what's the deal? this is the way i test fl:
i light a candle, set it on the kitchen counter. pull about 20 inches out on the tape measure, set on countertop align it with candle. then i hold my triplet or whatever lens i'm testing between candle and far wall. when large focused upside down flame is on the wall i check distance between flame and lens. 13" approximate 330mm. this works for me.
double checking, i test on adjacent wall, which is about 5 feet from candle. now i've got a distance of about 18" between flame and lens before big flame is focused on wall.
also, checking the relationship between fresnel and triplet, the triplet has to be about 18" from fresnel before producing a sharp image of flame right side up. this is on either wall. the fresnel is double sided 3M 310x310mm supposed to be approximately 13" fl on both sides, but, candle on one side of fresnel, piece of paper on other side. when candle is about 220mm from fresnel, upside down image on paper becomes focused at about 13" from fresnel. i move the candle to 13" away from fresnel and have to move paper closer to about 220mm to focus image. is this right?
now that all the info is here i'll try to throw out all my questions.
first, is the 18" between fresnel and triplet going to be a problem since triplet fl is approximately 330mm?
because i'm not trying to project the light source on the wall, but the lcd image, is focusing the light source an accurate or pointless test/what does this prove?
i have another 220mm fl collinating fresnel. what would be the advantages/disadvantages of building a split design using this lens with the other lens?
i think that's it for now. i know somebody will post saying this has already been discussed and to search before posting. i know this because i already searched and every thread that looked promising had that response or the thread disintegrated into "how's that Ushio working for you? any more problems?" before my questions were answered. thanks for any help. i'll try to keep any more posts to a minimum.
i light a candle, set it on the kitchen counter. pull about 20 inches out on the tape measure, set on countertop align it with candle. then i hold my triplet or whatever lens i'm testing between candle and far wall. when large focused upside down flame is on the wall i check distance between flame and lens. 13" approximate 330mm. this works for me.
double checking, i test on adjacent wall, which is about 5 feet from candle. now i've got a distance of about 18" between flame and lens before big flame is focused on wall.
also, checking the relationship between fresnel and triplet, the triplet has to be about 18" from fresnel before producing a sharp image of flame right side up. this is on either wall. the fresnel is double sided 3M 310x310mm supposed to be approximately 13" fl on both sides, but, candle on one side of fresnel, piece of paper on other side. when candle is about 220mm from fresnel, upside down image on paper becomes focused at about 13" from fresnel. i move the candle to 13" away from fresnel and have to move paper closer to about 220mm to focus image. is this right?
now that all the info is here i'll try to throw out all my questions.
first, is the 18" between fresnel and triplet going to be a problem since triplet fl is approximately 330mm?
because i'm not trying to project the light source on the wall, but the lcd image, is focusing the light source an accurate or pointless test/what does this prove?
i have another 220mm fl collinating fresnel. what would be the advantages/disadvantages of building a split design using this lens with the other lens?
i think that's it for now. i know somebody will post saying this has already been discussed and to search before posting. i know this because i already searched and every thread that looked promising had that response or the thread disintegrated into "how's that Ushio working for you? any more problems?" before my questions were answered. thanks for any help. i'll try to keep any more posts to a minimum.
focal length
Here's how to find out what the focal length of your lens is.
The distance from the lens to the object you are projecting
is called the object distance and given the symbol o.
The picture on the screen is called the image. The distance
from the lens to the image is called the image distance and
given the symbol i.
The focal length is given the symbol f.
The formula you have to use is : 1/o + 1/i = 1/f
So in your case the formula would be (1 divided by the distance from the candle to the lens) + (1 divided by the distance from the lens to the image) = (1 divided by the focal length).
Using your example,
1/o + 1/i = 1/f
(1 divided by 18 inches) + (1 divided by 42 inches) = 1/f
0.0555 + 0.0238 = 1/f
0.0793 = 1/f
12.6 inches = f ( 320 mm )
If you hate math as much as most people, here's an easier way.
🙂 Use sunlight.
Since the sun is so far away, the part of the formula 1/o is so close to zero that you can ignore it. The formula then becomes 1/i = 1/f.
You can go outside on a sunny day with your lens and a tape measure, focus the sun on the ground (concrete
) with your lens and measure the distance from your lens to the ground. That distance is the focal length of the lens.
Here's how to find out what the focal length of your lens is.
The distance from the lens to the object you are projecting
is called the object distance and given the symbol o.
The picture on the screen is called the image. The distance
from the lens to the image is called the image distance and
given the symbol i.
The focal length is given the symbol f.
The formula you have to use is : 1/o + 1/i = 1/f
So in your case the formula would be (1 divided by the distance from the candle to the lens) + (1 divided by the distance from the lens to the image) = (1 divided by the focal length).
Using your example,
1/o + 1/i = 1/f
(1 divided by 18 inches) + (1 divided by 42 inches) = 1/f
0.0555 + 0.0238 = 1/f
0.0793 = 1/f
12.6 inches = f ( 320 mm )
If you hate math as much as most people, here's an easier way.
