The first method seems to reflect the manufacturer's specs. Since we're dealing with beams of light in our PJs, does the measurement from the "sun method" have any bearing on our PJ's design?
Full Version: Proper Way To Measure Fresnel's Focal Length
In searching the forums, I've come across two different ways to measure a fresel's focal length. The problem is that the two methods give different results. Using fresnels of known focal length, I've focused the lenses on a distant objects and projected the images on the wall. Measuring the distance from the wall to the fresnel, I get a figure approximately equal to the advertised focal length of the fresnel. However, I've also read you should go outside, point the fresnel at the ground, focus the sun to a pinpoint, and measure the distance from the fesnel to the ground. This gives me a much shorter reading. For instance, a known 220mm fresnel gives a measurement of about 220mm using the first method, focusing an image on the wall. But, the same fresnel creates a pinpoint from the sun at only 140mm from the ground. Other fresnels I have produce similar results.
The first method seems to reflect the manufacturer's specs. Since we're dealing with beams of light in our PJs, does the measurement from the "sun method" have any bearing on our PJ's design?
The first method seems to reflect the manufacturer's specs. Since we're dealing with beams of light in our PJs, does the measurement from the "sun method" have any bearing on our PJ's design?
To be honest, I don't think the fist method you mention is an accepted way to find the focal point. At least, I don't think I've heard of it.
The sun method should yeild accurate results. Make sure you're measuring with the grooves facing the sun.
As I understand it, the "focal point" is quite literially the point at which parallel light is focused to a point. Sun light is probably just about as parallel as we're going to get (without specialized gear). So, take the sun's parallel light, find out how far away the light is focused and you've got the focal point.
The main concern for most people here is determining which fresnel is which. The "put 2 similar objects behind the lenses and see which one looks bigger" test is the easiest, but you don't get to start fires instantly unless you're using the sun... life is a compromise.
The sun method should yeild accurate results. Make sure you're measuring with the grooves facing the sun.
As I understand it, the "focal point" is quite literially the point at which parallel light is focused to a point. Sun light is probably just about as parallel as we're going to get (without specialized gear). So, take the sun's parallel light, find out how far away the light is focused and you've got the focal point.
The main concern for most people here is determining which fresnel is which. The "put 2 similar objects behind the lenses and see which one looks bigger" test is the easiest, but you don't get to start fires instantly unless you're using the sun... life is a compromise.
I've got a problem then. I have 2 Lumenlab 220mm fresnels, and I can focus sunlight to a point with the fresnel ~140mm from the ground. I literally burned asphalt with them! 
My Lumenlab 317mm fresnel focuses sunlight to a point at way under 300mm. I have the grooves facing the sun. What, if anything, am I doing wrong. I have some unmarked fresnels and am trying to determine their focal lengths. I was using my LL fresnels for reference.
My Lumenlab 317mm fresnel focuses sunlight to a point at way under 300mm. I have the grooves facing the sun. What, if anything, am I doing wrong. I have some unmarked fresnels and am trying to determine their focal lengths. I was using my LL fresnels for reference.
The sun method will be the closest. The small point of light on the ground is an image of the sun so the two methods are actually the same except the object, the sun, is much further away. Try again but use a welding mask or very dark sun glasses to look at the suns image. You can also try stopping it down so that you are only using the center 50mm of the fresnel.
Also if you are using a 220mm pro fresnel, I measured it at 200mm.
DJ
Also if you are using a 220mm pro fresnel, I measured it at 200mm.
DJ
Along these lines, I had a thought about measuring the FOV of a triplet.
I know, I know, thinking will just get me in trouble.
Find yourself a brightly lit window with sheer or lace drapes or such. Set up the triplet on the arm of a chair or what have you. Move closer to/farther from wall or whatever surface you're "projecting" upon.
When the image is in focus, shouldn't you just be able to measure the diameter of the circle of light to get the FOV? Or am I up too late?
The drapes are just to give some reference to focus.
I know, I know, thinking will just get me in trouble.
Find yourself a brightly lit window with sheer or lace drapes or such. Set up the triplet on the arm of a chair or what have you. Move closer to/farther from wall or whatever surface you're "projecting" upon.
When the image is in focus, shouldn't you just be able to measure the diameter of the circle of light to get the FOV? Or am I up too late?
The drapes are just to give some reference to focus.
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