A photographer and educator whom I respect and I got into a conversation on the web about the viability of filters on lens for purposes other than creativity. His contention is that a filter for protection is unnecessary and will degrade image quality. I found that I agree. Mostly. Let me explain why.
For many decades, photographers have had the option to modify the scene through the use of filters on their lenses. The most popular creative filters are the polarizer and the neutral density, with graduated filters and tints following closely. In the days of film, there were also Skylights (technically a 1A) with a subtle pink cast to improve the pop of blues, and UV (ultraviolet) filters to cut UV rays from hitting the film. Many photographers, myself included would use other filters for other purposes. An 81A really made skin look lovely, especially if it tended to pale and would also prevent black skin from bluing out too much. 82A filters could cool off a scene. An 85A corrected tungsten film to daylight. An 80B converted daylight film white balance for tungsten. We all carried a pouch of filters. Black and white photographers added a pack of coloured filters to change how certain colours rendered as grey.
Somewhere along the way, it was determined that some filters also provided protection for the front element of the lens against dirt and scratches, as well as environmental hazards. When I was shooting commercial work in factories, filters saved my lenses more than once. When the transition to digital happened, the need for Skylights and 81As tended to vanish with the advent of automatic white balance but I chose to maintain the presence of a UV filter on all my lenses to cut UV rays and to provide some minor level of front element protection.
In the olden days, camera manufacturers produced filters. Nikon's L37 was a highly respected UV filter and still exists. German companies produced filters made from the same glass as went into Zeiss and Leitz lenses. Two of them are still around, Heliopan and B+W. You may also still find real Schneider filters if you look carefully. Back in the day both Fujifilm and Minolta had their own internal glass factories making glass for their own lenses and selling blanks to other manufacturers for grinding and coating in their own lenses. There was a great American brand called Tiffen that used the unique process of sandwiching optical gels between optical glass. I used Tiffen correction and B&W filters regularly. Tiffen is still a substantial provider to the cinema industry but their low end camera filters are not the same as they once were and current UV and Protector filters deliver the same degradation as most of the other filter makers.
In today's marketplace, we hear a steady press from sellers to buyers that a protective filter is a good idea. Conceptually, I agree with this, although my peer does not. His fear is that the quality of these protective filters degrades the image quality more than is reasonable. In many cases he is absolutely right. I only use Heliopan or B+W filters and in over 40 years of photography both with and without filters have never found evidence of image degradation through the use of filters made with true optical glass, that are ground properly and coated to reduce the effects of refraction when light passes from air to glass to air, as well as internal reflections. I have however seen very expensive filters from third party lens makers and companies who seem to only make filters have negative impact on image quality. I own a certain manufacturer's 120mm-300mm / 2.8 zoom that is decent but not wonderful. I bought the same manufacturer's filter for that lens because of its odd size. The filter degrades image quality to the point that I now shoot that lens naked.
Sellers are motivated by margin, it's how they stay in business. There is more margin in filters churned off a line in China or southeast Asia than in a German filter. A lot more. There are now many offerings of "Protection" filters, basically a piece of glass of dubious quality in a poorly constructed metal ring sold to protect the front element of lenses. These are, to the majority, abhorrent in quality and serve only to reduce the image quality of the picture. Moreover, if one breaks, removing it from the front of the lens is a process fraught with peril. Some say that because the glass in front of the sensor is UV resistant, a UV filter is unnecessary. First, not all sensors have a UV impeding glass cover and secondly, even if there is such UV blocking, no harm is done by blocking UV rays prior to their entering the lens at all.
This article was spawned because a friend of mine is a professional seller who strives to do a great job for his clients. He read the initial conversation and was very concerned that he had been misled by things he had heard and been told, and that he was doing a disservice to his clients by selling them certain filters. He is a good man, and I believe that most professional sellers who see sales as a relationship business would have the same questions.
A cheap lens with a cheap filter is bad enough, a great lens with a cheap filter is a waste. So if you won't put the cash into top end filters, go without. When shooting into the sun, remove any filter. Reduce the number of air-glass-air transitions if you can in highly directional lighting situations.
So what should you do? Make your own decision. Shoot naked. Or shoot with a top line UV filter. Don't waste your money on cheap junk that will only hurt image quality. Avoid "house brand" filters of any type, but especially those called Variable Neutral Density filters. I see these hunks of dung sold regularly and wince knowing that yet another photographer is being introduced to a sickly green tint and a bucket of moire. When you buy a polarizer, buy a top end polarizer. Just as there are polarized sunglasses that sell for $3 and those that sell for $300, not all polarizers are created equal. A filter should offer enhancement, not pain.
For each new lens that I acquire that can accept a front element filter, I will buy a UV from Heliopan or B+W and put it on. If one is not available, I will go without, and worry a bit each time I use the lens but it won't stop me from making photographs. If I put some cheap piece of crap on the lens, why did I spend good money on a top line lens in the first place?
Today's lens coatings from the top manufacturers are better and harder than ever. I still choose to protect them with an optically correct UV when I can. You should do what makes sense for you, but don't compromise on filter quality if you choose to use one.