There has been several posts regarding different means for composition without lugging the huge camera to every location.... I recently bought the Linhof Multi focal length viewfinder. Although the viewfinder is quite unique and does work as a composition tool, IMO, it's a bit limited and cumbersome. The fl's are limited and the aspect ratios are limited. If you want to change formats, you must change out the masks. The viewfinder was clearly designed for its intended purpose - to sit atop a Linhof field camera. As a composition tool, I would consider it OK at best, as its heavy, not pocket friendly, not easy on the hands, ice cold in cold weather, hot when the sun beats on it. Its also a bit cumbersome to continously change fl settings, as it was designed more for, set it and leave it. Its not really a zoom type lens such as 35mm zoom lenses.
I recently came accross two interesting composition tools that may work well. The first is small, compact, and dirt cheap, its called the ViewCatcher. I just ordered one at Amazon.com for $9 + shipping.
The beauty of this little gem, is its ability to show any fl, AND any aspect ratio in the same tool with ease of changing between the two. AND it fits in your shirt pocket! You can mark the exterior with formats you shoot, 24x36, 6x9, 4x5, 5x7,6x12, 6x17, 4x10, etc. Then just slide the insert to suit your composition and see what format best suits the scene. Then look at your format markings. To determine fl, install a thin tape onto the frame, mark the lens fl's you own, (for example, 240mm lens = 240mm distance from eye to View Catcher) pull the tape to your eye, and see the fl required for that composure at infinity focus. Slightly closer focus will not change the fl, assuming its not macro type shooting.... For a small, compact and low cost system, this looks like a winner...
The second item is a movie directors viewfinder, or composing tool. If you shoot wide to normal only, this viewfinder would suffice well. In 1.5 aspect ratio (which you can mask down to 4x5) it will show 18 - 200 mm fl's. If you shoot with long lenses, you could still simulate them by masking down further.
Hopefully this will add to the list of options, for composing tools....