Hey everybody. My wife and I sat down and worked up some equations for determining picture size, lens focal length etc. She scripted it into the swiss army knife calculator tool, very useful. If you need any measurements in mm simply multiply an inch value by 25.4.

http://www.lumenlab.com/focal_calc.php

Very nice! By leaving all the fields variable you never have to back into a number. Thanks!

Thanks brainchild (and wife).. This will be quite useful!

One question though, I'm assuming that it calculates the relational size based on what measurement is given. Meaning, if I give the horizontal measurement of my lcd, I'd get out the horizontal measurement of the screen, alternatively if I use the diagonal, I would get the diagonal. Is that correct?

Yes, as long as you know the aspect ratio of your panel (4:3, 16:9) the diagonal measurement is equally relevant.

i take it that its using the thin lens equation and linear magnification equations?

1/f = 1/distance to lcd + 1/distance to screen

and

Magnification = image size/lcd size = image distance/lcd distance



can the calculator also calculate how far the lens should be from the lcd?

The calculator can not calculate the distance from the LCD panel to the lens... that would be nice though.

It is assumed that the object (LCD panel) is more than one focal length from the lens.

The calculator is based on the following formulas:

O = size of object (LCD panel)
D = distance from PJ lens to screen
S = size of projected image
f = focal length

f = ((distance from PJ to screen) / (size of projected image / size of object (LCD panel))) +1

or

f = (D / (S / O)) +1

D = ((S * f) / O) + f

S = O * (D - f) / f

O = (S * f) / (D - f)

The results are rounded to 2 decimal places.

So, what you're saying is that the calculator is giving the 4:3 measurement?

Hmmm.

Nope the calculator gives the diagonal measurement. The aspect ratio will be the same as whatever you measured to come up with your diagonal measurement.

I see okay. Thanks, gotta do some calculations then...

Maybe it's just me, but it seems easier working with screen width instead of the screen diagonal. So just use 12" at the LCD panel box (instead of 15").

A very good point jones rush.

I second that.

However I don't think it's any easier. It is just you jones

It seems the new zoom lens is going to roughly have the same zoom capability as my InFocus X1, which has a 1:1.2 ratio.

I've doubled checked my figures, and I am only able to set the new zoom DIY back 4 inches further than my X1.

I thought we were shooting for 1.6? Maybe I don't understand what the ratio thing refers to.

I'll move this to the zoom thread later.

The ratio to which we were referring is the screen size to seating distance ratio. I can't put my varifocus in the zoom because the field lens is static, which narrows the range we can move the pj lens.

thought everyone might get a kick out of this.

lens program

Question
I just bought a Barco HD(1.5-3:1) zoom lens that I would like to use in my pj. How do you know what mm fresnel lens to use when you are dealing with a zoom projector lens? I would like to project an +80" image from +216" back

HD(1.5-3:1) Wide Angle
Diameter 150mm
Projection Distance (PD ) 1.6 - 18 m / 64 - 884” ) Telephoto 3.1 - 45 m / 125 - 1756”
Screen Width (min/max) (SW) 1 - 15 m / 40 - 590”
Relative Luminance(2) 95%
Resolution MTF > 25% @5.5 lp/mm 14 lp/”
Distortion < 2%
Lens Aperture F / 4.5
Focal Length EFL 165 - 346 mm / 6.5 - 13.6”

The focal length of that lens isn't very good for your desired throw distance.

Did you use the focal calculator?

http://lumenlab.com/focal_calc.php

For your desired throw and screen size (using a 15" panel) you would need a lens with an 863mm focal length. The lens you bought tops out at 346mm.

I wasn't sure how it would work when figuring out FL of zoom lenses. I did use it and that is why I started my search for a zoom lens to compensate for the needed +800mm triplet lens.

what about
Projection Distance (PD ) 1.6 - 18 m / 64 - 884” ) Telephoto 3.1 - 45 m / 125 - 1756”
isn't this the actual throw w/ the zoom?

would you like to see the actuall document from Barco on the lens?

You have to remember that the Barco lens was made for a much smaller objective (lcd). If you are throwing a smaller objective you can use a smaller focal length (because the object being magnified is smaller the magnification is smaller). That said the beginning and ending focal length of any varible focal length (zoom) lens tells you everything you need to know. If you are attempting to use the lens with a 15" panel you can simply run the numbers through the calc to get your final screen size.