🙂 Use sunlight.
Since the sun is so far away, the part of the formula 1/o is so close to zero that you can ignore it. The formula then becomes 1/i = 1/f.
You can go outside on a sunny day with your lens and a tape measure, focus the sun on the ground (concrete
) with your lens and measure the distance from your lens to the ground. That distance is the focal length of the lens.i love math. and i love you. this helps me greatly. so, my method just happened to be coincidental and not at all accurate? the relationship between the countertop and the wall just happened to give me o as 13" which i mistook for the fl because that was the number i was looking for.
so, what about the distance from the triplet to the fresnel? that becomes o, right? i still don't understand why i'm getting 18" between the two if both fl are around 320mm. unless the distance for o is still from the candle to the triplet even though the fresnel is between them. i think i may have just answered my own question. thanks again for helping so quickly. hopefully the next thread i start will have something to do with results.
so, what about the distance from the triplet to the fresnel? that becomes o, right? i still don't understand why i'm getting 18" between the two if both fl are around 320mm. unless the distance for o is still from the candle to the triplet even though the fresnel is between them. i think i may have just answered my own question. thanks again for helping so quickly. hopefully the next thread i start will have something to do with results.
two different systems
It is too confusing to work out the whole lamp-to-screen as one system. Think of it as two different simple systems:
system 1) The lamp arc is the object, the fresnels are the lens, and the projected image is the image of the lamp arc. If you place your lamp arc 220 mm from the fresnels, then you will get an arc image 330 mm from the fresnels.
system 2) The LCD is the object, the 320 mm triplet is the lens, and the screen is the plane of the image. The same equation applies to each of these systems. So with a throw distance (lens to screen) of 10 feet, you get:
1/320 mm = 1/3048 mm + 1/lcd-to-lens
lcd-to-lens = 358 mm
To get the most light through the projection lens, you need to focus the arc image at the center of the projection lens. The 330 mm focal length of the field fresnel almost gets it there, but not quite. If you move the lamp arc closer to the condensor fresnel, then that will make the arc image focus at a greater distance from the field fresnel. Since you only want to move it out to about (358 + 20) = 378 mm, you will not have to move the lamp arc by much. You can calculate the correct distance with that same equation:
1/combined-fresnel-focal-length = 1/fl1 + 1/fl2 = 1/220 mm + 1/330 mm = 1/132 mm
1/132 mm = 1/378 mm + 1/arc-to-fresnel
arc-to-fresnel distance = 203 mm
Or you can make a split-fresnel design: You put the 220 mm fl fresnel 220 mm from the lamp arc, then 10-20 mm of air space to the LCD. Then you put the 330 mm fl fresnel about 20 mm after the LCD. That would put the arc image about (330 + 20) = 350 mm from the LCD. That is very close to the 358 mm LCD-to-lens distance. It might work perfectly just the way it is, or you might get a bit more light on the screen by moving the lamp arc a few mm closer to the condensor fresnel.
You can try it both ways to see which one you like better. Just make sure you adjust the lamp arc position for optimum light.
It is too confusing to work out the whole lamp-to-screen as one system. Think of it as two different simple systems:
system 1) The lamp arc is the object, the fresnels are the lens, and the projected image is the image of the lamp arc. If you place your lamp arc 220 mm from the fresnels, then you will get an arc image 330 mm from the fresnels.
system 2) The LCD is the object, the 320 mm triplet is the lens, and the screen is the plane of the image. The same equation applies to each of these systems. So with a throw distance (lens to screen) of 10 feet, you get:
1/320 mm = 1/3048 mm + 1/lcd-to-lens
lcd-to-lens = 358 mm
To get the most light through the projection lens, you need to focus the arc image at the center of the projection lens. The 330 mm focal length of the field fresnel almost gets it there, but not quite. If you move the lamp arc closer to the condensor fresnel, then that will make the arc image focus at a greater distance from the field fresnel. Since you only want to move it out to about (358 + 20) = 378 mm, you will not have to move the lamp arc by much. You can calculate the correct distance with that same equation:
1/combined-fresnel-focal-length = 1/fl1 + 1/fl2 = 1/220 mm + 1/330 mm = 1/132 mm
1/132 mm = 1/378 mm + 1/arc-to-fresnel
arc-to-fresnel distance = 203 mm
Or you can make a split-fresnel design: You put the 220 mm fl fresnel 220 mm from the lamp arc, then 10-20 mm of air space to the LCD. Then you put the 330 mm fl fresnel about 20 mm after the LCD. That would put the arc image about (330 + 20) = 350 mm from the LCD. That is very close to the 358 mm LCD-to-lens distance. It might work perfectly just the way it is, or you might get a bit more light on the screen by moving the lamp arc a few mm closer to the condensor fresnel.
You can try it both ways to see which one you like better. Just make sure you adjust the lamp arc position for optimum light.
- Status
- Not open for further replies.