Lens Protection Filters: A Cautionary Tale

I am sure opinions vary about the merits of using protection filters (UV/haze filters) on photography lenses. Back in the film era UV/haze filters were used to cut out a spectrum of light that was invisible to the human eye, but would register on the film and decrease contrast or sharpness slightly. Digital sensors have UV filters pre-installed, so with digital cameras these clear add-on lens filters, now more commonly called protection filters, are simply a piece of glass that keeps water, oil, dust, and scratches from marring the front element of the lens.

Lens designers go to great lengths to design an optical system that is as sharp and as free as possible from optical aberrations (given the target retail price of the lens… and physics). So what happens when you stick a $50 (or $250) piece of glass in front of that $1500 lens? Well, at the very least, it won’t improve the quality of the image. In the worst case, that (not-so-cheap) protection filter might actually make your images look worse than if you just let your lens glass get covered in dust and fingerprints. Don’t believe me? Check out the following example.

I acquired my Canon EF 280-200 f/2.8L sometime before the advent of the internet (1993-ish), which makes me feel old. This lens was released in 1989 and a pretty phenomenal piece of glass at the time. It was replaced in 1995 by the EF 70-200 f/2.8L USM which is still produced and sold today.

As I high school student, I probably spent every last penny acquiring this professional-quality fast zoom lens, so when it came time to put on a protection filter I didn’t get the most expensive option. I bought a Heliopan filter that probably doesn’t have any anti-reflective coatings. It says “Made in Germany” though, so it must be good, right?

I spent the 1990s shooting sports and outdoor adventures with the 80-200mm lens so I was always glad to have the protection filter in place. And, honestly, any issues with the protection filter may not have been apparent on film with its much lower effective resolution when compared to my current 24 megapixel Canon EOS M5. When I started shooting digital, in the early 2000s, the 80-200mm was starting to feel old, so it mostly just sat in a spare camera bag. I “replaced” it with a the Canon EF 70-300mm f/4-5.6 IS USM, which is lighter, more compact, has more reach, and is image stabilized (I always missed the 80-200mm’s fast aperture and solid build quality though). Later, I replaced/supplemented all my EF lenses with EF-M lenses.

In The Field

I recently purchases a wide-angle L-series zoom and became nostalgic about the 80-200mm red stripe lens (the “Magic Drainpipe” as it is affectionately called, and which still sells used for $500 to $700). I took the telephoto zoom out with me shooting in the foothills during “golden hour” one evening and I was shocked. I thought there was something wrong with me. I was using a shutter speed of 1/400 second, and the images looked motion-blurred when reviewing them in my M5 electronic viewfinder. Okay, maybe 1/(2xfocal-length) is not a fast enough shutter speed on a crop-sensor body when handholding a heavy lens. I increased the shutter speed to 1/800 second and then 1/1000 second, cranking up the ISO along the way (gotta love modern sensors). Nothing improved. I put the camera on a monopod. The images looked the same. I thought maybe I was seeing things and decided to check the images in more detail back home on the computer at 1:1.

Back at the computer, sure enough, all the images were affected with what looked like camera-shake induced motion blur. I started to suspect that the 25 year-old filter I always had mounted on the lens might be the culprit. I wanted to eliminate it as a variable anyway, so I shot some test images with and without the filter in place. Bingo! The unfiltered images are tack sharp. (All samples shown are 1:1 pixel crops, but may display at reduced sized depending on your browser.)

Canon EF 80-200mm f/2.8L w/ Heliopan S 72
Canon EF 80-200mm f/2.8L @ 200mm f/2.8 1/640sec ISO1000, Heliopan S 72
Canon EF 80-200mm f/2.8L w/o Filter
Canon EF 80-200mm f/2.8L @ 200mm f/8 1/500sec ISO1000, No Filter

The filter is introducing a strong amount of astigmatism. The filtered images are blurry in one direction, but relatively sharp in the perpendicular direction. Odd, don’t you think, for a supposedly flat piece of glass?

The 80-200mm is an old lens, but I still would like to keep a filter on to protect the front element. I’m having fun shooting with it and don’t want to hesitate to take it out with me. So, I bought a new filter: a Hoya Fusion Antistatic Super Multi Coating (SMC) protector. This is the series of filters I have been using lately, for both protectors and circular polarizers. They are decently priced and well rated.

Canon EF 80-200mm f/2.8L w/ Heliopan S 72
Canon EF 80-200mm f/2.8L @ 200mm f/5.6 1/400sec ISO2500, Heliopan S 72
Canon EF 80-200mm f/2.8L w/o Filter
Canon EF 80-200mm f/2.8L @ 200mm f/5.6 1/640sec ISO4000, No Filter

In The Lab

I shot a controlled comparison through the EF 80-200mm f/2.8L without a filter, with the problematic old Heliopan UV, and with the Hoya Fusion Antistatic SMC protector. I first shot with the bare lens at every full aperture stop to find the sharpest native aperture for this lens. The aperture of f/5.6 is the winner. Then I shot at f/5.6 with each of the filters. The image from the lens without any filters is obviously the sharpest. The Hoya image is almost as sharp, but with a little bit more chromatic aberration showing. The Heliopan images is, well, just gross.

