Hi Guys/gals?!
Fantastic resource here. However cant seem to find anything on measuring the focal length of a Projection Lens. I have a Projection Lens taken out from an unused OHP but cannot plan my diy projector without this information.

Help please...?
thanks!

so take a 2D image (preferably an overhead), project it through the lens to the wall. move the object until you get a sharp image on the wall. You have the object-to-lens distance, lens-to-wall distance.
from here:

One thing you can use to do this is a lamp with a low-watt clear bulb, like 60 or less watt. Move lamp and triplet until you get a sharp image of the lamp's filament on the wall, then measure distances.

Although this is a good way to get a FL of a lens, it won’t be accurate to find the exact FL of a multi-element lens like a triplet. The problem is that the thin lens equations don’t take into consideration the distance between the lens elements so there will be some error. But it will be close.
To get a bit closer you can use the object and image distance you have suggested but also use a known dimension marked on the 2D image, perhaps 100mm or so, and then measure how much magnification has occurred. Then with some extrapolation between the two equations you can calculate the FL. It takes quite a few measurements though.

DJ

DAZZZLA your method is far way better. other thing is, that of course, several measurements should be taken, i.e. 30-60, then compute mean value and deviation. if deviation is low, the mean value is somewhat accurate

Hey..You guys are great. Thanks very much for the help..now just need to understand it and then try it out. I've tried the wall method, i think its much easier altho not as 'accurate' ,but what the h*** we're suppose tweak tune and adjust until its perfect., right?

Now, how do I measure the FL for an UNsplit fresnel? So if I dont have that measurement, how do I position the unsplit fresnel vis-a-vis the light source? I will be putting it before the lcd right?

Since my fresnel set came from an OHP, its stuck together, I'll go with unsplit design and see how it goes.

So useing the formula 1/s1+1/s2=1/f
If s1=144" and s2=14"
then should it be 1/144"+1/14"=1/f

so 1/f=0.078373015873015873015873015873016?
and f=12.759493670886075949367088607598?

or does 1/f=0.012046485260770975056689342403628?
and f=83.011764705882352941176470592564?

Did I miss something? Am I not getting this?

I feel You have used the diagonal of your LCD and the diagonal of the projected screen. The formula was about LCD-to-lens distance and lens-to-wall distance. You may use DAZZLA's method if you already have the diagonals. Let's Have your LCD's diagonal DLCD and your your projection's diagonal Dproj. And let S is the distance between the LCD and the wall. From here:
f=S*DLCD*Dproj/((DLCD+Dproj)*(DLCD+Dproj))

Important note. DLCD, Dproj, S should be measured in the same units, i.e. both of them should be millimeters, inches or feets - and you will get the focal length according to.

That's correct. 12.75 in focal length.



Nope. Where did these numbers come from?

Tgreenwood

1/s1 + 1/s2 = 1/f is very true.
easier would be take the projection lens or the fresnel in daylight (preferably at noon) and try to get a clear image of sun on the ground and then measure the distance of the lens from ground. it is very easy but the results are not accurate but acceptable. donot focus it on a flamble object, like dry grass, paper or wood you might end up burning it.

Just to say that fresnel lenses from OHP, are ALWAYS(i havent seen any other) combination of 220 for back, and 330 for front fresnel, the only diference is output lens(triplet, or a single).Fokus for certain size is determined by the distance of the lamp from the first fresnel.Usualy (90%)lenses for OHP are adjusted for close distances, so it,s hard if you want to make smaler picture projection on longer distances(more than 3-4 meters).