I've always wanted to make some pictures to help explain the difference between using 16:9 projectors, and 4:3 projectors. The discussion came up here recently, so I thought, what better time than now. Also, in this post I refer to 16:9, but an almost identical ratio is 16:10. So, I'm really referring to both when I say "16:9".
Since most movies are now 16:9 (or wider) it makes sense to assume that most movie parties will be using 16:9 DVD's. The bottom line is that both 16:9 and 4:3 projectors can show 16:9 movies, and they both can fill an entire 16:9 screen. The difference is that a similarly spec'd 4:3 projector will lose some brightness and clarity along the way.
So let's start with an outdoor inflatable screen (I know, bright sunny day, huh?):
So now, lets shine a 16:9 projector, showing a 16:9 movie at the screen (assume we're using front projection here). And lets position the projector at the correct distance to fill the entire screen area with the movie image (represented by the yellow area). Also, lets say this projector is rated at 2500 lumens. In this case all 2500 lumens are shining on the screen. So far, so good.
Now, lets shine a similar 2500 lumens 4:3 projector at the screen, this time showing a 4:3 movie (again, movie image represented by the yellow). Since the 4:3 image is more "square", we would need to move this projector closer to the screen than the 16:9 projector above in order to fit the image on the screen, as shown in the next picture.
This would result in "black bars" on the left and right sides of the screen (as seen above). "Black bars" can refer to 2 things. In this case, it's an empty part of the inflatable screen without part of the movie being shown on it. The next image shows the second type of "black bars".
This next picture shows what would happen if you took the exact same projector and setup above, but instead used a 16:9 movie. Instead of using the full "square", the wider 16:9 movie is now squeezed into a smaller area. It's using only the middle area of the projector's image, as compared to the previous image. The areas labeled as "Black bar 2" on the next image represent the 2nd type of "black bar". In the first case, it was an unused area of the INFLATABLE SCREEN. In this case, it is an unused area of the PROJECTOR'S IMAGE.
Notice in the previous image, the lumens dropped to 1875 instead of 2500. This is because the yellow area (the part that makes up the movie image) is using a smaller part of the projector's full image. The full 2500 lumens that the 4:3 projector has, is only when using the full 4:3 image. In fact, it is now using exactly 75% of it's full lumens.
But, would you want to show a movie like this? Probably not. You would want the movie to fill the whole screen. It's easy to do, just move the projector further away from the screen (basically near where the 16:9 projector was from the first example). That makes the image larger, and now it will nicely fit the screen.
The problem is that the "Black bar 2" parts of the projector's image are now projecting above and below the 16:9 image area of the screen. In some cases you will see these "Black bar 2" areas being projected on the screen if using front projection. If using rear projection it would not be visible from the audience side of the screen at all. But a lot of people have used 4:3 projectors this way, so it can be done.
Where you lose out is in 2 places. First, compared to the 16:9 2500 lumens projector, your picture will only appear about 75% as bright. Also, since you're only using the central part of the projector's image, you are losing some resolution as well.
To keep things simple, it's best to just go for 16:9 (or 16:10) projectors and screens. If you do decide to go with a 4:3 projector, remember to subtract 25% of the stated lumens when comparing to 16:9 projectors.