I'm thinking of getting one of these for my 19" lcd monitor and would
appreciate some information.
What is the best groove pitch and focal length to get for this?
Thank you
MILO

Most people go with the F550 from 3dlens.com
Here are the specs. The smaller the groove pitch the better, but I
don't know how easy it will be to find less than 0.2mm.

Overall size: 320mm x 400mm
Fresnel size: 320mm x 400mm
Thickness: 2mm
Material: optical acrylic
Groove pitch: 0.2mm (127 grooves per inch)
Focal length: 550mm

Milo Johnson wrote:

rqk,
Thank you very much for the info as it answered my question 100% Not often
that happens

MILO
"rqk" <na (AT) na (DOT) com> wrote

I just want to point out that smaller pitch is not necessarily
better. In fact it's probably worse. More grooves mean more
divisions in the lens, and each division scatters light. There is no
way around it. I've learned a bit about lenses while pursuing another
of my hobbies - astronomy

Honestly, the 8.5x11 full page magnifier you can get from Staples or
any office supply store with probably 50 grooves per inch is more
pleasant to look through than the F550. But, the F550 is not bad. I
have one and used it all the time before I got a TH2Go. If I was
going to get another lens I'd be looking for a larger groove pitch.

Pat Dotson


On Mar 17, 3:28 pm, "Milo Johnson" <Mil... (AT) bresnan (DOT) net> wrote:

I thought that smaller pitch was better because it would be altering
smaller divisions of the screen and make the transition smoother, almost
as if the lens were completely smooth as if it had an "infinite" number
of grooves. I thought a bigger dot pitch would make things more grainy.

Oh well, learn something new everyday. Thanks.




pdotson (AT) mindspring (DOT) com wrote:

On Mar 18, 11:46 am, rqk <n... (AT) na (DOT) com> wrote:

A normal lens is not segmented like a Fresnel lens.

Imagine you had a top quality lens or mirror for a telescope. Now, if
it were possible, you start slicing that perfect lens up into
concentric circles to create a Fresnel lens. Each slice you put into
the lens will reduce the quality of the resulting view through the
lens because each new division is going to scatter the light particles
that hit it.

More divisions = more scattered light = more glare and less clarity in
the view.

Now, you could have a super-high quality fine pitch Fresnel that could
look better than a poorly made coarse pitch Fresnel, but that's a
different issue. Assuming decent quality, a coarser pitch Fresnel
lens should look better with less glare.

Pat Dotson