I use both lens hoods and protective filters. Protective filters are less needed for my longer lenses. But I still do not want finger smudges on my long lenses when the hood is in storage position or off altogether. More importantly, I have many shorter lenses, normal and wide angle that use very shallow lens hoods, these hoods to not protect the lens from dirty fingers, sticks, rocks, etc. I try to avoid ever having to clean the elements of my lenses beyond dusting off with a brush. I try to avoid using solvents and lens cloths. So I keep high quality B&W or similar protective filters on all my lenses. Shallow hoods protect only for their intended use, possibly stray light and flare.

It's a good thing that my study showed that a plain piece of window glass did not show any significant increase in the edge widths. Maybe you can consider that as a lens protector.

I should point out that window glass has undergone several improvements over the last couple centuries. Reealy old glass looks like the bottom of a bottle since it was probably blown, then cut. Glass from the 1800s to maybe mid 1900s show variations in thickness that can be seen to affect images. My house was built in 1960 and most of the windows show those variations. Moving your head back and forth will result in clear variations of the scene outside (or inside). New processes were developed that made glass quite flat and those effects do not show now. https://en.wikipedia.org/wiki/Float_glass

The piece of window glass I used in the study was purchased at a local hardware store from their scrap barrel, so it was probably float glass. My house has storm windows that do not show the variations I see in the smaller windows, so either they came from a different source or were added later than 1960.

I am not a dedicated protective filter user. My lenses do not have filters on them for protection most of the time. I depend on a lens hood for protection from ordinary hazards such as having the camera bump into things. However there are situations where I use protective filters. Usually those situations involve environmental hazards such as salt spray, blowing sand, flying mud, or small hazardous things flying around (e.g. welding).

There are many instances in which someone claims that his lens was saved by something hitting the front of his lens and breaking the filter but not the lens. I believe that something hit their filter and broke it but I do not believe that their lens would have necessarily been broken in the same situation without the filter. After all, the front element of a lens is probably several millimeters thick while a filter is generally going to be a millimeter or less in thickness. This opinion is based on Steve Perry's post in which he performs extreme tests of things hitting lenses with and without filters. (I emphasize here that this is my opinion. YMMV).

On the other side of the coin there are a lot of people who claim that the excessive use of filters can affect image quality. While it is true that adding surfaces to an optical path increases the chance of reflections causing flare and such things, well made filters will have antireflective coatings which will reduce (not eliminate) this possibility.

So, being an experimentalist, I decided to do the experiment.

I did a study of edges to look at image sharpness. It was a limited study, using only one lens, but results to date show no significant effect of filters on the sharpness of an image. I also used a piece of regular window glass as one of the test filters. No significant effect was seen there. I concluded from this study that using a filter will not significantly affect the sharpness of objects in the image. The study did NOT include the possibility of flare or other effects. A PDF of the study is attached below.

I am not a dedicated protective filter user. My lenses do not have filters on them for protection most of the time. I depend on a lens hood for protection from ordinary hazards such as having the camera bump into things. However there are situations where I use protective filters. Usually those situations involve environmental hazards such as salt spray, blowing sand, flying mud, or small hazardous things flying around (e.g. welding).

There are many instances in which someone claims that his lens was saved by something hitting the front of his lens and breaking the filter but not the lens. I believe that something hit their filter and broke it but I do not believe that their lens would have necessarily been broken in the same situation without the filter. After all, the front element of a lens is probably several millimeters thick while a filter is generally going to be a millimeter or less in thickness. This opinion is based on Steve Perry's post in which he performs extreme tests of things hitting lenses with and without filters. (I emphasize here that this is my opinion. YMMV).

On the other side of the coin there are a lot of people who claim that the excessive use of filters can affect image quality. While it is true that adding surfaces to an optical path increases the chance of reflections causing flare and such things, well made filters will have antireflective coatings which will reduce (not eliminate) this possibility.

So, being an experimentalist, I decided to do the experiment.

I did a study of edges to look at image sharpness. It was a limited study, using only one lens, but results to date show no significant effect of filters on the sharpness of an image. I also used a piece of regular window glass as one of the test filters. No significant effect was seen there. I concluded from this study that using a filter will not significantly affect the sharpness of objects in the image. The study did NOT include the possibility of flare or other effects. A PDF of the study is attached below.