The Great "Protective Filters" Debate
Good lenses cost good money, and no one wants to see their expensive lens get damaged in a fall. I doubt I'll get much argument on this point. But disagreement does arise on how best to protect them.
It's good advice to take care of your lenses to avoid damaging them. That's something we can all probably agree on. Such advice is no different from what you'd receive for protecting anything valuable but potentially fragile. Lenses are built to take a reasonable amount of abuse, but there are limits. These things do contain a fair amount of glass, after all.
One common method advocated for protecting that glass is the use of a "protective" filter screwed over the front element. By covering the lens glass with a filter, it is believed that, should anything unfortunate happen, it would happen to the filter and not the lens being protected. We'll get to just how effective this truly is in a bit here, but to begin with, it's worth looking at the history of these things.
An entire industry has built up to provide photographic filters that can be screwed onto the front of lens. Back in the era of film photography, such filters were a necessity for multiple reasons. Color film came in two basic varieties: daylight balanced for use outdoors, and tungsten balanced for use when shooting indoors. Even though human vision automatically corrects for the color differences between the two common light sources, film could not. All I had to do was see the shockingly orange result that came from shooting indoors with daylight slide film to realize I needed to do something get better color balance. Of course, had I used tungsten film, I would have gotten much closer, but with the simple addition of a screw-on filter, I could get closer without changing film. Today, this whole thing seems quite silly given that white balance can easily be fine tuned with no filter needed. And no film. And tungsten lighting has given way to more efficient LED lighting that itself comes in a variety of color temperatures including "warm" to simulate the old-timey look of tungsten. Hurray for progress.
Colored filters were also common when shooting with black and white film as it allowed the user to alter tone mapping that dictated how various colors were rendered. A red filter would darken skies and make clouds stand out more. A yellow filter would add contrast to vegetation by more clearly delineating shades of green based on how whether they leaned more towards yellow or blue. You get the idea. Of course, today, this seems a bit antiquated too since we can mix color channels freely after the fact to create just the ideal conversion to black and white from any color source image.
Filters have been designed to solve other problems too. Of all of them, the polarizer has had the greatest staying power as technology continues to advance. There's just no good way to emulate the ability of a polarizer to minimize reflections and alter tonality based on the direction of light.
Just how filters intended as protection came about is a bit of a mystery. Photographers themselves started realizing that the skylight filter they used at higher elevations to cut down on the transmission of UV light could be left on all the time since they were basically clear. In part, this was simply a matter of convenience. But somewhere along the way, it became apparent that an argument could be made that they helped protect the lens underneath. But over the same period, filter manufacturers started marketing the sales of such filters for this purpose solely to sell more filters. There was a period when camera stores were pushing filters for protection with the sale of every lens much the way burger joints pushed French fries. "Would you like fries with that?" And if you said "no" you were somehow cheap or at least missing out on a good thing. At least some of those guys work on commission you know.
My point is, the need for protective filters had to have been the result of someone figuring out a new use for a filter they already owned. I don't think anyone ever sat down to solve the problem of protecting lenses and came up with a filter. A filter is basically a pane of plate glass cut into a circle. Sure, the expensive ones are higher quality glass and all that, but to remain useful, filters are of necessity thin sheets of glass. If you really wanted to protect a lens, you'd wrap it in bubble wrap or leave it at home, but such extreme options aren't viable when you need to be able to take pictures.
Let's try a thought experiment since none of us is likely to try this for real. Buy two identical lenses and screw a filter to just one. Then drop both from various heights onto various surfaces and so on and observe the results. Short of that, we're all just making educated guesses here as to the effectiveness of filters for protection. It's possible to conclude from anecdotal evidence that there may be something to this. Even if it's never happened to you personally, most of us have at least heard of cases where a lens fall resulted in a broken filter but with no visible damage to the lens itself. But lacking a control in such real-life experiments, it has to remain conjecture what would have happened in alternate configurations.
As another thought experiment, suppose you had a piece of plate glass, and a piece of thicker glass such as a lens element. Without actually trying it, I'm sure you'd agree that the thicker the glass is the harder it would be to break. I can snap a sheet of window glass with little effort, but a heavy glass paperweight or one of those newfangled clear glass "lens balls" the size of tennis balls would present a bigger challenge. Throw it against a brick wall and it would shatter, but a drop on a carpeted floor would result in little more than a dense "thud." Just because a thin glass filter breaks is not evidence that a thicker glass lens element would. Even though both are made of glass, the two are not created equal. One is inherently more breakable than the other.
It is true that a filter will prevent getting fingerprints on a lens front element, but this seems of little real benefit if those fingerprints now end up on the filter instead. You still have to shoot through fingerprints or clean them. And using a real filter would force you to choose between leaving your protective filter on and stacking filters, or temporarily swapping filters and risk getting fingerprints both on the lens front element and the backside of your protective filter. Both would become exposed in the process of swapping filters. Yet the only alternative risks vignetting, lens flare and image softness from shooting through so many added glass-to-air interfaces. There's just no practical way to use other filters if you rely on the continuous use a protective filter.
Now, there are times when the risks your lens may warrant taking extra precautions, even given all the above. For example, when I shoot at the beach, I use a filter, especially if the wind is blowing at all. Or were I to be shooting at thermal features in Yellowstone I'd want the extra protection of a filter despite the nuisance in can be. But under normal circumstances where even modest precautions will provide all the cover that should reasonably be required, I want to get the best images I can. I don't use a filter for protection unless I've assessed the risks and what I may need protection from. I can't see using a filter unless I've determined that doing so makes reasonable sense given particular circumstances. It's definitely the exception and not the rule.
Before anyone assumes that I'm advocating going naked with your lenses, rest assured. It's just that I prefer to rely on other means of prudence. Rather than putting my faith in a filter for protection, I tend to go with a lens cap when not shooting, and a lens hood when actually putting my gear to use.
Just checking if you're still paying attention out there.
But for those keeping tally on the great "protective filters" debate, you can put me down in the "no" column. There are better ways to protect lens front elements that avoid the demonstrable problems that come with their use.
But when it comes to your lenses, it doesn't ultimately matter what I think. Don't just do what I say; it's up to you to decide.
But definitely don't just do what the camera store salesman tells you to do.