135mm long-throw lense kit from diylabs

[ace3000_1]

In my test i did not measure till it get distorted, i measured till it stoped (mean nome degrees more ligth was stoped, no light passing throw the lens). My measurement was 120 full angle (60 + 60 half angles) the same 32cm focal triplet as you. Well, can we conclude anyting? i would say no.

LOL, i think mines fairly right, from memory the specs on the diylabs triplet has somthing like a 70 or 80deg FOV so i must be close. I dont do it till it like starts to distort or where the light cuts out, you see a sweet spot that all of a sudden drops away very rapidly and thats where i took my measurment, they wer combined angles aswell not per side.

Its a shame your not in madrid as i could show you what i mean and how i do it. Ive got some big condensers here that you can clearly see the FOV with your eyes, that same FOV i see with my eyes i soon see on the screen , and see what the lens has as in limits in the real world environment. Prety much the shorter the focal and the larger the lens is the wider the FOV (not counting projection lenses with varible FOV's ). One thing i think you may be mixed up on is the clear arperature and the FOV in your measuring, to this day i havnt found a lens with a full side to side FOV and you wont either, its typical of them having a ratio of 3 arperature and 2 FOV so 2/3 of the lens is FOV. Though not aways somtimes its higher other times its less, it just depends on the design and type of the lens.

In the manufactures test labs im sure alot of them use lasers for the fov (did read that somwhere though ages ago), they use paralelle light for the focal finding. There is a formula for the FOV on a single lens, stuffed if i know what it is on a multi ellement lens, kinda gets complicated then.

Trev

[Guy Grotke]

Rox,

You DID determine the maximum field angle of the lens with your laser test. You know that light can get through the lens at a certain angle. It just may not give you very good performance at that angle. But "good performance" is a relative term.

LCD projection lens don't have to be all that good! The maximum spot sizes used for triplet design are good enough for photography. Film grain sizes are very small compared to the size of an LCD pixel. If a projection lens can get most of a pixel's light within the 2 mm pixel space on the screen, then it is good enough.

If you can get all the light through a corner pixel into the lens, so the most extreme ray is less than the maximum field angle, then all of that light will get through the lens. Here is a drawing of the first lens surface of the triplet. The red oval is the light that comes through one corner pixel. (It has the oval shape because it comes from all the areas of the lamp arc.) Since the part of the oval closest to the edge of the lens has a lower angle from the central axis than the maximum angle, all of this light will get through the lens.

[Rox]

ace my good friend, if it was 70 degrees field angle, we could use 19" lcd with 32cm triplet and you know that 15" is the largest we can use with it.

where did you see it is somewhere 70 degrees the field angle for this 80 triplet? i would say it is far from reality. (i stimate 50 degrees in the best case for this lens)

Why don´t we trust on manufacturers specs? mean who thought it is not 450 mm the focal of the 135 triplet? why shouldent the filed of view be 24?

[Guy Grotke]

Here is what happens if the fresnels are not working right. The blue oval is all of the light going through a corner pixel. Half of it is falling outside the triplet, so it will not get to the screen.

There are other factors that all contribute to making the corners dim:

1) The condensor fresnel geometry: A 220 mm fl fresnel for a 15" LCD makes the corners 1.8 times dimmer than the center, just by the inverse square law of light intensity.

2) In a non-split design, the LCD may not pass as much light at the corners because the viewing angle is higher.

Many people have built great projectors using the 450 mm triplet and 15" LCDs, so I think your 24 degree FOV figure is too low.

[Rox]

well, i am all the time using the same example;

My good friend diyeitor, the famous 135 triplet srtipper, has better image with the same 330 field fresnell when he modified the triplet. We don´t know the actual focal lengh, in fact it is very easy to measure the projection size and the trhow and we can determine the focal, but the only important thing is that the larger focal triplet (lets say 50cm focal) works very well with 330 field fresnell, you only need to check his last results.

then, please start thinking about the field of view angle istead of the field mismaching fresnell.

You know what?, i think the 450mm focal is not true. i think the manufacturers lie on this as well (ironic).

[ace3000_1]

ace my good friend, if it was 70 degrees field angle, we could use 19" lcd with 32cm triplet and you know that 15" is the largest we can use with it.

The highest lcd size on the triplet i have and tested works on a 17inch lcd. If you wer to get a longer focal one which are readily avail you can run a 19inch through em no probs. The ohp lenses are designed to have a wide feild of veiw, they are made that way for the stage they have to illuminate, also they would make the FOV wider on a ohp lens to elliminate distortions on the edges for when projecting text.

With my findings of the rear lens being about 80deg FOV and the top being about 40deg FOV, that doesnt neserserally mean that the overal FOV of the lens is 80deg, maybe they combine it ?

You got any answers guy?


The stated FOV for a typical ohp projection lens like mine is about 65-75deg. Ill try to find you a page that states this.

Imo its interesting now that we may have found a differing FOV triplet lens, now we can go and make stuff .

Trev

[Inkog]

Imo its interesting now that we may have found a differing FOV triplet lens, now we can go and make stuff .

What kind of stuff?!?

[Guy Grotke]

>You got any answers guy?

Nope, sorry! If I had one of those 450 mm fl 135 mm diameter triplets then I would try some FOV tests with a thin laser beam and some projection tests with diffuse backlighting on a 17" transparency.

But I don't have one of them.

I guess the folks that have them will just have to experiment a bit to see what they can actually do with different size LCDs and fresnel systems. (I know that most of the people happy with them are using a lumenlab long fl field fresnel.)

I do have a 600 mm fl Rodenstock process lens that has a terrific maximum field angle. I had to block bits of light leaking around the frame holding my 15" LCD, because this lens projected them on the wall (off the normal screen). My frame is about 1 3/4" wider than the LCD, so I think this lens would work for a 19" LCD.

[ace3000_1]

Originally posted by Inkog


What kind of stuff?!?

LOL, ive got a few tricks up my sleeve, including a new light engine that recycles lost light, ill try that in a few days .

I basically made that comment as the more we know or discover things, it opens up our horizons in constructing our own custimizable lens system with available on the shelf parts. Making a projection lens isnt that hard, and the more we learn about them, the closer we come to customising/building our own.

Trev

[Rox]

Originally posted by ace3000_1

The stated FOV for a typical ohp projection lens like mine is about 65-75deg. Ill try to find you a page that states this.

Yes please.

And to you that would like to meassure the field angle, if we don´t know the criteria, how could we measure the angle?

I mean I could say; This is the point where I CONSIDER THE MAXIMAL ABERRATIONS FOR MY SETUP, SO HERE IS MY MAXIMAL FIELD ANGLE. But this is not the criteria manufacturers use.

For instance, Ace says that 32focal tirplet does good work with 17" (we should know the throw he uses, since is very important as well) then others say that it is 16" the maximun LCD we can use with it. The only value i would agree with is the one stated by the manufacturer specs, this is the functionality of the specs, so enginers do know if it will work or not before they try it.

If someone finds the exact criteria to determine the Field angle, then ok, you can meassure it using the same criteria as manufacturers but until then i will not trust on any subjetive meassurement of field angle.