A lens will always alter the image in the same manner.
For example, what is called a "16:9" lens, like the ISCO I and ISCO II ; will stretch the image
horizontally by 33%. That will convert a 4:3 AR into a 16:9 AR. Coincidentally, it will also
convert a 16:9 AR into a 2.35 AR.
A lens that is labeled as a "2.35 AR" lens or a so-called "scope" lens takes a 4:3 AR into a 2.35 AR.
You have a 2.35 AR screen. The D-ILA has a 4:3 AR panel - so you could use a 2.35 "scope" lens to
alter the 4:3 image coming out of the projector to make the 2.35 image for the screen.
This is where the HTPC comes in. A 2.35 AR movie has an aspect ratio of 2.35, of course. The D-ILA
has this 4:3 panel that you want to use all of. The HTPC stretches the 2.35 image vertically, so
that it fills the 4:3 panel - people would look tall and thin. This image is put through the lens,
which stretches horizontally, thus restoring the original aspect ratio - and you have a 2.35 AR image
on your 2.35 AR screen.
Now if you have a 16:9 image - the HTPC will again stretch it vertically to fill the panel - but since
it is not as wide as a 2.35 image, it won't fill the panel horizontally, so when your scope lens
expands it horizontally it won't fill the 2.35 screen either - and you will have black pillars
left and right. Same for 4:3 - but the pillars are even thicker.
The above assumes you have a 2.35 lens. You could
also do the same with a 16:9 lens. For your 2.35
movies - you would use a 16:9 AR subset of your D-ILA pixels and have the HTPC compress the 2.35
image to fit into that. Then when the lens exanded it - it would be back to 2.35 AR. A 16:9
AR movie would use the whole panel of the D-ILA, but would not fill the whole 2.35 screen.
Although both movies would come out looking correct in both these scenarios - a 2.35 AR movie
using a 2.35 lens would use all the D-ILA pixels whereas the 16:9 AR movie would use a subset.
If you use a 16:9 lens, the 16:9 AR movie will use all the pixels, while the 2.35 AR movie will use a
So if you want to get best performance on 2.35 AR movies, and a little less on 16:9 movies, then get
the 2.35 "scope" lens. If you want best performance on 16:9 AR movies, and a little less on
2.35 AR movies - then get a 16:9 lens.
Since you have a 2.35 screen, I would guess that the former option is the one you would prefer
[ otherwise you would have a 16:9 screen ].
The choice of lens is based on which AR ratio you want to do "best". Get the lens for that AR.
For all other aspect ratios, the HTPC can scale and stretch the image so that what ever aspect
ratio the image starts with - it will come out of your chosen lens with the proper aspect ratio.