Imaging optics and lowest possible angle video projection?

[mudicokula]

I have a very very unusual project requirement where I need a pocket video projection with the lowest possible projection angle or "longest possible throw-ratio".

It's a pocket DLP projector so goal is to have the beam remain ,as much as possible, almost the same size as the DLP matrix, for a 15 cm projection distance.

I had some luck with ordinary "12x" attachment telephoto lens for iphone and moving the stock projector lens way farther from the DLP matrix than originally inteded and having the telephoto attachment lens right after that. Results were good but projection still bigger than the DLP matrix but focusing was only achieved with a 50cm distance, not closer. I assume the inability to focus at closer distances could also have been from the positioning limits of the focus lens element on the telephoto attachment lens and the stock projector lens.

I guess my first question is what minimum projection angle is possible with DLP projections? I don't suppose it can be made to be the same size or even less than the DLP micromirror array itself for a 15cm throw distance?

Second question, is a telephoto lens the correct approach for achieving this desired result?

 

[Tom.G]

Here is a step-by-step description of how to find your lens requirement.


Above was found with: https://www.google.com/search?&q=lens+formula+calculator
(which found over 200 000 hits). Also try a search the "Lens Formula", which is the common name.

And welcome to PF!

Cheers,
Tom

 

[mudicokula]

imaging non-monochrome lenses are complicated, projector and camera lenses can have 6-8 or more elements. Sadly you can't just put one lens element to focus the projection beam to infinity.

 

[sophiecentaur]

I need a pocket video projection with the lowest possible projection angle or "longest possible throw-ratio".

IF you want a 1:1 magnification ratio of object to image size, the ratio of distances from object to lens and lens to screen need to be similar. A telephoto lens can reduce this a bit by shortening the projector lens tube a bit but it could be hard to beat the basic geometry.
Your Micromirror device is probably not all that cheap and you perhaps should consider spending a bit on a lens. It is possible that a 'Macro' lens would do the job of a 1:1 ratio. Second hand camera lenses are all over eBay. It could be worth a look.