Originally Posted by WillyJ
I don't have my 5020 up yet but I am wondering about these convergence reports - don't these projectors have convergence adjustment like the old rear projection sets or is the convergence problem so bad on some units that the built-in adjustment is not sufficient (and the units have to be exchanged)?
If you consider the physical construction of the projector and how it works (You can google for images of the inside of the projector, or I have pictures of one here: http://www.avsforum.com/t/1458911/epson-8100-replaced-bulb-and-now-pink-blue-bands#post_23003902
), you may better understand.
There are several lenses in the light path from the lamp to the LCD screens. There are 3 physical LCD screens. These of course do not mechanically move in these projectors (imagine added cost if they added mechanical adjustment of each LCD's position) The LCDs are very tiny and a single pixel is 1/1920th the width of the LCD.... A panel being "misaligned" by less than 1 pixel is a very small distance.
DLP projectors do not have this issue because there's only one DLP chip where the light is reflecting off of, so it's "perfectly aligned" with itself. However, I'd imagine the more expensive 3DLP projectors could possibly have similar convergence issues because they have 3 DLP chips that must be aligned.
As said already, they don't have tiny motors in these projectors to move each LCD screen, so essentially the built-in adjustment uses software/firmware to shift which pixels are used. For example, if Red is off by 1 pixel to the right of Green in the projected image, it can be adjusted to just "move" the image 1 pixel to the left. For non-whole pixel adjustments, it will do things like (just an example, probably not the correct %'s) for an exactly half-pixel adjustment it will "split" 50% of the pixel's brightness in one pixel and 50% of it in the adjacent pixel. When you do this type of non-whole pixel adjustment there is some degradation to the image (debateable how noticeable). As a direct answer to your last question, with the built-in alignment you can adjust to less than 1/2 pixel misconvergence at-worst if you stick to whole-pixel increments.
Some people are very picky and put their face 1 ft from the screen, and it's off by 1/2 a pixel each on Red/Blue and its unusable convergence to them. Other projectors really have issues (misalignment by several pixels is a real problem if it occurs, and can be seen from viewing distance). You might get lucky and get a projector where the three panels appear to be near-perfectly aligned, but more likely you'll get one that has the panels at least off by a fraction of a pixel. To many people this is acceptable and expected, but to others it is not. Some people may have exceptional eyesight and see the misconvergence when watching movies or reading text from viewing distance.
It's debatable how much a small misconvergence will actually affect the image quality that you will actually see. At a first thought someone might say that one of the other panels were off by 1 pixel to the right (and not adjusted for), that you'd have a projector with equivalent to half the native resolution. In reality, this is not the case as your eye is more sensitive to yellow-green than the other colors. This is why when you do the LCD alignment adjustment it leaves the Green alone and you adjust the Blue/Red and it won't "split" the Green pixels. This will result in you still being able to perceive each individual pixel (assuming you're close enough to the screen, you're eyesight's good enough, etc), and not seeing as large a difference from the split pixels for Red/Blue.
Also, if you wear eyeglasses with high-index lenses, you're likely to see much more distortion at viewing distance due to your eyeglass lenses than due to misconvergence. In my specific case (around -5 prescription with 1.61 index lenses) the effect is comparable to more than 1 pixel misconvergence on the edges of the screen.
--As a side benefit of slight misconvergence, if the other panels are very slightly off in vertical and horizontal (1/3 pixel?), I think that you can effectively nearly eliminate the "screen-door-effect" at close viewing range that the people who prefer DLP seem to complain about most in LCD projectors.