Hey guys,

I've asked this before and got some OK feedback, but now I really want to move forward with it and really want hard core mathematical proof if it can work and how. This is for two reasons. First, I don't want to try it unless it will work. Second, it will help flesh out my senior project report if I can at least explain it in lamens terms.

Heres what I want: Current FL of triplet 457mm. Target FL 400-415.

I'll take whatever improvement I can get, but those numbers would allow me to fill the screen entirely.

Thanks for your help,
Brian

Some have moved the rear leans closer to the middle lens to make the fl longer so moving the rear lens further should make the fl a little shorter. You can't really make large changes without having bad side effects.

There is a little extra room in the lens housing so adding a small spacer might get you close to 400mm fl.

Mark the lens so you don't flip them.

So does anyone know of a good site with triplet theory... or is there a ray trace of how light goes through triplets out there? I want to understand a little about why moving the lens this way or that has a particular effect.

The EFL of multiple singlet lens elements is additive based on the individual FLs, see the "Multiple Lenses" section on this page.
http://en.wikipedia.org/wiki/Lens_(optics)

A typical PJ triplet consists of a postive FL lens, a negative FL lens, and another positive FL lens.
Designing a lens using multiple elements(aside from simple or symmetric doublets) is very complex.

Here's some stuff to help you out.

Here's a basic way to change the focal length of a lens.
Click to view attachment
It came from Anchoroptics.com in the download section. It is from the PDF titled "How to Build Opaque Projectors". That PDF is full of great information about optics. If you are wondering why Anchor has all of the old Edmund Scientific stuff, it is because they are a subsidiary of Edmund Optics, and sells their seconds and over-runs at a deep discount.

There is a three element (triplet) focal length calculator at the bottom of this page.....3-Element Focal Length Calculator. You give it the focal lengths of the three lenses, the distances between them and it tells you what the effective focal length of the triplet is. I know it says it is an eyepiece calculator, but the formulas are correct no matter what the end use is.

One cool tool is this Geometric optics applet. It lets you add lenses and change their focal length and positions with click-and-drag and see how the path of the light is affected. Here's what it looks like....
Click to view attachment

Have fun.

Tgreenwood

Thanks for the info and the sites guys.... on vacation in Daytona right now, but I will look into it deeper when I get back.