Hi all. I've been lurking for a few weeks now, trying to get a feel for the general design of these DIY projectors. While I feel like I'm developing a good understanding of the science, there's still one big question that I can't seem to find an answer to.
While I understand the science behind the collimator Fresnel (how you decide on a focal length), I'm still confused on the exact relationship between the size of your panel, the focal length of your objective Fresnel, the focal length of your objective lens, the throw distance of your projection, and the final projected image size...
Here's what I (think I) understand so far:
To decide which objective lens to use, you first decide on how far a throw and how large a projected image you'd like. You then use the formula 1/(focal length of objective) = 1/(projected image size) + 1/(panel size) to calculate the focal length of your objective. Now, I guess what I'm confused about is:
1. How is throw factored into all of this?
and
2. How do you calculate the focal length of your objective Fresnel?
Thanks to everyone in advance.
While I understand the science behind the collimator Fresnel (how you decide on a focal length), I'm still confused on the exact relationship between the size of your panel, the focal length of your objective Fresnel, the focal length of your objective lens, the throw distance of your projection, and the final projected image size...
Here's what I (think I) understand so far:
To decide which objective lens to use, you first decide on how far a throw and how large a projected image you'd like. You then use the formula 1/(focal length of objective) = 1/(projected image size) + 1/(panel size) to calculate the focal length of your objective. Now, I guess what I'm confused about is:
1. How is throw factored into all of this?
and
2. How do you calculate the focal length of your objective Fresnel?
Thanks to everyone in advance.
1 / (projector lens focal length) = 1 / (distance from LCD to projection lens) + 1 / (throw)
These are the equations you want to work with (first one is the equation you mentioned just named differently). They share these two variables: projector lens focal length (objective lens FL) and throw (aka Distance to Screen or Distance). This is how they depend on each other. LCD diag size doesn't appear in the first equation and distance from LCD to projection lens doesn't appear in the last two equations.
So start with the second equation or third equation. They are both the same, just written differently. Find the throw you want with the projected screen size you want. Match an LCD and a objective lens (projection lens) that makes these two equations work with the throw and screen size you desire. Due to limited supply of LCDs and lenses, you might have to compramise a little with screen size and throw.
Once you have that, take the projection lens' focal length and the throw and put them into the first equation. Solve for distance from LCD to projection lens. This number will most likely be different than your projection lens focal length. Take that number and find an objective (front) fresnel that is within +25mm for an unsplit design and one that is within -25mm for a split design.
For example, I use a 457mm FL projection lens. My throw is about 2540mm and my screen size is about 2000mm. I use a 17" (432mm) screen.
Second Equation:
Projection Diag Size = (2540 - 457)*(432) / (457)
Projection Diag Size = 1969mm; round up to 2000mm (my numbers are estimates)
First equation:
1 / 457 = 1 / (distance from LCD to projection lens) + 1 / (2540)
Distance from LCD to projection lens = 558mm
So I bought a 550mm fresnel lens to match this since I am using a split design and my fresnel needs to be between 558mm and 533mm. Generally, you want to keep a little bit more space between the LCD and the fresnels to prevent the fresnel rings from showing on the screen. Usually around 15mm of space. My projector ACTUALLY has about 20mm of space and the Distance from LCD to projection lens numbers is more like 570mm. Again, I was using rough estimates.
For the physical size of the frensels, a good rule of thumb is 20mm wider and taller than the LCD on all sides (so 40mm wider total and 40mm taller total). Hope this helps!
An externally hosted image should be here but it was not working when we last tested it.
These are the equations you want to work with (first one is the equation you mentioned just named differently). They share these two variables: projector lens focal length (objective lens FL) and throw (aka Distance to Screen or Distance). This is how they depend on each other. LCD diag size doesn't appear in the first equation and distance from LCD to projection lens doesn't appear in the last two equations.
So start with the second equation or third equation. They are both the same, just written differently. Find the throw you want with the projected screen size you want. Match an LCD and a objective lens (projection lens) that makes these two equations work with the throw and screen size you desire. Due to limited supply of LCDs and lenses, you might have to compramise a little with screen size and throw.
Once you have that, take the projection lens' focal length and the throw and put them into the first equation. Solve for distance from LCD to projection lens. This number will most likely be different than your projection lens focal length. Take that number and find an objective (front) fresnel that is within +25mm for an unsplit design and one that is within -25mm for a split design.
For example, I use a 457mm FL projection lens. My throw is about 2540mm and my screen size is about 2000mm. I use a 17" (432mm) screen.
Second Equation:
Projection Diag Size = (2540 - 457)*(432) / (457)
Projection Diag Size = 1969mm; round up to 2000mm (my numbers are estimates)
First equation:
1 / 457 = 1 / (distance from LCD to projection lens) + 1 / (2540)
Distance from LCD to projection lens = 558mm
So I bought a 550mm fresnel lens to match this since I am using a split design and my fresnel needs to be between 558mm and 533mm. Generally, you want to keep a little bit more space between the LCD and the fresnels to prevent the fresnel rings from showing on the screen. Usually around 15mm of space. My projector ACTUALLY has about 20mm of space and the Distance from LCD to projection lens numbers is more like 570mm. Again, I was using rough estimates.
For the physical size of the frensels, a good rule of thumb is 20mm wider and taller than the LCD on all sides (so 40mm wider total and 40mm taller total). Hope this helps!
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