Ok. I have a standard unsplit OHP fresnel lens. I once tried to measure its FL buy doing this.
1. Turn on the overhead lamp in my room
2. Place the fresnel about 15 inches off the ground holding in the air directly below the lamp.
3. Start droping the fresnel slowing until you see the bulb arc sharpest on the floor.
4. The distance from the floor to the fresnel is the fresnels combined FL.
So as I understand this is the ideal FL of my objective to match correct? There is some give here in the FL of the objective assuming the beam fits inside the clear diameter of the objective lens. Correct?
Ok. So who do i determine the back FL or the FL between lamp and fresnel?
1. Turn on the overhead lamp in my room
2. Place the fresnel about 15 inches off the ground holding in the air directly below the lamp.
3. Start droping the fresnel slowing until you see the bulb arc sharpest on the floor.
4. The distance from the floor to the fresnel is the fresnels combined FL.
So as I understand this is the ideal FL of my objective to match correct? There is some give here in the FL of the objective assuming the beam fits inside the clear diameter of the objective lens. Correct?
Ok. So who do i determine the back FL or the FL between lamp and fresnel?
better way for unsplit fresnels
If it is from an OHP, the upper fresnel probably has a fl of 330 mm. Put a small (ie flashlight) bulb 330 mm from the fresnel. Move a piece of white paper around on the other side. See if you get a focussed image of the bulb 200 mm or 220 mm from the fresnels. If not, try flipping the fresnels over.
You can experiment with other distances, but a pair of fresnels will work best with the object or image at the focal length from each fresnel. If you try to go far from those angles, you will lose a lot of your light.
Let us know what you observe!
If it is from an OHP, the upper fresnel probably has a fl of 330 mm. Put a small (ie flashlight) bulb 330 mm from the fresnel. Move a piece of white paper around on the other side. See if you get a focussed image of the bulb 200 mm or 220 mm from the fresnels. If not, try flipping the fresnels over.
You can experiment with other distances, but a pair of fresnels will work best with the object or image at the focal length from each fresnel. If you try to go far from those angles, you will lose a lot of your light.
Let us know what you observe!
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