Do camera lens filters cause a softer image?

The trusty camera filter, designed to protect whilst altering your image through changing light levels, causing punchier images, different colors or even less reflection. Does this extra piece of glass take sharpness away from your image?

Whilst the filter has minimal space between it and the front element of the lens, there is still a small gap which perhaps could even the slightest distort an image.

I did some examples to check if any obvious changes where present from a filter compared to no filter on a lens:

Left image is with filter, right image without it. Images taken in manual focus with same settings. Lens 18-105mm f4.The filter was a minimal UV style, more a protector than anything else.

It’s a small sample size but it shows one thing, there essentially is no difference. There is some slight light changes in the lock shot 1 but as for sharpness once the RAW files are turned into JPG you cannot notice any difference. It is consistent throughout just some highlighting and shadow differences depending on the shot.

This eases any stress I had that using a filter may be weakening my photos.