I've built my pj with standard Lumenlab triplet. As far as I remember, its "brightness" (or whatever this feature is called in English), is something about 1:3.5. Is there a chance to find something significantly brighter, with similar focal lenght? Am I right, that it needs to be larger (wider) to obtain higher brightness? If so, what would be a possible source of such lens?
Regards
Regards
My understanding is that, for our projecting arrangement, a lens significantly larger than the lamp arc will not necessarily result in a brighter image. A larger lens will help most for projecting a diffuse light source, as in an opaque projector.
Try this experiment: set up the projector for correct focus and distance. Then open it up and tape a circle of paper to the inside of the projection lens. If the condensed light coming from the fresnel is fully within the circle, a larger lens will not significantly increase the brightness.
Try this experiment: set up the projector for correct focus and distance. Then open it up and tape a circle of paper to the inside of the projection lens. If the condensed light coming from the fresnel is fully within the circle, a larger lens will not significantly increase the brightness.
Another way to figure out if the size of your projection lens is big enough is to calculate the magnification the arc goes through. Like this:
field fresnel / back fresnel = magnification of arc
Field fresnel is the one closest to the projection lens. Back fresnel is closer to the bulb.
Measure the arc length, multiply it by the magnification. Your projection lens should be bigger than this number to get the most light.
Ex:
220 Back Fresnel Lens
550 Field Fresnel Lens
27mm Arc length
550 / 220 = 2.5
27mm * 2.5 = 67.5mm
So a 135mm lens obviously (if aligned correctly) is big enough to capture all the light. 80mm would probably be good too. Just remember that as you move towards the edge of the projection lens, light becomes less focused. So a 68mm lens wouldn't do the job well.
field fresnel / back fresnel = magnification of arc
Field fresnel is the one closest to the projection lens. Back fresnel is closer to the bulb.
Measure the arc length, multiply it by the magnification. Your projection lens should be bigger than this number to get the most light.
Ex:
220 Back Fresnel Lens
550 Field Fresnel Lens
27mm Arc length
550 / 220 = 2.5
27mm * 2.5 = 67.5mm
So a 135mm lens obviously (if aligned correctly) is big enough to capture all the light. 80mm would probably be good too. Just remember that as you move towards the edge of the projection lens, light becomes less focused. So a 68mm lens wouldn't do the job well.
Thanks for responding. But my lens is not 135mm in diameter, it's about 60mm. I mentioned the number 1:3.5, which is the "optical brightness", or "f-stop" value for this lens, not its dimension.
I've made a test replacing a whole front cover with a piece of paper, light spot seems to be small enough to fit this 60mm lens. But I don't know the exact meaning of this f-stop value. Is it possible, that two similar lens, with same dimensions, same number of glass pieces, same focal lenghts, etc., would vary significantly, regarding this "brightness" parameter? May be I'm totally wrong and looking for something, that doesn't exist.
Regards
Pawel
I've made a test replacing a whole front cover with a piece of paper, light spot seems to be small enough to fit this 60mm lens. But I don't know the exact meaning of this f-stop value. Is it possible, that two similar lens, with same dimensions, same number of glass pieces, same focal lenghts, etc., would vary significantly, regarding this "brightness" parameter? May be I'm totally wrong and looking for something, that doesn't exist.
Regards
Pawel
From what I understand, F-Stop is the widest your lens' iris will open up (ie the diameter of the lens). The wider the (F-stop) iris, the more light comes into the lens -> the brighter the image.
As far as I know:
f-stop = (lens diameter) / (lens focal length)
So in your case:
1 / 3.5 = 60 / FL
So your focal length would be 210mm.
I'm not an expert on camera lenses, but I have been reading about them extensively. I think this is correct. Please correct me if I'm wrong.
As far as I know:
f-stop = (lens diameter) / (lens focal length)
So in your case:
1 / 3.5 = 60 / FL
So your focal length would be 210mm.
I'm not an expert on camera lenses, but I have been reading about them extensively. I think this is correct. Please correct me if I'm wrong.
I was wondering superdavumo about the effect of the condenser lens on the image arc size.
considering
field fresnel / back fresnel = magnification of arc
does the addition of a condenser lens modify this magnification?
I think so
and how to calculate? we need to take into account the distance between condenser and back fresnel??
If you have an idea on that particular point this would be great
considering
field fresnel / back fresnel = magnification of arc
does the addition of a condenser lens modify this magnification?
I think so
and how to calculate? we need to take into account the distance between condenser and back fresnel??
If you have an idea on that particular point this would be great
Well, I honestly don't remember (took physics a while ago). At first I thought that multiplying each lens' magnification would do it. But, it doesn't come out right for an odd number of lenses:
27mm * (1 / 220mm) * (550mm) = 67.5mm (original without condenser lens)
27mm * (1 / 220mm) * (550mm) * (1 / 25mm) = 2.7 but with no units (ie, mm) using dimensional analysis. Assuming the FL of the condenser lens was 25mm.
This leads me to believe that the math gets more complicated when you introduce a third lens. My GUESS, since the purpose of the condenser lens is to spread the light out on the back fresnel lens, is that the arc would be magnified even more.
Someone who knows the math behind 3+ lenses please jump in here.
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