Keep your lenses clean. Don't keep cleaning your lenses. And for God's sake don't stick a filter in front of them!!!!

See how blurry the photo got over on the left side?  That's because I had a UV filter on the front of my lens for protection and..........

Okay, not true, but..... I've experimented many times over the last few decades and I've proven to myself that filters in front of lenses degrade the quality of the final images.  Here's how I understand it all:  Every air to glass interface causes a slight loss of resolution and contrast.  This tends to make a lens look "flatter" and less sharp than it could be.  Lens designers have understood for over a century that adding more glass elements increases the compromise.  In the 1940's and 1950's they were willing to compromise things like corner sharpness and flatness of field so that they could design lenses with fewer corrective optics that had much more "snap" and "sparkle" than lenses of equivalent focal length designed with more elements.  

Everything in lens design and manufacture is a compromise.  If you add more elements you can correct for more distortions but you inevitably compromise contrast or resolution.  And contrast/resolution is an equation.  You can have one or the other or a mix but not high apparent acutance and high resolution in the same design.  Really.  Macro lenses need to have flatter fields and greater correction of geometric distortion.  They have more elements.  But in order to keep the image quality very high they have slower f-stops and smaller elements.  Smaller lens elements are easier to machine with high accuracy than larger elements.  They are easier to correctly assemble in barrels.  Faster lenses have bigger elements.  According to optics expert, Erwin Puts, every time you double the diameter of an optical glass element you increase the manufacturing complexity by something like a factor of 8X.

The creation of the "cemented pair;" two elements bonded together, is an attempt to reduce the number of air/glass interfaces to cut down on light loss and the tendency to increase "veiling flare" at each intersection.  Lens coatings are also an attempt to cut down on light lost at the interface of each element. They also prevent (by the process of wavelength interference) light from hitting the element and bouncing back to cause ghosting on the surface of the glass element thru which it just emerged.  Yikes.  A lot of design goes into making glass and coatings that nullify various wavelength "bouncebacks".  

Practically speaking, when you buy "L" glass or premium Nikon or Leica or Zeiss glass you are buying a system that's tweaked like a race car.  Really.  Like a Formula One race car.  It's optimized to produce stunning images as part of an overall optical system.

So you drop a few grand on your dream lens, put it on a tripod, lock up the mirror and trigger the shutter with an electronic cable release and.........you don't see the huge difference between the deluxe optic and the old beater you've had in the bag for years.  You know why?  To use the race car analogy you just put on aftermarket hood scoops, spoilers and fancy wheel covers on your race car when you stuck the damn filter on the front!!!  You introduced two air/glass interfaces that the lens designer didn't include in his calculations.  His computer didn't compute for them either.  You added weight and drag to your race car.  

And to make matters worse the coatings on the filter may interfere with the coatings on the lens and cancel out parts of the spectrum that you might really like to have on your imaging sensor.  They also introduce more chromatic aberrations because now the various color frequencies don't line up as well on the imaging plane.  

The idea behind the desire to use a filter is to protect the front element of the lens.  In days of old, when people would sit around on their davenports and immerse themselves in the latest novels of Nabokov and Kerouac while sipping cognac,  the coatings and the glass used on lenses was......soft.  Rigorous and frequent cleaning degraded the coatings and could scratch the front surface of the glass which led to flare and other nasty optical business.

But lenses have been hardcoated for years and years (five decades?).  And the infinitely expensive fast telephoto lenses from Nikon, Canon and Leica are designed with a neutral front element that is, essentially a built in protective filter.  The difference being that the systems were designed with that component as an integral part.  Not an after thought that's only benefit is to increase the commission of your camera sales person or to increase the margin on your internet purchase.

Finally, too many people who decide they must have the glass make the stupid decision to save money and buy cheap filters.  Back to our analogy, it's like putting retreads on a Ferrari.  You might be able to go but you won't be able to go fast.  

If you live in constant fear that your lens will become damaged you have obviously spent too much money on your lens and should return it and buy something that won't cause you unbearable emotional distress should it become damaged.  Really.  Like buying a nice car and always having to park it across three spaces because you don't want it to get door dinged.  It's karmic.  It's the quickest way to get your car "keyed."  And your fear for your lens attracts calamity to your lens like a magnet.

Stop.  Take the filters off the lenses.  Shoot like a real man.  Or a real woman.  And if your lens is destroyed then make sure you have a good story to go along with the loss.  That's the way it's done.

I've been doing this for 25 years and I've never had a front element damaged.  The protective filter is an urban legend.  It's also a huge profit add-on for the camera sellers.  My advice?  The only filter we really need in the digital days is a circular polarizer.  And that should stay in your pocket til you need it for something aesthetic.  

added after:  Here's what Lloyd Chambers, noted writer about optics, has to say about the filter imbroglio:  http://diglloyd.com/articles/Filters/quality.html

A second chance at writing a competent review of the Zeiss 21mm lens.

