There has been lots of debates online discussing whether UV filters affect IQ or not. Some manufacturers also claim that it improves IQ, this may have been true back in the film era when UV filter affects the film, however this is not the case with digital sensors.
DRTV even made an episode about this topic.
So I decided to perform a simple test this morning with a Canon 5D Mark 2 +EF 100-400L and a Tiffen 77mm UV filter to personally see whether it has any effect.
All shots were taken at Av mode, ISO 100 and f/11.
A tripod and Mirror Lockup was also used.
Taken with the UV filter
Taken without the UV filter
There's a flare that happens because of the extra layer of glass on the front.
When you zoom in, you will notice that the image looks sharper without the UV filter, both on the center and corners.
Here's a crop of the cap with the UV filter
Needless to say, after this quick test, I will never use a UV filter again.
The only benefits on having one is for protection to your glass as it does create an extra layer of protection on the front from elements.
Also most Canon L lenses need one to be fully weather sealed.
However it's not something that I will always be putting on my lens, especially on primes and I will still keep some special filters with me like ND, CL-Pol and GND, just not UV filters.