For this test a high contrast target is made from a light box and a small slide box lined with black felt. The test setup is photographed with no filter on the lens, a Hoya S-HMC UV filter (a multicoated filter) and a Tiffen Haze 2A filter (an uncoated filter as far as I can tell from Tiffen marketing material). A 70-200/4L lens is used since it is a good constrast lens that has good tolerance for flare. Most importantly I own this lens and happen to have 67mm filters of both types available. For each condition three shots were taken to verify there was no shot to shot variance. Only one of each is shown here.
So here we have the 70mm/4.0 shot with no filter. The average RGB value of the center of the dark box is 53,51,46. This is the base contrast level for this lens at this exposure. For those who care these were all shot in sRGB with daylight WB and parameter 2 set on the XT (that is, contrast, saturation and sharpening at 0).
From Filter Tests