I'm assuming for this post that you've read the bonus chapter on basic cinematography concepts that's included in the free expanded DSLR Cinematography Guide. Anamorphic lenses are used for CinemaScope productions, wherein an oval lens
stretches the image vertically compresses the image horizontally in order to fully utilize the imaging sensor. To achieve a 2.35:1 aspect ratio during projection, a corresponding anamorphic lens must be used on the projector. This is all well and good, but is it possible to use an anamorophic lens on a DSLR, instead of just cropping the image in post?
It's possible, but I'm not sure I'd recommend it. Vid-Atlantic productions makes a $65 "clamp" that allows you to mount an anamorphic adapter to your regular lens (with up to a 52mm lens thread). While the lens gives the characteristic lens flares that we associate with big budget productions, it also distorts subjects near the edge. Check it out:
However, while the clamp is cheap, the anamorphic lenses themselves can run $2k per lens. With this problem in mind, via planet5D we have word that DP Timur Civan has been working on a cheaper solution of his own. No word from Timur on exactly what the device is or how it works, but he has applied for a patent, so presuably that's the reason for his being coy. Here's a demo of his homemade device in action:
I get the appeal of shooting in the 2.35:1 aspect ratio, but it's certainly not easy (unless you just want to mark your viewfinder/monitor and then crop in post). If you'd like to know more about anamorphic lenses, check out this post at planet5D.