Originally Posted by Toe
I use low lamp Craig. My 1.78 screen is small though at 94" diagonal, so combined with the HP gain I can get away with low lamp and iris at -15 (I bumped this up recently to -13 as I have ~400 hours on the bulb). For my 9' wide 2.35 image doing the zoom method, I use low lamp and -7 iris right now which puts the brightness approximately the same as my 1.78.
I'm not sure whether you have a single iris model or one of the higher dual iris models, but I was curious about what the native CRs might do between using the manual iris to get close to max native on/off CR and using the auto iris. This year's models are supposed to be brighter than last year's, so seems like you could use -15 for a while for 1.78:1 content. This year's should also have a little bit better native on/off CR, but we of course don't have numbers for this year yet.
According to the numbers from Cine4home at http://cine4home.de/tests/projektoren/JVC_X-Serie_2012/JVC_DILA_2012.htm
for the X55 model some example CRs are:
Max throw, manual iris at -15: 48k:1
Max throw, manual iris at 0: 28k:1
Min throw, manual iris at -15: 42k:1
Min throw, manual iris at 0: 24k:1
So, going from a case like your 1.78:1 situation a person would lose about 40% of the native on/off CR in order in order to gain the higher dynamic on/off CR. For a person who could go with a darker screen to help ANSI CR retention in their room that may work well, but for others this reduction in native on/off CR alone might make them decide not to enable the dynamic iris (not even counting that they may not want brighter whites that happen with the iris open).
Looking at the X75 from the same link some values are:
Max throw, manual iris at -15: 83k:1
Max throw, manual iris at 0: 38k:1
Min throw, manual iris at -15: 63k:1
Min throw, manual iris at 0: 31k:1
As I mentioned earlier, I don't know what the back iris does in auto mode, but assuming that the back iris opens up for auto mode then a person who could use the projector with both irises shut down could lose over 50% of their starting native on/off CR in order to use the dynamic iris.
On the subject of the CRs, from looking at the specs of the new models it looks like JVC is using a 10x multiplier from native on/off CR to dynamic on/off CR. However, since their native spec isn't with the irises open they would need to use more like a 20x multiplier from their starting native on/off CR to get to their claimed dynamic on/off CR.