(There is also another issue to consider, FOV field of view.)

understood, I'm going to look into the 7" LCD project, I don't think it will get me exactly what I'm looking for but it is closer. Accourding to the FL calulator w/ a 7" LCD I could have a throw of 14' and obtain and 80" image.

as FOV, are you talking about the large diameter of my zoom lens? I've thought about that and I figured I would black out the excess so I don't wind up projecting the inside of my box.

The field of view is how much of the panel the lens can 'see'. If the lens barrel is too long or the lens diameter/focal length is too small you won't be able to project the entire panel.

Hello, I'm new to this DIY PJ and was wondering if i might be able to use an lcd projector lense i have. The lense is from a sharp xg-1500u. The lense's stats are 135-270mm 1:4.5. The primary (input?) lense is about the size of a memory stick wide and the output lense is about 1.25x the size of a memory stick. I apologize for not having measurments in mm or inches but no tools are available at the moment. I have ordered all the material necesary from the store and lighting unlimited. If i cant use this lense it ok but i wanted to know because this pj will be used in church for songs/lessons/seminary/ and powerppoint sermons. The pj currently in use is a Nfinity
that is about 15 ft from the screen and projects an image about 8'*6'. I would like to match or surpass this size but not have to be 15' away Could I achieve this by using this zoom lense?

Ezra

PS: box construction begins later today!

Sorry but you can't use that lens. It sounds like the regular lumenlab projection lens will do what you need.

I was wondering about the focal length of lens in the focal calculator. Which lens is it refering to and what does that measurment mean? Is that the distance between two points? And if so, what two points are they?

The projection lens is the lens that is measured. By inputting 3 of 4 variables you can ascertain the 4th. You can figure out your throw length, your projection lens focal length, or your screen size.

I have read these calculations up and down, back and forth and have done nothing but confuse myself. I am planning on building the basic pj. If I want a certain size screen approximately 94" X 50", this is just a ballpark figure, using a window in my house that looks like a good size screen, how do I come up with the other figures.

Is this a realistic size?

And, How do you convert the focal length mm to inches?

Any info would be greatly appreciated!

25.4mm = 1"

I can tell you that with the standard lenses your box will be about 94" away from the screen.

What options do I have available to make it so I can relocate the pj. I move around alot and need some flexibility with the distance betwen the pj and screen.

Thanks for the help.

This site is great by the way.

Well, the distance is flexible per se...you can focus the image at virtually any throw distance but you can't control the picture size. The pro lens offers a longer throw and some variability in picture size.

Brain, Thanks for the assistance. What is the projected image diminsions with the standard lens kit? I'm going to start ordering my supplies soon, I have ordered the Benq FP557S-V2 15IN LCD already. I am pretty excited about getting started I just want to make sure I have all my ducks in a row before I do.
Thank you.

The projection ratio is about 1:1, meaning if the projector is 10' away from the screen, the image will be about 10' wide.

I'm not sure if anyone is aware of this but these formulas don't seem to be correct. f=(D/(S/O))+1 will only agree with Pythagoras if the +1 part is removed. They all don’t transpose with each. The last 3 do but not with the first. It appears as if one form of the last 3 formulas has been used in the LL focal calc, and I have no idea where this formula came from. Here is what I thought the formula should have been, S/D=O/f. If I’m wrong please except my epology.

DJ

Here's something I don't get.

The 1:1 ratio as stated above will yield that with the standard lens kit on a 15" panel, you'd want to sit something like 15 feet away from the screen if the proj is 10' away from the screen (yielding approx 10 foot picture).

The problem I see is that given only 10 degrees of keystone adjustment, how can you possibly mount this on the floor or ceiling where it doesn't block the view?

Looking at my own basement, where I'd like to mount this... My ceiling is approximately 7 feet high. I would like a picture that is roughly 4 feet high, to leave a gap on the top and the bottom. With a 4:3 ratio, the screen width would then be 6 feet. This would give a diagonal of just over 7 feet (did I do that right?).

Given this size, and that I would need to sit about 10.5 feet away for optimum viewing experience... it would seem that a floor mounted projector would need to be 3.5 feet off the floor, or a ceiling mounted would need to be 3.5 feet off the ceiling.

Is this correct?

Now the problem is that the ceiling is only about 7 feet high. I would prefer to have the projector higher than 3.5 feet off the floor/ceiling. First I will project at the upper edge of the wall (for ceiling mount), which would buy another foot higher (2.5 feet from ceiling). Then I could use 10 degrees of keystoning to get a decent picture - is there a calculator anywhere to assist with determining how high off the ceiling I would then be able to mount my projector?

Thanks
-Alfisti

Could someone help me out with the calculation? I would like to mount the pj 18' from the screen and I have a space about 9' wide by 7' high in which to project the image on. I plan on using a 15" monitor. If I use the pro lens kit will these measurements work?

