When stacking two images, it is always preferable to align them optically as much as possible before using any digital alignment (including only sending the
projector its native resolution). So, to answer your question, my preference is to have the two projectors as close to each other as possible (one physically stacked on top of the other is ideal) and as close to perpindicular and centered to the
screen as possible. When the two lenses are almost in the exact same starting position, then aligning them will be considerably easier. Using the internal
grid test
pattern, first fully align one
projector (if
ground supported, align the bottom
projector first, or if hanging, align the top
projector first); focus then
lens shift. When you have this aligned properly, then open the
shutter on the second
projector (you may want to close the
shutter on the first while you focus the second) and focus then
lens shift to match. If you have the ability to have your
grid be green on one and red on the other, then you will notice the
grid turn yellow as they
line up.
I should note that you do not want to do any alignment until the
projector is fully warmed up. I will usually wait at least 20 minutes prior to alignment but they should definitely be at full operational temperature by 30 minutes. This is essential as the optics will
shift slightly as the
lens mount warms up.
A couple of reasons why you want to optically align instead of using the "chip set" or digitally aligning. The first reason is that the less processing the
projector has to do, the less possibility of
latency in the signal. This doesn't always matter with strict playback, but each part of the
system that adds
latency can make a difference when doing a live
image or timing with audio. The second reason why I prefer the
lens shift over digital alignment is the quality of the
image. For instance, if I am sending the native resolution to the
projector, but now I need to
keystone correct, I will see that straight lines become jagged. If I have to
shift the
image or resize the
image then I am not fully using each
pixel which can make the
image apear out of focus in parts. This becomes more apparent when the
image is larger and you are closer to it. Also, when you
shift an
image in one direction, it will end up blanking a portion of the signal on one side and repeat the last
line on the other side (sometimes) as it only has a certain amount of information being
fed to it. This often does not do much to the overall
image as the audience mainly pays attention to the center, but can be distracting depending on content.
There are going to be times when optical alignment is not possible or you cannot get the two projectors to perfectly
line up. That's when I will use the digital alignment for the fine tuning.
Does this make sense? Did I explain the difference between
lens shift and digital alignment that you understand the difference between the two?