Parameters of Lens Specification and the Properties of Lenses
I'm going to indulge myself a bit here and assume you may not be a grizzled
old veteran enthusiast with stacks of Camera 35 and Modern
Photography in your closet or basement, and therefore might
like a bit of an explanation of some of the properties of lenses. For those
to whom these notions are old hat, my apologies in advance.
I've been known to have a pretty jaded and cynical and sometimes even
(heavens!) sarcastic view of "lens tests." I feel for the most part that
they're relatively useless. It's not just that lens tests are sops to
insecurity, although they are that. It's not that most of them are silly, or
misleading, or wrong, either, although they are often that, too. It's that,
often, they're just not helpful at all.
One of the things that troubles my hearers when I go off on one of these
rants is my scorn for single-number ratings. You know, this lens is a 3.8,
this lens is a 4. The anxious lens-owner, looking for ego-bolstering or
purchase justification, will feel pleased if he owns the 4 lens and may
start biting his nails with a distracted look in his eye if his is the 3.8.
I hope by the end of all this, you'll have an idea why I think this is silly
— and why I think you should think so, too.
I don't want to talk about lens design per se (I don't really know much
about it, to be honest. Talk to Ron Wisner or Erwin Puts). What I want to
look at is how a lens is specified. When most experts talk about how lenses
are made, they seem to start from the cozy assumption that the problem is
primarily (or entirely) a technical one, that the goal is simply to make it
as good as possible, and that the designer has carte blanche in terms of
resources. Such articles are loaded to the rafters with happy tech-talk
about aspheric elements and how multicoating defeats reflections and so
forth.
All that's well and good, but as you've probably already guessed, the
technical part of the design is not the whole story.
Over the years, there's been a gradual, herky-jerky shift in the business
model of how companies come up with products. To simplify it a lot, under
the old assumptions, the engineers got ideas for products and built them,
and the poor beleaguered marketing departments then took whatever they were
handed and attempted to sell it. Under the new paradigm, it's the other way
around — the marketing department decrees what it wants to sell,
and the engineering slaves burn the midnight oil until they come up with
what they've been asked for.
I'm sure a book-length exegesis could be written expanding on the preceding
paragraph (and perhaps has been). But let's move on. The fact is that the
specification of the product when the product is a lens begins not with a
technical problem, but with a marketing one. As such, the chief puzzles
faced by the engineers is one of dealing with limitations that may have
little to do with absolute exercises in design. Only seldom is the marketing
problem to make something of higher quality than the competition has.
Let's look at some of the limitations:
- MONEY. This is the biggie. If you could put it on a
scale, it might outweigh all the others combined. Hobbyists typically have
in mind a set hierarchy of who's best — Leica first, then Zeiss,
Canon and Nikon tie for third, then Pentax and Minolta, and so on down the
line. I get outraged stares when I say that any competent lensmaker could
build the best lens in the world it it had enough money. But it's the case.
All this earwash about "we have the best glass" and "we have the most
expertise" and "we use the best computers" and so forth is marketing
posturing. There may be optical sweatshops that simply don't have the
expertise or the equipment to do well, but believe me, most lensmaking
companies could build the world's best lens if it knew in advance it could
sell 3,000 of them at $5,000 each.
My enlarging lens for 35mm, a Carl Zeiss S-Orthoplanar, is discontinued now,
but when it last appeared in the Zeiss catalogue it listed for more than
$3,000. Obviously, with the precipitous decline of interest in traditional
darkroom equipment in recent years, this is not a product that would be
viable today.
The actual cost constraints are considerable. Let's take a brief look at two
lenses that have appeared in recent years, the Leica 50mm Elmar-M and the
Nikon Nikkor-P 45mm f/2.8. Both are derived from a simple 4-element Tessar
design (originally developed by Zeiss) that's more than a century old. Both
are easy as pie to manufacture. But at $700, the Elmar-M is the cheapest
rangefinder lens Leica offers and (well, sometimes) gets talked up as a good
value, while at half that price, $350 or so, Nikon users grump and grouse
about how costly the Nikkor-P is "for what you get." So tell me, what's the
price you think the market would bear for a Tessar-type labeled Phoenix or
Samyang? Think those companies could find a market for a $700 one? But if
you went to Schneider or Elcan or Perkin Elmer or Cosina and asked if they'd
like to build 100 Tessar-types for you for $200,000, you think you wouldn't
get a lens at least as good as any on the market?
In fact, a good deal of the engineering problem addresses questions the
manufacturers would rather you not hear them ask, such as, How poorly can it
perform before buyers start to bitch? How few elements can we get away with?
How little coating can we get away with? How cheap can we churn 'em out? How
much profit can we build in? Well, perhaps not these actual questions, but
you can be sure the issues behind them are not far from the corporate
consciousness.
- Size, weight, and physical lensmount constraints.
Size is a major design constraint. In general, the larger the designer is
allowed to design a lens to be, the easier it will be to make a good one.
Pros have learned over the years that they're going to have to lug some
weight if they want the best performance. Consumers aren't so well trained.
