ND's are awesome. They balance the image so much more, cut down bleeding, and open the aperature more which decreases the dof. If you can screw on filters, Id suggest a 2x and a 4x ND.
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I bought my K3 from ebay. There was a problem in shipping where Russian customs opened the box and replaced all of the packaging with little styrofoam balls which found their way into the leather case and into the camera body. This caused the camera not to run. After tearing the camera's face plate off (4 screws) I was able to shake all of them out. I verified this by opening the baffle plate when removing the loop formers (a HIGHLY recommended modification), there were no more balls in there. I cleaned everything with alcohol and qtips, and have just shot 4 test reels with it. It is wonderful image. Even from a one-light it is absolutely beautiful. I would only use it for silent or B-roll stuff as is, though because after inverse tk'ing the footage, the framerate is off a slight amount. I would hate to have to manually synch up 90 minutes of footage with constant adjustments. It could be done BUT the optional crystal synch motor from TCS is a fantastic solution to that problem. tobincinemasystems.com is the website I think. The motor is coming supposedly May 1st, at $495. This would make it the least expensive Crystal Synch camera out there, and by a good margin. The major downside will still be the 100' loads though, which will last only ~2:45 . That means about 1:45 - 2:10 of screen time depending on how quick you are with the slate, loading, and how many setups/takes you do.