I frequently and happily use the Horseman 6x12 rollfilm back on a couple of different 4x5's, especially for emulsions not available in sheet film (e.g., Reala) and for particularly vertical or repetitious subjects (like stained glass windows; I hate to bracket 4x5 when there are 20 or 30 tall thin windows in a church!).
The Horseman is exactly 2:1 (112x56mm) so it produces negs that are about .33" shorter than the Linhof 12x6 negatives (which are 120x56), but in 4+ inches per frame that 1/3" is not a big deal to me and nobody has complained yet about the graininess of my 6x12 enlargements.
6x12 backs tend to cost a lot more than 6x9 and 6x7 (say, twice as much) but I figured that there are many times the additional area would come in handy. Best price I've seen lately on the Horseman 6x12 is from www.keh.com: $695 (B&H lists it for $770).
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