(The target is something I drew in Illustrator, printed on a not-high-quality laser printer, taped outside my bedroom window, and photographed on a cold and cloudy day. Not laboratory quality, but it does the trick.)

Canon EF 80-200mm f/2.8L w/o filter
Canon EF 80-200mm f/2.8L @ 200mm f/5.6, no filter
Canon EF 80-200mm f/2.8L w/ Hoya Fusion SMC 72mm
Canon EF 80-200mm f/2.8L @ 200mm f/5.6, Hoya Fusion SMC 72mm
Canon EF 80-200mm f/2.8L w/ Heliopan S 72
Canon EF 80-200mm f/2.8L @ 200mm f/5.6, Heliopan S 72mm

To UV or Not To UV

So, should you use a protection filter on your lens? It’s not an easy question to answer. For some lenses, the risk of damage, cost of repair, irreplaceability, or prospect of incessant cleaning, are good reasons to use a filter. In other cases, you might not want to degrade the image quality.

Buying filters is a bit of an art. A $250 filter may or may not be better than a $50 filter, but buying from the bargain bin is a sure route to disappointment as is buying from a questionable eBay seller. As a rule, I steer clear of really expense filters. I’ve heard too many stories of people getting absolutely fleeced with super expensive filters that just don’t deliver what they promise. Additionally, every other month a manufacturer comes out with a new filter series featuring some whizzbang feature. Is this feature beneficial to your photography? And is it worth the price?

Here is my policy in regards to protection filters…

  • For a lens under $600 dollars, I don’t bother. This usually means a lens with a fairly small front piece of glass that is not as exposed to the elements or potential damage. In the unlikely event that the lens gets wrecked I will just replace it or take the opportunity to upgrade. Most EF-M lenses fit into this category: they are great lenses, but easily replaced.
  • For a lens between $600 and $800, I make a judgment call. If the lens is not likely to be used in a destructive setting and/or I want to maximize the image quality (a portrait lens, for example) I will forego a filter.
  • For a lens costing over $800, I add a protection filter. I try to read technical reviews before choosing a filter, and will always buy a multi-coated filter. Other features are nice to have if they don’t add too much cost. The Hoya Antistatic filters have been great, and probably attract less dust. Maybe. I also try to maximize my quality per dollar. I’m not making money from my photography at the moment, but I also believe there is no point in owning a state-of-the-art $1700 professional camera lens only to hamper it with a $15 subpar pane of glass.
  • If I am shooting in a controlled environment I might take a filter off for a short period to maximize image quality.

That’s about it. I have a few other policies that, in practice, supplement or obviate the use of a protection filter.

  • I use a lens hood on every lens, all the time. A hood not only keeps stray light from refracting off the lens surface and causing flare, but is a great first defence for keeping rain, snow, kid fingers, photographer fingers, dust, rocks, branches, door knobs, the ground, and anything else from touching the front lens element.
  • In storage I keep each lens in a camera bag or in a lens pouch and plastic bin.
  • I keep the lens cap on unless I am actively shooting. If the the lens hood is nice and deep, I will will occasionally walk around with the lens cap off (for example, if I am walking for less than two minutes between outdoor shooting locations). But if the lens is really wide and the petal hood not very good protection I will put the lens cap on every time I lower the camera (especially if no protection filter is in place). Setting the camera on the kitchen table, I put the lens cap on. Laying the camera on the seat of the car, I put the lens cap on. Putting the camera back into the camera bag, I put the lens cap on. Get the idea?

In 30 years taking photos I have never damaged a front lens element, knock on wood. I’ve dropped a lens into the ocean (it died), driven over one with a van (the lens and the van survived), and sheared the mount off several plastic lenses (a bit of Crazy Glue does the trick for a little while). I hate cleaning lenses so I try very, very, very hard to keep them free of dust, water spots, and finger prints. I would rather clean a protection filter which is flat and relatively disposable than an irreplaceable large curved lens element.

The arguments for or against protection filters can reach a religious fervour. I choose to be pragmatic. Roger Cicala at LensRentals tries to be scientific about these things and recently posted another example of a bad protection filter messing up an otherwise good lens. Its worth a read.

What’s Next?

Since I had a target set up and the tripod out, I got out every lens I use regularly (10 lenses, if you must know), mounted each on the camera, and shot a series of images at every full aperture (and at both the wide and long end of each zoom lens). It is good to know the sharpest aperture for each lens. In many cases the results are as I expected, but there are also some amazing surprises. I’ll try to post those results when I have time to generate the 120 1:1 crop thumbnails.