Peter, there are a lot of ways to meter and to process, so practical film rating is very personal and elastic,
what surprises to me is that with same metering and (I guess) similar processing you find objectively TMY twice faster than HP5, because (at least with xtol) speed point of HP5 it's very close to the TMY one, in lux·seconds units, and both films are well linear from that point...
I'm confident you know what you do with the meter and with the calibration, but I don't understand how you find such an 1 stop difference...
Thanks all for this discussion! I have 100 sheets of 11x14 HP5+ that I will be using in Zion in April (tho it is not my goal to use all of it!). It does not easily build up the density range I like to work with, but should work nicely for me with scenes that already have a large brightness range. I'll be using staining developers and perhaps my standard Ilford PQ Universal Developer in some cases.
I have a limited amount of Efke 100IR in 11x14...as well as quite a few fogged sheets (>30) of it that I can use as 11x11, and I like the square. Expired, I rate it at ISO25 and it behaves well. I have FP4+, Acros, and a few others in 8x10 and smaller LF.
"Landscapes exist in the material world yet soar in the realms of the spirit..." Tsung Ping, 5th Century China
Quickest way to taste them would probably be to shoot a roll of each, widely bracketed & then process to a contrast index in the 0.52-0.58 range, contact print them well & see which general characteristics you prefer. The differences will be pretty obvious side by side, even on a 35mm contact sheet. TXP & HP5+ will be rather closer in character than TMY-II & HP5+.
I prefaced my own comment with "high contrast" applications. HP5 will not yield good shadow gradation below Zone 3, TMY will because it has greater native contrast in the toe. In this respect, Ilford box speeds are routinely overly-optimistic. If you're only talking Z 3 to 8 they aren't. FP4 is steeper than HP5, but almost a zone or EV stop less than either TMax film. The other thing I like about TMax is that you can enlarge it a lot more without getting grainy or mushy. HP5 is a great film, but I don't like enlarging it more than 3X. With TMax I can not only use my 8X10, but if there's a nervous portrait sitter, or they want their dog in the shot, I can have a backup MF camera loaded with the same film, or even 35mm with an analogous perspective focal-length lens.
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