Having been a Boy Scout for many years, and well used to fitting kit into numerous rucksacks in my time, it is no surprise that in my Camera bag there are many useless and redundant articles... like midge repellant in winter (just in case I forget to put it back in) ... however, many photographers return from a days shoot having failed to get a decent image for the want of a bit of kit that is a permanent fixture in my bag and has got me out of blank days on several occasions.
It is the ten stop neutral density filter (mine by B+W)... but there are many and various types.
On a wet day, they can transform a seemingly contrast free image into a decent shot by smoothing the water and allowing the clouds to develop a pleasing pattern... on a bright day, allowing longer exposures than would be possible...
All in all, when they are mastered, and this itself takes time, they can provide an extension to a dull or over bright scene, and the chance to get at least one image to take home...
All the images below, from Millarochy Bay, Loch Lomond, were taken within ten minutes of each other and show the effects, moods and variety that can be achieved with these filters.
All images are taken on a tripod, with a cable shutter release, all at f16, mirror up with the longer exposure ones, and several attempts to get the timing right...
Original (no filter) 0.5 seconds, F16
Long Exposure 33 seconds at f16
Long exposure 25 seconds at f16