Start by opening the back and firing a couple of shots to physically see if the aparature blades open correctly by looking into the lens from the back. If you try different setups (wide open or fastest apparature and do it indoor when you get longer shutter times and outdoors with faster ones etc) you should be able to determin if the exposures work fine. If yes - try another role of no - then it needs service!
Nikon RFs ---- S, S2, S3, S3-2000 & SP-2005, NHS R2S with Nikon 35 (f/2,5 & 3,5), 50 (H, S & Mill ed), 85, 105 & 135, Zeiss Sonnar 50 (2008 ed), CV 21, 50 & 85, adjusted Helios 103 & Jupier 12 lenses. Nikon D90 with numerous lenses & Fuji X100
Personal photo blog: risfelt.blogsome.com