Why filter decrease the photo quality?

I don't really add filters on my photos most of the time. Unless its for Facebook just to show I'm real.

Technically, filters blocks some and allow others. With photo filters, the one you attach in front of the lens, some parts of the light is blocked and other parts pass through. This means not all of the lights gets to reach the sensor so again, technically, the decrease in light causes a decrease in photo quality.

Digital filters on the other hand, as with other seemingly normal terms nowadays, have taken a whole new meaning. Anything used automatically to purposely alter an image is now termed a filter. There is no need to remove something as in most cases, digital filters even add. As to why the decrease in quality happens is because the original photo has to follow certain qualities the originator of the filter deems necessary to achieve the effect. Since most filters are done online, the photo is reduced in quality for faster speeds in internet transmission to an offline site for processing.

I assume you mean optical filters, not the cheesy digital filters.

Light transmission is reduced. Using coated filters reduces the transmission loss.

Reflections. Filters will cause reflections which are obvious with strong point sources of light. Again, using coated filters helps a lot.

Vignetting. With wide angle lenses the filter will cause vignetting. Using thin glass will help to minimize this.

I use decent quality UV filters on most of my lenses except the lens I mostly use for studio work. I use the filters for protection since I end up in situations where the lens is subject to being scratched and the dust factor is high. The filters I use have coatings that minimize dust sticking to the glass making it easier to clean. I have done comparisons and can't detect any noticeable difference in image quality with and without a UV filter except on my ultra-wide. Using junky filters will cause noticeable issues.

I also use a polarizing filter when needed and take the hit in light transmission which is usually not a problem.

We are talking about optical filters that are used to modify the incoming light rays. Optical filters are a mainstay used to enhance or otherwise adjust the resulting picture. Now all such filters have advantages and disadvantages. We mount them when the good outweighs the bad.

Filters that we mount to intercept image forming rays are expected to be optically flat --otherwise they contribute to distortion. In many ways, making optical flat is somewhat difficult. We expect name brand filters to meet this criteria.

Optical filters have a minimum of two polished surfaces. These, by their very nature, reflect away some of the light. Reflected light from the front surface is lost. Reflected light from the rear surface is light that is reflected into it by the camera lens itself. The unavoidable light loss averages about 5% if the filter is uncoated. We coat lenses and filters as a countermeasure against unwanted reflection. A coated lens element / filter reflects about 2% or less. Note: the purpose and goal of lens coating is reduction of surface reflections. These reflections are devastating, as they cause loss of light; plus the reflected light from all inside lens elements will be misdirected rays that comingle with the image forming rays. A high percentage of these misdirected rays bathe the image during exposure and contribute flare, and this action reduces image contrast.

Thus --- Never add a filter unless the good it fashions outweighs the bad. Keep in mind, most times, even a poorly made filter might not induce enough image degradation to be noticeable. In other words, an image displayed on a computer screen might be quire acceptable, whereas a large paper print from this same image might show unacceptable degrading.

Let me add, the UV filter is a blocker of Ultraviolet light, while it passes all other frequencies. Its initial utility is based on the sensitivity of chemical-based photo films, which are most sensitive to UV, violet, and blue light. UV light not seen by us humans; it records on film, and thus the images made are not faithful. Mainly the UV light appears as a haze seen in distant landscapes and aerial photographic images. The UV filter was popular in the film era and a necessity when doing aerial photography.

The digital camera, on the other hand, has a fixed UV filter, mounted as a protective cover glass over the image sensor. Mounting an external UV is redundant and unnecessarily induces degradation.

Bottom line – mounting a UV filter on a digital camera serves mainly to protect the camera lens from scratches and other physical damage. The main objective is to line the pockets of the filter manufacturers and their agents.

I rarely use them except for UV filters to protect the front element. I should use them more often. They improve the images for example by eliminating reflections when polarizing filters are used.