Hello
I am thinking that this is not possible but what i need is:
A long throw of around 144 inches.
A smaller than normal image of 72" across.
Now a rough calculation will mean I need a fl lens of 27"
Now I can get Achromatic Doublets with that fl BUT they are around 2" in diameter.
Are these 2 small to project an image from a 15" ???
I was thinking of this kind of arrangement: see below
Am I going totally doem the wrong path here and will a 2" lens be too small.
I am thinking that this is not possible but what i need is:
A long throw of around 144 inches.
A smaller than normal image of 72" across.
Now a rough calculation will mean I need a fl lens of 27"
Now I can get Achromatic Doublets with that fl BUT they are around 2" in diameter.
Are these 2 small to project an image from a 15" ???
I was thinking of this kind of arrangement: see below
Am I going totally doem the wrong path here and will a 2" lens be too small.
Attachments
Fizzer said:
An externally hosted image should be here but it was not working when we last tested it.
Sorry had image attached BUT it got lost on the way
The focal length calculator I use gives the following distances for a 15 inch lcd in a split fresnel design:Fizzer said:
22 inch focal length lens
Throw distance 12.5 feet
Screen size 72w x 54h 90 diagonal
Distance from center of lens to lcd 654mm
The lens you use will need a field angle of 33.5 degrees. A 2 inch diameter lens is to small to for this application. Using a 650/220 fresnel combination with a bulb that has a 27mm arc, the minimum diameter lens would have to be 3.25 inches to get all the light through the lens. I hope this helps.
I think he was looking for a 72" diagonal. If so, you still wouldn't need a 27" FL proj lens. 25" would do it. There are large format lenses (copy lenses) that have focal lengths that long.
Yeah, the diameter of the proj lens is going to matter with respect to your arc size (like Mikey P was saying). Take your field fresnel (condensing) and divide it by your back fresnel (collimating). That will give you the magnification. Take that magnification and multiply it by the bulb's arc size (like Mikey P did). That will give you the absolute minimum diameter your proj lens needs to be in order to theoretically get all of the light into the lens.
Yeah, the diameter of the proj lens is going to matter with respect to your arc size (like Mikey P was saying). Take your field fresnel (condensing) and divide it by your back fresnel (collimating). That will give you the magnification. Take that magnification and multiply it by the bulb's arc size (like Mikey P did). That will give you the absolute minimum diameter your proj lens needs to be in order to theoretically get all of the light into the lens.
Many thanks. 😀
This does help alot.
I am quite new to this and still getting my head around the lens / fresnel stuff.
I want to get this bit a perfect as possible to facilatate the best image possible.
Those posts really help, as for the calculations those I posted were just a rough idea. Will have to look at the on-line calculating tools.
This does help alot.
I am quite new to this and still getting my head around the lens / fresnel stuff.
I want to get this bit a perfect as possible to facilatate the best image possible.
Those posts really help, as for the calculations those I posted were just a rough idea. Will have to look at the on-line calculating tools.
Hello
So if I used a 550/330 combination and a 27mm arc this would give a 1 3/4" lens ????
Is their any other considerations apart from light loss for the lens Dia.
I am finding it difficult to find Opaque Projector Lenses here in UK.
I really would like my projector as far back as possible BUT i only have a fairly small (relatively speaking) projection area.
P.S. I have spent ALOT of time reading the forums but I think I can't see the woods for the trees now
So if I used a 550/330 combination and a 27mm arc this would give a 1 3/4" lens ????
Is their any other considerations apart from light loss for the lens Dia.
I am finding it difficult to find Opaque Projector Lenses here in UK.
I really would like my projector as far back as possible BUT i only have a fairly small (relatively speaking) projection area.
P.S. I have spent ALOT of time reading the forums but I think I can't see the woods for the trees now
550 / 330 = 1.67 magnification. So a 27mm arc would be 45mm (1.77 inches) by the time it hit the projection lens. This is in a perfect setup, which probably won't happen. So you want to make it much bigger to compensate for all the imperfections in the parts and the design.
So yes, a smaller diameter proj lens will give you a dimmer picture. On top of that, the smaller the diameter, the higher the quality of the lens you need. This is because there is less glass to take in the picture. Therefore, you have more picture per field of view (FOV) degree of glass.
So this picture assumes that the lenses' specs in each projection lens (there are 3 of them in a triplet) are matched except their diameters.
You can see that the coverage area (the amount of the brown wall the lens can "see") is much smaller for the smaller proj lens. To make area covered by the proj lens with the smaller diameter equal the larger proj lens, the smaller lens needs to have a wider FOV (ie wide angle lenses). This usually makes the lens more expensive and harder to make. This would also shorten the overall length of the projection lens; hence why wide angle lenses are much shorter than telescopic lenses.
So yes, a smaller diameter proj lens will give you a dimmer picture. On top of that, the smaller the diameter, the higher the quality of the lens you need. This is because there is less glass to take in the picture. Therefore, you have more picture per field of view (FOV) degree of glass.
An externally hosted image should be here but it was not working when we last tested it.
So this picture assumes that the lenses' specs in each projection lens (there are 3 of them in a triplet) are matched except their diameters.
You can see that the coverage area (the amount of the brown wall the lens can "see") is much smaller for the smaller proj lens. To make area covered by the proj lens with the smaller diameter equal the larger proj lens, the smaller lens needs to have a wider FOV (ie wide angle lenses). This usually makes the lens more expensive and harder to make. This would also shorten the overall length of the projection lens; hence why wide angle lenses are much shorter than telescopic lenses.
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