Consumers just don't care for great big heavy lenses, and they tend not to
buy them. Again, a brief example: a number of years ago, Contax (Kyocera)
commissioned a 35-135mm zoom from Zeiss To keep standards up, Zeiss
delivered a lens that was approximately the size and weight of a mortar
barrel filled with bricks. A magnificent performer, it sells at the rate of
approximately four per year. (Okay, I'm exaggerating. I'm weak.) Ten or a
dozen years later (I'm not looking it up...told you I was weak), Contax
brought out the nifty little Aria, a camera more or less expressly designed
for Japanese females. The winds of the industry were changing by that time,
so direct comparison would be meaningless, but Contax had learned its
lesson: the 28-70mm marketed with the Aria had lots of polycarbonate in it
and, while not wee small, it was indeed wee small for a Zeiss —
a lot lighter at 11.5 oz. than the older 35-70mm, which is 17.5 oz., not to
mention the aforementioned 35-135mm, which weighs 34 pounds. (Okay, actually
25.5 oz.). Over the years, a great deal of optical and engineering expertise
has gone into making lenses "just as good, but smaller."
Physical lensmount constraints are another often inflexible limitation. Just
as long lenses are often limited by the allowable size of the objective
(outermost) element, fast lenses are often limited by the size of the exit
pupil (the element you see when you look at the back of the lens). Nikon
designers might like to make a fast lens with a two-and-a-half-inch exit
pupil, for instance, but the project is not likely to gain approval since
the Nikon F-mount is one and seven-eighths inches across. Wide lenses are
sometimes constrained by the amount of backfocus that is or is not needed,
and leaf-shutter lenses by the size and the speed of the lens and shutter.
Why shutter speed? Because making a leaf shutter open very wide for very
short amounts of time requires a more expensive shutter mechanism. Case in
point are the slow "fastest" normals for medium-format rangefinders such as
the Mamiya 7 and Bronica RF645. It's not that the lens's maximum apertures
couldn't be bigger, it's that the leaf shutters would also have to be.
Interestingly, this is a limitation for leaf-shutter rangefinders in that
buyers expect normal lenses to be fast, small, and inexpensive, because
that's the way it is with focal-plane-shutter cameras. Really, it would make
the most sense for the normal lens for a leaf-shutter rangefinder to be the
fastest and the most expensive in the camera's lens lineup,
and probably also not the lightest, but that's just not what buyers expect,
and the manufacturers of such niche cameras know better than to try to
re-educate the entire market.
- Mechanical robustness, manufacturability, and durability.
Many years ago, in the salad days, when cameras were sold in
camera stores and camera manufacturers had reps and all was right with the
world, a friend of mine attended a demonstration by a Leica rep. According
to my friend, this man gave a short talk on mechanical robustness, during
which he took a short section of barbed wire and rubbed the barb against the
outermost element of the lens. Then he took the lens in his hand, crouched,
and launched it like a bowling ball across the floor, where it skittered and
bounced until it banged into the wall. He walked calmly over, picked up the
lens, snapped it into his camera, and said, "Ready to take pictures."
That's mechanical robustness. Old lenses are often more robust than the
cameras they fitted. In a trend that is likely to continue, Cosina /
Voigtländer has recently introduced several cameras intended solely to make
use of old lenses. The Bessaflex takes M42 screwmount SLR lenses, the Bessa
R2C takes classic Zeiss Contax rangefinder lenses, and the Bessa R2S takes
classic Nikon rangefinder lenses.
Manufacturability is another design issue that has seen incremental
improvement over the years. Obviously, a product that can be made by
semi-skilled labor in 10 hours is going to sell for a lower price and have
more profitability built in than a similar one that requires highly trained
workers 20 hours to make. Some of this ease of manufacture can be "designed
in" — in the case of a lens, for instance, element edges that
are fatter and have more contact area are likely to be easier to collimate
(align) properly. Lenses with fewer elements and fewer moving groups are
also likely to be easier to make. Complex shapes molded from plastic or
magnesium may be easier to make than the same shape machined out of a billet
of metal. You get the picture.
In the case of cameras, manufacturability has a lot to do with economies of
scale — the number of units of a lens projected to be sold. A
soap manufacturer with a $250,000 high-speed boxing machine is likely to be
more cost-efficient than a soap manufacturer that employs thirty people in a
room boxing the soap by hand, assuming it sells more.
More in a couple...
Well, hmm. I notice this is getting long, and you must be getting tired,
because I am. And I haven't even really begun to cover the subject I started
out to cover, because what's still to come are all the properties of lenses
that photographers, as opposed to designers, engineers, and manufacturers,
really care about. This will continue two weeks from now. Or maybe, given my
tendency to procrastinate, three.
— Mike Johnston
Every week, I ask people who enjoyed this column to shoot me 53 cents for
it. Of that, 32 cents goes to PayPal, and 21 cents goes to me. You've got to
know I'm not getting rich 21 cents at a time, don't you? So c'mon,
contribute 53 measly cents.
Where else can you get so much for your entertainment half dollar?
Mike Johnston writes and publishes an old-fashioned, entertaining quarterly
ink-on-paper newsletter called The 37th Frame (
www.37thframe.com). He has a
B.F.A. in Photography from the Corcoran College of Art and Design in
Washington, D.C., where he was a student of the late Steve Szabo and of Joe
Cameron.
He was East Coast Editor of Camera & Darkroom magazine from 1988 to 1994 and
Editor-in-Chief of PHOTO Techniques magazine from 1994-2000, where his
editorial column "The 37th Frame" was a popular feature and where he
presented, among other things, a set of three articles on "bokeh" by John
Kennerdell, Oren Grad, and Harold Merklinger that were subsequently widely
discussed among photographers.
His critical and technical writings have appeared in various publications
and newsletters such as The Washington Review and D-Max. A number of his
articles written under the pseudonym "L. T. Gray" (el Tigre) appeared in the
English magazine Darkroom User.
|
