For everyone (all of us) trying to pump up the brightness of their display, you may also want to take a look at your optics and see if you can get a similar focal length lens to whatever you're currently using, but with a shorter F/ratio.
Case in point is going from an F/5.6 projection lens to an F/4 lens means your projected image is twice as bright. If you aren't familiar with lens F/ratios here is a quick summary of major F/ratios. For each successive ratio going down, the light is cut in half, so F/5.6 delivers only 25% of the light of F/2.8 F/ratio is easy to figure out, you just divide the focal length of the lens by the diameter of the lens, so a 300mm focal length, 60mm diameter lens is F/5.0
F/1.0
F/1.4
F/2.0
F/2.8
F/4.0
F/5.6
F/8.0
F/11.0
I realize many of you probably know all this but I haven't really seen much discussion on it so I post it here for anyone that may not.
Case in point is going from an F/5.6 projection lens to an F/4 lens means your projected image is twice as bright. If you aren't familiar with lens F/ratios here is a quick summary of major F/ratios. For each successive ratio going down, the light is cut in half, so F/5.6 delivers only 25% of the light of F/2.8 F/ratio is easy to figure out, you just divide the focal length of the lens by the diameter of the lens, so a 300mm focal length, 60mm diameter lens is F/5.0
F/1.0
F/1.4
F/2.0
F/2.8
F/4.0
F/5.6
F/8.0
F/11.0
I realize many of you probably know all this but I haven't really seen much discussion on it so I post it here for anyone that may not.
LuisGonzalezLT
I find your information very interesting and easy to understand.
Optics is a very dry subject for some and not all that easy to follow.
In your example above if one replaced the 60mm diameter lens with a 150 mm dia lens one could expect an F2.0 which presumably would pass considerably more light than an F5.0
I find your information very interesting and easy to understand.
Optics is a very dry subject for some and not all that easy to follow.
In your example above if one replaced the 60mm diameter lens with a 150 mm dia lens one could expect an F2.0 which presumably would pass considerably more light than an F5.0
Your argument is not really true for a projector. It is of course absolutely true for a camera.... well in general at least. In a camera, the lower the F-Stop, the more light you are collecting. That assumption is of course cased on the light radiating out evenly from a point and of course extending beyond the edges of the lens...... if you were to shine a laser into a camera, the F-Stop may have absolutely no effect on how bright that laser appears when imaged. Think of a simple single lens with F-stop controlled by an iris. If the laser is shining on the center of the lens, the iris will not effect the brightness unless the iris actually starts to block the laser.
You can now think of projectors as working in reverse. The condensing optics, i.e. a Fresnel lens, will act to condense all the light onto the lens. If your system is able to direct all the generated light into the output beam by way of design, the actual speed (F-stop) of the projection lens will not be a critical factor.
There are other factors that effect F-Stop including transmissivity, surface reflectance, etc. Of course optimizing these figures will increase the overall light output, possibly significantly and may also result in a better image (lower surface reflectivity can lead to higher contrast ration as can higher transmissivity).
In the example above, if the 60mm lens had a really good AR coating, and was high transmissivity glass, in a properly designed projection system, it may actually pass more light than a 150mm lens.
Alvaius
You can now think of projectors as working in reverse. The condensing optics, i.e. a Fresnel lens, will act to condense all the light onto the lens. If your system is able to direct all the generated light into the output beam by way of design, the actual speed (F-stop) of the projection lens will not be a critical factor.
There are other factors that effect F-Stop including transmissivity, surface reflectance, etc. Of course optimizing these figures will increase the overall light output, possibly significantly and may also result in a better image (lower surface reflectivity can lead to higher contrast ration as can higher transmissivity).
In the example above, if the 60mm lens had a really good AR coating, and was high transmissivity glass, in a properly designed projection system, it may actually pass more light than a 150mm lens.
Alvaius
the f/Stop is the ratio of the lens diameter to the focal length. the iris reduces the effective lens diameter.
well, they used to teach optics in high school and college in the States, now it's not part of the syllabus. It is dry, methodical, mechanical...
well, they used to teach optics in high school and college in the States, now it's not part of the syllabus. It is dry, methodical, mechanical...
Aaahhhh
when the light is being collimated then condensed into the lens...
If your projection system isn't a condensing design, such as using a CRT, which I have done before, it makes quite a difference. A remember an old projector I used to have 15 years ago that used a 9" TV, the lens was made of two 8" diameter fresnels spaced about 12 inches apart, with a very fast F/ratio. Plopping a little 60mm diameter lens on that system wouldn't get you very much light output. (aka. those dastardly 100" TV ads all over ebay - ack)
Although I still don't think I would stick a 5mm diameter F/60 lens on my projector.
I haven't finished making a projector so I'm new to these types of optics - but having been a photographer for decades I guess I have a slant toward applying inverse square to every optical system.
I do have a 9" F.L. F/5.6 lens I was going to use on the projector that has a built-in iris that stops it down to F/11 Once I have it built I'll play around with the iris and make some measurements with the light meter at the display end and see what kind of an effect, if any, it has.
when the light is being collimated then condensed into the lens...
If your projection system isn't a condensing design, such as using a CRT, which I have done before, it makes quite a difference. A remember an old projector I used to have 15 years ago that used a 9" TV, the lens was made of two 8" diameter fresnels spaced about 12 inches apart, with a very fast F/ratio. Plopping a little 60mm diameter lens on that system wouldn't get you very much light output. (aka. those dastardly 100" TV ads all over ebay - ack)
Although I still don't think I would stick a 5mm diameter F/60 lens on my projector.
I haven't finished making a projector so I'm new to these types of optics - but having been a photographer for decades I guess I have a slant toward applying inverse square to every optical system.
I do have a 9" F.L. F/5.6 lens I was going to use on the projector that has a built-in iris that stops it down to F/11 Once I have it built I'll play around with the iris and make some measurements with the light meter at the display end and see what kind of an effect, if any, it has.
Luis
if you are in Cleveland, Electronic Surplus on Broadway near 55th has had some great buys on lenses -- for macro etc. I don't know about color correction etc. The lens may be valuable just for the Iris assembly.
if you are in Cleveland, Electronic Surplus on Broadway near 55th has had some great buys on lenses -- for macro etc. I don't know about color correction etc. The lens may be valuable just for the Iris assembly.
Lens with Iris
I got the thing for only $20. It's an Ilex Paragon 9" F/5.6. They usually go for $60 or so. Got lucky on it. I will be reselling it, though, since I picked up a very nice Carl Zeiss S-Tessar 300mm F/5.6 lens which will be my permanent projector lens. The 300mm has an image circle large enough to easily cover a 15" diagonal panel and it has excellent coatings and performance. That one ran me $51 but it is a much better lens than the Ilex.
I got the thing for only $20. It's an Ilex Paragon 9" F/5.6. They usually go for $60 or so. Got lucky on it. I will be reselling it, though, since I picked up a very nice Carl Zeiss S-Tessar 300mm F/5.6 lens which will be my permanent projector lens. The 300mm has an image circle large enough to easily cover a 15" diagonal panel and it has excellent coatings and performance. That one ran me $51 but it is a much better lens than the Ilex.
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