How a Hat solved my photography achilles heel

Since my trip to Australia in March last year, I've noticed that this line/purple hue appears whenever I use my Hoya ND400 filter (its a 9 stop filter which allows 1/500 of light through - to allow you to shoot long exposures) - see example above. Initially, it only appeared occasionally. Eventually, the filter fell apart, and I replaced it. I also upgraded my wide angle lens from Tokina 11-16 (DX lens) to Nikon 16-35 (Full Frame) lens. The frequency of the purple banding/line - became almost 100% - and asking my photography friends in Indonesia, in Singapore - no one had ever seen it. I didn't think it was the filter (since it was new, and also happened with the old filter), the lens or camera (since it doesn't appear when I don't use the filter).

This recent trip to Lombok, it appeared again! And I was determined to eliminate as many variables as possible, before I ditched the filter and adopt a new system.

So here's the normal 16mm shot, without filter.

The shot with the filter is right on top. Argh!
So, I tried it with a different lens - the 24-70mm. And this is the result:

ok, no problem right? Here's the same shot, with the ND filter.

Argh! So it definitely isn't a lens problem. Maybe a filter problem? I try the same shot, but this time, the filter is turned 90 degrees... to see if the line changes.

?!!! erm, three's no difference! I'm stumped. It isn't a problem with the filter, not the lens. I try searching on the internet again, and finally stumble on the problem - light leakage from the view finder! Its when you have light going through the view finder, leaking into the sensor. Its more pronounced during long exposures.

And so, since I didn't bring the view finder blocker (which I've never used) - I used my good old trust-worthy hat- and just hung it over the view-finder...

Voila! It works! Trying about 10 more different shots, the line is gone! Hurray!
Light leakage - it seems to be more camera specific, and happens with the D600 (didn't happen with my D90). So if it happens, use your hat!