At that distance your projection would be 10' wide. But it depends on whether the projection lens is 18' away or the back of the box is. If the lens is not truly 18' you can come in at 9'.

Thanks for the help. The back of the wall is actually 18' so the lens will be closer to 16' from the screen. With the built in adjustment of the pro lens, is 9' the maximum width and then I can adjust down to a smaller size picture from there? Lastly, do I need the pro lens setup for these distances or would the standard setup work. If the standard setup would work what would be the advantages to the pro lens.

Thanks

if i buy the big sheet lense that is sold here with the 220mm(?) and 320mm(?) but im cutting it for a 5" screen-do i still need to put it 220 mm away?
Also-would a smaller focal point lense allow me to make a smaller box and still produce a big image?-for instance-a smaller lense than the 310mm focla lense from the overhead projector would allow me to make a smaller box but would i get a pic thats njust as big?
thanks!! im still learning here!

The distances are the same how ever small piece you use.
(Sheet lense-->fresnel lens)

Small focal distance makes a bigger picture with smaller lens-wall-distnce BUT you will get problems:
1. Difficult/impossible to get a sharp pic at corners and center at the same time. The distance differs so much.
2. The center is also much brighter than corners.
3. Remember that for a big picture you need more watts in the lamp.

My problem is to get as SMALL pic as possible, with a throw distance of 14 feet. I'v got a Pro lens and 15 inch lcd. Pic width about 80 inch would be good.

For a amall box start thinking an lcd of 8 or 11 inches. The resolution is just enough for movies, but not for PC-games etc. through.

There're nasty pop-ups, but a nice distance and pic-size calculator:
http://www.gwidijanto.fcpages.com/index.htm
It needs MS Excel (or maybe OpenOffice.org could be used?)

Tux

I'm lost.

To project a 10'-12' screen from 18', I need a ~500 FL fresnel w/ a 15" LCD?

Where can one find a long throw pj lens??

TIA

It's not working for me.... I type in the size of my panel and the size of my screen and it comes up as "Na".

Brainchild is working on fixing the PRO triplet which was 450-500mm, maybe only 450 or 500 after the fix. Its been removed from the store until the fix is final, expect to pay ~250$

Nevermind, got it working, my stupid mistake.

So, if I’m using a 17” LCD with a 15’ throw and the new, new pro lens…... What size screen do I need? Will this even work? Any help would be appreciated! I am new to this...

Two good links, anyone come across them b4?

(Edited, links bad)

These are no longer valid, server error. Makey could you post a working link or post/attach copies of the files?

Hoping that I understand this right. I'll be using a 17" panel roughly 14.5 ft away and using the pro lens (not sure if I use 450mm or 500mm). The projected image I get using the calculator would be 133" for 500mm and 155" for 450mm. I guess my question is #1 Is this right(taking into account rough estimate) and #2 do I need to do a different calculation if I want to change the aspect ratio of the panel to get the theatre screen look? I guess knowing how many mm the new pro lense will be would help but if the calculations are correct it will be np for me to get my desired projected image size for the room.

The PRO triplet is 500mm.
133inches at 14.5 feet, 17" lcd is correct.
The calculator assumes that what ever you use for a source is what the screen will be so if you input a diagonal for a 4:3 LCD you will get diagonal for a 4:3. If you want to run the LCD as a 16:9 then you need to figure out the new diagonal of the smaller screen which would scale the output image, now a 16:9 etc... If you input width or hieght of the LCD you get width or height of the projection etc...

Ahh I see. So the 133" would be the diagonal which is perfect and gives me plenty of leeway to make whatever adjustments necessary for final placement. The wait on the new triplett will give me time to make sure I get everything together and get my box planned out. Thanks for the help pager

I'm completely new to this.... I'm wanting to build my own projector but I'm wondering where to begin. This projector would be used in an auditorium, and I'd need the throw distance to be 35 feet and the width of the image to be 12 feet. What are my options? Can I use the Pro Triplet for this? From my reading it looks like I can't, but do you have any recommendations for what I could use?

To use a 17" LCD you would need a triplet with a focal length of atleast 900mm, these do exist but you would also need a fresnel with a 1000-1200mm FL which maybe harder to find. This sort of design isn't really meant for 35' throws, find another place for the projector, 19 feet with the PRO would be about right.

Thanks for all your work brain! Unfortunately, the link appears to be broken; http://www.lumenlab.com/focal_calc.php. Anyone know where I can get this aid?