Can a filter make an image sharper?

Question

A recent social media campaign by Urth, promoting their new magnetic filters, claims to increase sharpness (emphasis my own)

Our professional Ultraviolet Filter Plus+ cuts UV light to make your images clearer and sharper and protects your lens from scratches, oil, dirt and water.

I was always told UV filters can reduce sharpness, and are really only any good as a temporary form of protection.

I asked Google Bard the question "In photography, can a UV filter increase the sharpness of an image?". It replied with

No, a UV filter cannot increase the sharpness of an image. In fact, it can actually decrease sharpness slightly. This is because a UV filter adds an extra layer of glass in front of the lens, which can cause diffraction and reduce contrast.

UV filters are designed to protect lenses from scratches, dust, and other elements. They can also be used to reduce the amount of UV light that reaches the sensor, which can be helpful in high-altitude or other environments where there is a lot of UV radiation.

However, if you are concerned about image sharpness, it is best to avoid using a UV filter. If you need to protect your lens from the elements, you can use a lens hood or a camera bag with a padded compartment for your lens.

Is there evidence to prove that a filter can increase sharpness? Or is this disingenuous marketing/creative writing?

Answer 1

If you are shooting with color negative film during daylight hours outdoors over long distances a UV filter can reduce the effect of haze and make the image appear clearer by increasing contrast. So there is some truth to the claim with the unspoken qualification which eliminates pretty much anyone who isn't shooting long distances outdoors during daylight and using color negative film.

On the other hand, since they block UV light, UV filters prevent sunlight from inhibiting the growth of any fungal spores that might be in your lens when daylight is passing through it.

Practically all digital cameras used to take artistic, documentary, and historical photographs already have a UV filter in the filter stack directly in front of the image sensor. So in the case of digital cameras, a UV filter on the front of the lens will not provide the same benefit that such a filter would provide when using a film camera with color negative film.

It's mainly just marketing gobbledygoop.

Flat "protection" filters, which is what the sellers of UV filters try to use to sell them as these days since very few folks use film that needs a UV filter, don't provide nearly as much protection as they claim. Thin flat filters made of the materials used to make most UV filters are easier to scratch and shatter than the front elements of most lenses. Front elements tend to be thicker and made out of denser, more robust optical glass than that used in flat filters.

It takes a lot of abuse to scratch the front elements of many lenses, even cheap ones like the EF 50mm f/1.8 II in this video that is hit with a hammer without a visible scratch. The "tester" finally resorts to using the sharp claw end of the hammer to finally make a noticeable mark on the front of the (already unusable) lens!

For all but the widest angle lenses, lens hoods are much better at protecting the front elements of lenses than any flat filter can be. They also protect from stray off axis light without reducing optical quality at all. Lens hoods don't cause ghosting when shooting in dark environments with bright points of light inside the frame, either.