Since I've posted most of the Pentax Q system in the last couple of days, I thought I'd best find this one, too!
This adapter lets you use Pentax K lenses on your Q system camera. You have aperture control, even if your lens doesn't have an aperture ring.
The other plus with this adapter vs cheap third party ones, is that is has a leaf shutter built in, which means you can shoot a slower shutter speeds as slow as 30s, as well as getting flash sync up to 1/1000s
After this, there's also SRS at £199, then a jump up to Wex, at £249
Since this is to use other lenses on the Q, I thought I'd add the little ramble about focal length/f stop/DOF that I wrote in the comments of my Q10 deal, as it's relavent to this and hopefully useful!
The focal length and aperture remain unchanged - A 50mm f1.4 is always going to be a 50mm f1.4 - using it on a cropped sensor doesn't change that.
What does change is the field of view, which is where things change in relation to aperture - with a 50mm f1.4 on a cropped sensor, you get greater depth of field.
Basically, depth of field is inversely proportional to crop factor.
So, if you put a 50mm f1.4 on, say a micro four thirds camera (for easy maths!) and shoot wide open, you get an equivalent field of view of 100mm (2x50), the light gathering of an f1.4 lens (no change), and the depth of field of an f2.8 (2x1.4).
I'm practical terms on the Pentax Q, it means that where on a full frame camera shooting with a very fast telephoto lens you would have a very narrow area in focus (a person's eyes might be in focus, but the tip of their nose out of focus) you get the same view and light gathering (so the same shutter speed and ISO) but with more in focus (from in front of their nose to the back of their head).
That's why if you like blurred backgrounds, crop sensors aren't so popular - you need proportionally faster lenses to get the same amount of blur.