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.