Hi Guys.
So i found a couple of condeser lenses going cheap on e-bay the other day and figured what the hell i may aswell give them a go. as above however the fl wasn't put in the ad so they may be totaly unsuitable but for the price they where worth a shot.
now i'm sure this question has been asked before and answered before and i have spent the last couple of hours searching for a definative answer but most of what i came across went straight over my head! (yeah i am a little slow! )
so what i can say is that i have two lenses.

Lens 1
65mm in diameter
3mm thick at the edge
around 14mm thick at the center

Lens 2
65mm in diameter
5mm thick at the edge
around 16mm thick at the center.

Do these sound remotely useable?
How can i calculate the fl?

If it is of any help i will be using a 15.4" screen and the old 500mm pro triplet and currently have a 22mm rear fresnel but may up to a 330...

thanks for any help. as i said i did search but it is taking me a while to get my head around this stuff...

Calculate the FL by focusing an image of the sun on the sidewalk.

Thanks for the reply supraguy.
That is the technique i saw mentioned however i saw mentioned only using a small area of the lens?
i also assume i will need to hold the flat side of the lens toward the sun? and i will be measuring from the center of the spherical side to the image?

Thanks again for the help.

Ps.. Hows the car running? have you had it out yet this year or is it too cold up by you?

Actually, the flat side goes toward the ant (or whatever you're scorching.) The curved side towards the sun, measure from the flat side to the sidewalk.

The car's running fine. I drive it year-round, actually. The snow tires are still on (Except for one which got killed yesterday, so I guess it's 3 winter tires and the spare) I have different settings for things in winter too. Lower boost, less power. Kind of a shame sometimes with that nice cold, dense air, but it doesn't help to have a lot of power when there's little or no trction.

Ahh ok thats making a bit more sense now so i am goinjg to be focusing the sun into a point and measuring the distance..... ok i think i get it.
good to hear the cars running well. you gotta love turbo's for the flexibility they can give you wilth an ems/boost controller.
Its probab;ly down to the fact that we have quite mild winters in the UK but you dont find anyone using winter tires here. Which is a shame because i'm sure it would reduce the number of nice cars i see wrapped round trees/lamposts this time of year!

Anyway thanks again for the help.

Mark

No answers for months huy? The diameter is way too small. You need something like 5 inch diameter to cover the entire beam. Your going to be placing it about 1-1.5 inches from the arc. Im planning on doing the same thing using a 330 fresnel as collimator and even out the light from the side compared to the center (inverse square law)
So have you bought yourself the condenser yet?

LOL i just came across your reply i had pretty much lost track of this thread....

thanks for the advise.
I didn't find the right lens yet. i still have lots to buy for the projector aswell. right now i'm just taking my time keeping an eye out for bargains.
How bout you.. any progress?