Parameters of Lens Specification and the Properties of Lenses
by Mike Johnston
The Sunday Morning Photographer a weekly photo.net column:
Feb 22nd , 2004
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
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
• 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
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.
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