distance rear fresnel to center objective split design

[nitsua]

hi!

i've done a lot of searches on this one, but strangely i came up with two main contaradictions:

parts:

Lcd (8" but not important)
rear fresnel: FL 310.
objective varifocus gadaffy FL = 310 to 360
throw: 5 meter

situation 1:

i would like to have a screen size as big as possible with this objective, so i've to try to make the fl as short as possibe. To give myself some margin in constructing, i decided to stick to a fl of 330 and then focus on site to exact sharpness.

distances would then be as follows:
1/objevtive FL = 1/LCD to objective + 1/throw

1/330 = 1/lcd to objevtive + 1/5000
lcd to objective is then 319 mm

That would mean that the distance of my rear fresnel with FL of 310 should be placed on a distance of 9mm from lcd.

This all sounds logical...

but..

situation 2:

Using the FocalCalc 2 software from Lumenlab gives me the following distances:

An externally hosted image should be here but it was not working when we last tested it.

yes, it says as LCD to objective distance of 355mm!?

so, which one is right?

really hope that someone knows the truth on this one

 

[superdaveumo]

1 / (projector lens focal length) = 1 / (distance from LCD to projection lens) + 1 / (throw)

1 / 330mm = 1 / distance + 1 / 5000mm

So LCD to Projection Lens distance is: 353mm

This is to the field fresnel; the one between the LCD and the projection lens in a split configuration.

So you will have to place your 310mm fresnel 43mm from the LCD. That's a little too far IMHO. If you used your varifocal projection lens at the 310mm mark, you'd get a LCD to Projection Lens distance of 330mm. So a 310mm fresnel would be 20mm away from the LCD. That's a good number.

I think you just did a little algebra mistake.

 

[nitsua]

Hi superdaveumo,

yeah, your right, that was of course a stupid mistake , but what I do not understand about it is that your saying that a distance of 43mm is too much for a sharp image.. Why is that, because aren't the rays all parallel in between the Fresnel and the lcd?

The reason for asking is the following:

I have a fixed projector to screen distance of 4700mm.
I use a Hami 8 inch Lcd and the screen size I want to achieve is 200mm wide.. (that makes 250 diagonal).

I already have a rear Fresnel with FL 330, and I now can buy a objective with FL 400.

That would make:

distance from LCD to objective of:
1/400 = 1/dist - 1/4700
dist - 437 mm.

Using the rear Fresnel of 330 will make the distance between lcd and Fresnel: 437 - 330 = 107mm (yes, 10,7 cm!)

The projected screen size will be:
size = (4700 - 400)*20,32 / 400 = 218 mm.

So size would be acceptable.

Is this possible or just not at all? Please tell me the best solution if you have another 🙂

And second question, with the LL Focalcalc displayed above, i get a bigger screen size when i increase the distance between lcd and rear Fresnel. How is that possible since the rays al all parallel in that area!?

 

[superdaveumo]

Your calculations are correct except you placed the decimal point for the LCD's diagonal in the wrong place. 8" is about 20.32 cm not mm. Your LCD screen's diagonal is 203.2mm (203,2mm). That makes your screen MUCH bigger. In fact 10 times bigger to be exact: 2186mm (2.186 meters). If this is the screen size you are looking for and to me it sounds reasonable, then your calculations are correct except that one decimal place mistake.

So you have a 400mm projection lens and you need to place your LCD 437mm from the projection lens. Those are good numbers. You just have to find a fresnel lens with a focal length of around 400mm - 410mm (in a split config) or 450mm - 460mm (in an unsplit config).

Ok, now your question about positioning fresnels relative to the LCD. Ok see here:

An externally hosted image should be here but it was not working when we last tested it.

This is a SPLIT config. Study that picture a little bit and see if it makes sense to you. The 410mm I've put in there is assuming that the field fresnel lens you get has a focal length of 410mm.