One doesn't -- for even tray development, sheet film should be processed in trays at least the next size up. So you can do the 8x10 sheets in a 14x11 tray with no problems.
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I always buy 8x10 labeled film. Once I got some 10x8 film which I will never do again. I was able to get it into my 8x10 film holders by rotating the film holders 90 degrees, and fortunately my Shen-Hao has a rotating back, so I was able to slide the film holders in from the top to take pictures. However, when it came to processing the film, I found out the film was 2 inches too wide to roll up and fit into my 3005 Jobo Expert drum...
I also ran into the same issue as the original poster, my iPhone only has 4:5 aspect ratio for checking composition! The phone won't show 5:4!!
Another wrinkle in the subject matter, when trying to match the field of view and/or aspect ration between a 4x5 and/or 5x4 camera to the view thru the viewfinder of a DSLR the image recorded on the DSLR's sensor may be different, usually larger. My Sony A850's viewfinder shows about 95% of the sensors image whereas the Sony A900 had a 100% viewfinder. Most DSLR viewfinders are not 100% as far as I know.
Since the image formed by the pentaprism is less than that recorded by the sensor there is no white border only a little more image field recorded at the sensor. So what you see is not completely what you get, unless of course you are using a DSLR with 100% viewfinder like the Sony A900.
Most "pro-sumer" and higher digital cameras these days have 100% viewfinders. And you still see what you get on the screen after shooting on those older/lesser models, or in live view mode. If it is that much of a concern, one could factor in the % in their calculations of equivalencies.
Regarding the original question, I'm honestly not sure why one needs perfect 100% equivalency for lighting tests though. There's always the ground-glass and modeling lights...or perhaps some Instax? :)
I was wondering how this thread went on for six pages.
Now I know.
I'm still wondering.
Lets go on ... really a nice theme ...to talk about aspect ratio(s)....
Well I don't have the film for my full plate camera yet (and probably won't get it in the near future) but I - think - I like the aspect ratio of full (whole) plate the best.....
...but therefore mostly use still 2x3
...and always puzzled to read that someone apparently gets annoyed because he doesn't understand the answer served to him....