Lens Selection
Looks like you've got the math right, though my results are that you would need a 1.33:1-capable
lens for a 27'
throw to get an
image height of 27'. The 1.2:1 would be work at 24' but at 27' would fill your
screen with 3' of excess in the vertical. The excess in the horizontal doesn't matter because you'll blend them together.
When selecting a
lens you want to remember that you want to be on the bottom end of the
throw ratio with the widest angle as much as possible. In this case you could lose 10-20% of your
lumen output by being zoomed in on the
lens. This is because the natural function of most lenses is that as you
zoom in, the aperture closes down, reducing your light transmission permitted to travel through the
lens elements. In a short
throw lens this is less pronounced, but in a long
throw lens you could lose 50-60% of your brightness by being at the narrow end of the
lens'
zoom ratio. You may be able to press Panasonic to give you their aperture data for this series of lenses. The formula is pretty simple:
(F1/F2)^2 = % of remaining light permitted through
lens
F1 = Lowest f-stop aperture
F2 = Highest f-stop aperture
So let's say your
lens had an f-stop of 2.3 at narrow, and 2.6 at the widest. (2.3/2.6)^2 = 78.25% --- the same
image area is 78% as bright at the narrowest
zoom as it is bright at the widest
zoom. Your results may vary, but these f-stop values are in the neighborhood of similar short
throw zoom lenses I've seen by manufacturers who do publish their
lens data.
--
Image Brightness
12K/ea is in the ballpark of being appropriate. With a target contrast ratio of 10:1 and ambient light levels of 2fc at the
screen and 10% light loss in the
lens, my result is dead-on at 12082.5 lumens required for that
image area, producing 69 nits of brightness on the
screen. For reference, if you want to get licensed to stream The Met opera events in your theater or cinema, 55 nits is the minimum allowable brightness. Your average consumer TV is usually 300-400 nits, which is higher because you have to compete with
daylight coming in through windows and standard room lighting. In your case, if your RP
screen is good at rejecting ambient light and you don't let your
stage lights
wash all over your
screen, 69 nits should be adequate. If your
stage areas are typically lit up as bright as the sun, then you may want to consider something brighter so you have some
headroom to blend the appearance of your scenery surfaces with the appearance and brightness of your performers, set pieces, and
props.
Small footnote: All of my calculations all assume you have an RP
screen with a
gain of 1.0, which is likely not the case. Do you know what the
screen gain is for your RP
screen(s)?
