Hate to tell you that you are the lucky victim of the laws of optics - Given the same light source and lens, a bigger image will always be dimmer then a smaller one. (ie measure less lumens on the screen) While it is true that light falls off with distance, that is not what's happening here.
To measure the same brightness on a bigger image you need a bigger lamp and or a faster lens, and or a brighter screen. No way around it. Just like you will need more watts to achieve the same SPL in a bigger room.
The other thing that is going on is that every lens has a range. At x distance it
will project an image x size. Move it further back and the image gets bigger, closer and it gets smaller. This is why zooms are popular. The conversion lens you found is designed for a worst case scenario where there is enough height in the room to get a reasonably sized picture, but not enough depth to back up to throw it with the stock lens.
Also a wideangle adapter will quite probably not be as sharp as what you are used to - optics again - you are likely to notice curvature on the edges as well as the image being brighter in the middle then on the edges.
The right way to do it is to size the projector to provide a given lumen level at a specific image size, then find the right lens that can deliver that image from the desired projection distance. Of course a fast lens (low f stop) will deliver more of the light to the screen - sort of like a more efficient speaker will be louder with the same watts.
The other part of the equation is gain (reflectance) of the screen. The higher the gain, the narrower the viewing angle but the brighter the image for a given lumen output. Think of gain with light as something like gain with a cartridge - certain screens amplify it.
Did you buy the glass bead or the matte surface? This will be an important clue as to how bright the image will be - with a glass bead you will find its a whole lot brighter then your wall or your sheet.
So what to do?
Well one thing you could consider is cancelling the screen and paying the restocking fee. Then you can get some special paint - a bright white is good but you can get better stuff - and simply paint your wall. Just Google "screen paint" (Once you determine an optimum size you can put a nice frame around it or dress it up however you like.)
Then put the projector on a ladder and start backing up till you feel that you have reached the optimum compromise between brightness and size. Rehang the projector there.
Keep in mind that as the image gets bigger its also going to look progressively worse as the pixels become more and more apparent. This is a function of resolution. Based on a quick Google search your projector outputs an XGA picture - meaning 1024x768 pixels. Since the viewing distance (where you sit) will remain the same the image will appear progressively coarser as it gets bigger. Will you be sitting 20 feet away per your test? If not, you should sit at the actual viewing distance and see if you can live with the resolution.
The brighter image will be a lot punchier and more dramatic and by keeping the image size under control you will enjoy higher resolution for a more cinematic experience.
Then save your pennies for a better projector - one that can handle progressive scan or maybe even hi-def.
Sorry to be the messenger here - its why home theater guys get the big bux =) So you know, I spent thirty years designing and staging projection in everything from conference rooms to stadiums. this very basic advice is based on hard-earned experience...
To measure the same brightness on a bigger image you need a bigger lamp and or a faster lens, and or a brighter screen. No way around it. Just like you will need more watts to achieve the same SPL in a bigger room.
The other thing that is going on is that every lens has a range. At x distance it
will project an image x size. Move it further back and the image gets bigger, closer and it gets smaller. This is why zooms are popular. The conversion lens you found is designed for a worst case scenario where there is enough height in the room to get a reasonably sized picture, but not enough depth to back up to throw it with the stock lens.
Also a wideangle adapter will quite probably not be as sharp as what you are used to - optics again - you are likely to notice curvature on the edges as well as the image being brighter in the middle then on the edges.
The right way to do it is to size the projector to provide a given lumen level at a specific image size, then find the right lens that can deliver that image from the desired projection distance. Of course a fast lens (low f stop) will deliver more of the light to the screen - sort of like a more efficient speaker will be louder with the same watts.
The other part of the equation is gain (reflectance) of the screen. The higher the gain, the narrower the viewing angle but the brighter the image for a given lumen output. Think of gain with light as something like gain with a cartridge - certain screens amplify it.
Did you buy the glass bead or the matte surface? This will be an important clue as to how bright the image will be - with a glass bead you will find its a whole lot brighter then your wall or your sheet.
So what to do?
Well one thing you could consider is cancelling the screen and paying the restocking fee. Then you can get some special paint - a bright white is good but you can get better stuff - and simply paint your wall. Just Google "screen paint" (Once you determine an optimum size you can put a nice frame around it or dress it up however you like.)
Then put the projector on a ladder and start backing up till you feel that you have reached the optimum compromise between brightness and size. Rehang the projector there.
Keep in mind that as the image gets bigger its also going to look progressively worse as the pixels become more and more apparent. This is a function of resolution. Based on a quick Google search your projector outputs an XGA picture - meaning 1024x768 pixels. Since the viewing distance (where you sit) will remain the same the image will appear progressively coarser as it gets bigger. Will you be sitting 20 feet away per your test? If not, you should sit at the actual viewing distance and see if you can live with the resolution.
The brighter image will be a lot punchier and more dramatic and by keeping the image size under control you will enjoy higher resolution for a more cinematic experience.
Then save your pennies for a better projector - one that can handle progressive scan or maybe even hi-def.
Sorry to be the messenger here - its why home theater guys get the big bux =) So you know, I spent thirty years designing and staging projection in everything from conference rooms to stadiums. this very basic advice is based on hard-earned experience...