It always amazes me how people automatically associate plastic lenses with something inferior ("prosumer" - Davidde, are you listening?!).  I'll admit to owning a bunch of big, heavy prime lenses (for SLR and rangefinder), and at least one monster 28-70/2.8 zoom.  Sure, all that metal is reassuring, but I'd bet that the construction has something to do with the customer perception you need to sell a $1,800 lens.  Do you even know for sure that the inside of a 28m f/1.4D AF Nikkor is even metal? (Ok, it is so heavy, the lens elements must be made of metal too...). 

And now the mythology of the plastic fantastic...

Myth: plastic lenses are not durable.  In many ways, plastic is better.  It usually breaks before it bends.  While this could mean a total loss when you drop a lens, it also means that you won't be shooting all day with a lens that is hopelessly out of alignment.  If you think that plastic breaks more often, you would presumably find a lot of reports of it in internet searches.  But such reports are oddly absent.

Myth: it is easy to cross-thread plastic lens threads.  Someone on the internet constantly complains that plastic lens threads lead to cross-threading.  The only thing you need to know about mounting filters is that you turn the filter counterclockwise until it clicks, and then you spin it onto the lens clockwise.  I learned this working on machines much more fragile than plastic AF lenses.  The major benefit to plastic filter threads is that they are somewhat elastic and don't dent when you accidentally swing the camera into something.  That's why you never see used plastic lenses with dented filter threads.  An additional advantage is that plastic is not aluminum, so your aluminum filters don't cold-weld to your lens barrel.

Myth: plastic lens mounts are inferior.  While stainless looks really cool, the reality is that you really only need a metal lens mount in two circumstances: (1) if you have a heavy lens (in which case you also need a metal internal lens barrel - and a metal-framed camera) or (2) if you switch lenses on every shot.  Most of the lenses you see with plastic mounts are extremely lightweight and don't need the metal mounts.  Putting metal mounts on these light lenses has no benefits.

Myth: plastic lenses have more slop.  This is not a plastic issue; it's an AF issue.  AF lenses need to have as little mechanical resistance as possible.  This means they have none of the heavy lubricants that usually slow down movement in lenses.  Note also that older metal lenses have slop, too... you just can't tell because the focusing mechanism is filled with grease (Soviet lenses have a lot more play and tons of grease taking up the slack).  Even the best, most expensive internal-focus lenses have slop in them.  Pick one up and rock it end over end sometime.  Slop is something of a non-issue because an AF motor reaches the focusing point turning in one direction, i.e., tensioning the helicoid.

And now the benefits of lightweight plastic lenses...

Benefit: low cost, high performance for the money.  Some of the best lenses made have plastic parts.  The Fuji GSW lenses have plastic external barrels.  The 20, 35, and 50mm Nikon AF lenses are extremely high performance in small packages at low costs.  Canon's AF zooms are legendary.  Plastic saves weight, but it also saves die-cast tooling costs, which can be substantial.

Benefit: fast autofocusing.  The name of the game in AF is to get the moving mass as small as possible so that the same motive effort focuses the lens faster.  With screw-drive AF bodies like the Nikon, you can have light, unit-focusing prime lenses that focus very, very quickly so long as the moving parts are lightweight plastic.  The alternative is having huge heavy metal lenses with internal focus (used with technologies like Nikon AF-S and Canon USM).   The problem is that unless the optical design is very carefully controlled (as in very high end lenses), internal focus can introduce color fringing.

Benefit: less fatigue:  If your camera body is something like a Nikon F4 or F5, using a lighter lens makes long days feel a little better.  Even with an FG, lighter is better.  Every pound you carry feels like ten pounds at the end of a long day.

And now the pratfalls of photographic hubris...

It's just not good enough (for me).  You're so good.  That's why you shoot for Magnum.  That's why you're published.  That's why you were able to quit your day job.  Right.  One of the most shocking things is how much some people spend on equipment.  There are usually two justifications for this behavior.

1.  The photographer thinks it will take better pictures.

2.  The photographer thinks he already takes better pictures.

The first is the Johnston & Murphy effect.  It's the interviewee with his new shoes.  The second is pure hubris, as the ancient Greeks called excessive self-love - to the point of not being able to see one's own flaws. You can visualize this more concretely as a 250 pound man riding a 20 pound racing bicycle.

Let's take the Johnston & Murphy effect first.  This is phenomenon by which you buy a new pair of Johnston & Murphy shoes for an interview.  The idea is that you are getting a really solid pair of shoes, indistinguishable from the best in looks, and durable as hell.  That pair of Johnston & Murphy shoes gives you poise and confidence.  And it makes you forget that a few minutes ago you were wearing worn-out Hush Puppies with frayed laces.  In like fashion, some photographers need expensive equipment to make up for their own self-perceived lack of skill, to be inspired or at least unrestrained.  Usually this type of photographer is much more talented than he thinks he is, but his ability to actualize that talent is dependent on his perception of the equipment.  He is relunctant to buy plastic because it may not be professional enough.  A Maxxum lens just doesn't inspire confidence like a Summilux-M.

Then think about the hubris-infused photographer.  He is the 250 pound bicyclist.  As we all know, it is easy to take the weight out of a bicycle.  Six hundred bucks gets anyone an aluminum road bike. The problem is taking the weight off oneself.  Mr. Hubris knows deep down that it's not really the bike (or the camera) that's holding him back.  But he wants to think he is so good that he knows the difference.  I really like the APO-Samanthar because it is superior to the Theftar ED in the thingamabob test.  In reality, he is a bad photographer.   He takes uninspiring landscapes, baby pictures.  or any other number of unchallenging subjects.  He outputs his images badly.  He is taking his film to the minilab.  He scans negatives on a flatbed.  Or in a glassless carrrier.  Or prints them on a Beseler with the head out of alignment.  He is leaving the digital camera on the default settings.  Ultimately, he doesn't know the difference between a good lens and a bad one except that he read something or played with it or God forbid, shot a couple of frames.

Everyone loses track of the fact that any name-brand camera made after 1970 (note by this I mean West German or Japanese  - there are plenty of poorly-performing East German and Soviet products out there) is going to take great pictures unless it is grossly malfunctioning (wish I could say the same for the people behind the lens...).  On film (or even in digital), it is really surprising how little difference there is between an old lens and a new one.  Camera optics hit the top of their stride a long, long time ago.

There is also suprisingly little difference between a top-end lens and a cheap lens for any enlargement up to 8x10 (which is larger than most people ever do with most of their pictures).  For film, the enlargement factor is still relatively small (and the bar low for making a sharp-looking print).  Digital cameras almost invariably apply sharpening to make up for their anti-alias filters, and that sharpening helps level the playing field.   The exception is in the higher-end cameras that have high resolution and no anti-aliasing sensor -- you will see in no uncertain terms which lenses just don't cut it.  But I guess when you bought that $4,500 Kodak SLR/n body, you didn't expect to get away with using a $100 zoom lens, did you?

The really troubling thing, though, is just how little difference there is between an expensive lens, a cheap lens, and a disposable camera when you are out in bright sunlight and with apertures like f/8.  Luckily for the egos of those of use who use optics more expensive than the ones on Fun Savers, the world is not always bright and sunny!

...and the not-so-stunning conclusion.

So what is the answer, new or old, cheap or expensive?  After studying these issues in some detail, I have concluded that in general, newer equipment performs better.  Cheap equipment performs adequately in most situations.  Of course, if you think you're the next Robert Capa, get that wallet out.