Several weeks ago I borrowed a 21mm Zeiss ZE lens from the local Zeiss representative.  I'd been asked by a client to shoot interiors and equipment installations at a very private and very exclusive country club somewhere in central Texas.  I own a Canon lens but I'm all too familiar with its limitations.  I wanted to make really great images for this particular client and I thought the 21 Zeiss would do it for me.  Most of the images from that shoot were really good.  In fact, the art director said she could see a difference in quality and sharpness when she looked at the small images on the back screen of the camera.  I felt that the optic was very, very sharp but it a few of the images with bright light sources (sunlit windows especially) I saw too much flare.  Far more flare than I would have gotten with the Canon lens.  I wrote a brief paragraph or so on the blog and stumbled off to work on something else.  One afternoon my friend, Paul, who is a renowned architectural photographer, called to see what I thought about the lens.  I gave him my impressions and he said, "That doesn't sound right....do you mind if I take a look at it?"  I met him for coffee and handed the lens over.

Now, if most people questioned my judgement I'd take it with a grain of salt but Paul owns and uses the newest Canon 17t/s the 24t/s a bag full of Zeiss Biogons and other wide angles and just about every exceptional optic that's ever been pressed into the service of photographing the interior and exteriors of buildings.  Is he knowledgable?  Absolutely.  Lately Paul's upped his game by buying a complete Hasselblad MF digital system.  Five or six of their best lenses and a really nifty shift adapter.

In using the lens and in comparing it with the 18mm Zeiss Paul was able to duplicate my flare results.  We met at my studio to shoot some additional comparisons.  As we were putting the lens on the front of a Canon 5dmk2 Paul put on his reading glasses and looked carefully at the front of the lens.  There were two small spots on the front element.  Could have been water marks.  Or dried spit.  Or some outer space goo.  But we're talking maybe one or two millimeters in diameter, tops.  And quite transparent.  Paul wiped out a cleaning cloth and ministered to the front element.  Minutes later we were shooting amazing tests with absolutely none of the flare I'd seen previously.  As Paul explained (and I should have known) flaws on the lens surface are magnified with wide angle lenses.  It's imperative to keep the front element cleaned.  Does that mean hanging a filter on the front?  Absolutely not......but that's fodder for another post.

My next tests were night and day.  The lens performed the way Paul and I expected it would.  By f.5.6 it was sharp and detailed and bright in a way that zoom lenses and my 20mm Canon wide angle can never match.  The distortion is well controlled and there is a profile of this lens and most of the Canon pro cameras in the Camera Raw panel in both PS CS5 and in Lightroom.  That extra nudge from the profile makes the geometry pretty much perfect.  

The lens is both heavy and slow to focus.  Slow because you'll be doing it manually and it's harder to see the in and out of focus as easily with a wider angle optic.  The discrimination between zones of sharpness is less obvious.  But the way I used this lens was to put it on a tripod and use the magnifier in Live View with the Canon body.  Many decry the addition of live view as a gimmick or a "feature" that adds extra complexity and cost to a camera but Paul and I find that, for architecture and other carefully considered uses, the Live View transforms most cameras into miniature view cameras with most of the control that implies.

If I were to use the lens for traditional (I'll take the picture without telling anyone and I'll try not to get caught) street photography I would rely on zone focusing it and depending on the wide depth of field that results.  It's not a lens you whip up to your eye and whip into focus quickly.

The summation?  It's at least two levels of magnitude better than the Canon 20mm.  It's better at 21mm than any zoom I have used.  That includes the Nikon 14-24 and the Canon 16-35L.  If I decide to add more architectural photography to my menu of services this lens would be my "go to" lens.  Will I buy one?  I think so.  Should you care if I buy one?   Probably not.  As a matter of fact the previous review points out the foible of taking people's web opinions too literally.  My first impression was a lens with flare issues.  I was ready to give the lens back.  Obviously my test procedure was flawed.  I should have checked the front element carefully when I saw the first evidence of flare.  That's a mistake I probably won't make again.

It's an incredible lens.  Made in a fashion that is quickly becoming rarer than cheap, full frame camera bodies.  It's a sharp tool.  But a very specialized one.  If you need  that focal length you need this lens.  If you love shooting with longer lenses and you don't have clients to please then it's a lot of $$$$.

That's it. Off to lunch.  Today is Chinese food at Lotus Hunan on Bee Caves Rd.  If you live in Austin please try this place.  It's wonderful and the family that runs the place is sweet as can be.  Lots of interesting specials.