This is going to be another one of those posts where the seasoned veterans are like, "oh come on, thanks again Captain Obvious," but for someone just learning about lighting and cinematography and how they relate to proper exposure, or someone just entering the depth of field adapter market but not sure where or how to start, the following information will be of some assistance in determining:
(a) Whether a depth of field adapter is for you, and
(b) How to prepare yourself to properly take advantage of this tool if you do indeed acquire one.
So they're out there: The Redrock M2, the Letus Extreme and Ultimate, the Brevis, and a myriad of other adapters, most of them generally claiming that their adapter loses "only half a stop of light". While these claims themselves are the subject of much discussion and debate (we'll assume them to be true for our purposes), one fact that anyone new to the concept of adapters should consider is that even if your adapter brand of choice loses only half a stop of light...you will lose much more than a half-stop by incorporating a depth of field adapter into your filming.
For those without a photography background, a decrease of one f-stop means that half as much light is being allowed into the camera to reach your sensor. And in these terms, a decrease in f-stop would actually mean increasing the F value on your lens. So going from, say, F2.0 to F3.0 would cut your light in half.
So a half a stop of light... Not too big of a deal. Maybe you can just open the aperture on your camcorder itself if it wasn't wide open and you haven't lost anything, practically speaking. Certainly shouldn't be much of an issue shooting outdoors in the daytime.
But here is the reality with your adapter.
Not only will you lose the half-stop or so that is inherently lost by the adapter mechanism. You will also lose light in additional ways...
ONE
In order to focus your camcorder on the ground glass in the adapter (where the image is projected), you have to zoom in so that the ground glass fills the frame. If you have your camcorder fully zoomed out, your image will be small in the center of the frame, surrounded by a black rectangle (elements of the adapter). You have to zoom in until these black edges are not visible.
What happens when you zoom your camcorder in? The aperture shrinks and the f-stop value increases. And you lose light.
TWO
What do you attach onto the end of your depth of field adapter in order to film your nice, shallow depth images? An additional lens.
Depending on the lens (smaller minimum F-stop lenses increase greatly in price), you may have a very fast one (perhaps F1.4) or something slower, with minimum values from F2.8 to F4.0. That means that the lens itself is already costing you possibly two stops of light before the image even gets inside the adapter, where it loses more light and then hits the (zoomed in) camcorder sensors.
APPLICATIONS
So what does this mean, that adapters are crap and the manufacturers are liars? No, of course not. But it means that you will really need to augment your indoor shooting methods from what they may have been with a bare camcorder. You will be losing 2+ stops of light and getting some seriously underexposed images if you don't attend to lighting much more enthusiastically.
If you want to get a feel for why you will have new lighting requirements, set up your camcorder in a fairly dark setting that is still reasonably filmable with your bare camera. Maybe your living room at night with the main light on. Make sure your camera has its aperture wide open and no ND filters operating, pretty much the way it would be if you were trying to get usable footage in this situation. Then just start dialing down the aperture. Increase the f-stop by 1, then 2, then 3, and you'll get a rough feel for how you may end up fighting the darkness as you add all of this wonderful gear to the front of your camera.
And anyway, this "drawback" is actually not such a drawback, because if you haven't paid this level of attention to your lighting design so far, chances are that your images have suffered for it. Now, as you're making sure you fill your nighttime indoor set with enough light to get that image through all of that glass, it really isn't that much additional work to focus on some foreground / background separation, or toy with the contrast ratio. You're setting up the lights anyway...
