses and cameras, this advantage quickly
fades when we deal with lenses of an exotic nature that are sold in a much
smaller quantity and cannot be mass produced.
Suddenly we find lenses from competing companies to be equally expensive
than their Leica counterparts. Extreme
wide angle lenses and long telephoto lenses, especially those that have proven
to be close in performance to their Leica counterparts, usually sell for not
much less than their equivalent of Leica lenses. This is the case with most of the major
Japanese lens manufacturers. Yet when
dealing with absolutes in performance, none of them have anything to offer that
can compete on all levels with the newly designed Leica lenses featuring
apochromatic design and aspherical lens elements. The heart of any camera system is its line of
lenses, and this is still one of the main reasons for choosing a Leica.
For other articles on this blog scroll down in the column to the right to BLOG ARCHIVE
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