Never heard of a mirror filter? Well, there are two possibilities:

(a) You are just a pretty ignorant sort of person. Fancy not knowing what a mirror filter is!
(b) Nonsense, Tannin just made the term up. Of course I haven't heard of it!

I'll give you a hint: (a) is incorrect.

I've been playing with grads lately. (And CPLs too.) And they are a right royal pain to use. Sure, if you are a proper landscape person and think nothing of spending half an hour setting up the shot exactly the way you want it, something like a Cokin system or a Lee system is fine. They are fiddly and awkward and precise, and that's just the way you like it.

But for someone like me, my Cokin set is a horrorshow. Most of my landscapes are passing fancies - more documentary than artistic - taken in spare moments between birds. I already have a vast quantity of gear to lug around on my back, the filters are just an added problem. Mostly, I just want to control an over-bright sky, so all I really need is a grad or two, and a CPL. But I have to cart around the clumsy adaptor, step-down rings for different lenses, hard plastic cases for those fragile filters. The reality is, I just don't. They stay in the car. If I'm not near the car when I want to take a picture, tough luck. And if I'm parked somewhere dodgy where I can't stay (e.g., in the middle of a bridge during a break in the traffic) and have to snap and run, hard luck again.

Uncle Arthur taught me a naughty trick years ago: instead of all that rigmarole, just hold the filter in front of the lens with your hand. (You can grip a standard 100 x 150mm filter by the corner using finger and thumb. It doesn't smudge any of the part that the lens sees, you just have to be careful not to get your thumb in the picture if you are using a wide angle. But it's still a painful way to tame a sky.

What I want is a thing shaped like an old fashioned lady's hand mirror (or a table tennis bat) and about the same size. It has a firm outer ring, and two hard plastic covers, one on either side. (Like the rubbers on the table tennis bat, only rigid.) These are hinged, so that you can slide them to one side, revealing the filter glass that they protect. There is a hook or eye in the end of the handle so that with the covers closed you can dangle it from your belt. When you want to use a grad, simply unhook the gadget, slide the covers to one side, and hold it in your left hand, covering the front of the lens and adjusting it to suit. (Like a rectangular or square filter, it's big enough to move around to put the darker part just where you want it.)

That's the standard Mirror Filter (tm). You can get it in any of several grades of grad.

The Pro Model Mirror Filter is a bit thicker and heavier. It contains two or even three filters (just like a Cokin adaptor) which, like the covers, pivot out to one side when not in use. It comes with a CPL, and you can buy others to suit.

Where do you get it?

In my dreams.

I checked: not even B&H has anything like it. Damnit!