# Telex Firefly Magnification problem

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#### daveb

Hello,
I've been reading throught this forum, looking for an answer to my problem, but I've seen nothing. So, I'll ask y'all for your ideas.

I've got a Telex Firefly P250 projector, 800x600, not terribly bright, but it's not too bad. I want to put it in my 9' x 12' home theatre room (under construction in the basement) and mount the projector behind the wall, and throw the image 12' to the other side of the room.

However, since the unit has no zoom lens, the optimal distance from projector to screen is about 6' (which puts it in the middle of the room and that's not what I want.) At 12', the image is way too big!

I am hoping theres an optical way to reduce the magnification of the image, so that I can put the projector 12' away, but have a not-too-large image.

The lens that is in the unit has a focal length of 140mm (if that helps).

Thanks,
dave.

#### jamie

yes it helps if under stand it and i dont but i can tell what you need to do the real answer is change the lens the focal point on the lens you have wont work at 12 feet then try a lens with half that focal power 70mm? but im not sure maby double it 280mm one or the other should get you going if thats not an option then use mirrors they can be small and hidden

replace the lense....

just use a lense with double the focal point.

think of it like this,

for every 1x of the focal lenght past the focal lenght, the image will double in size.
ill convert to inches so that their is a standard measure
140mm = ~5.5inch
the focal lenght is ~5.5inch

at 12'(144inch) the image would be

144-5.5=138.5/5.5 = ~25.18 times the focal lenght

im assuming that your wall is ~8ft(96inch)
so you image should ideally be about 6'

you problably getting a ~138.5inch(11ft 6.5inch) image which sucks and is too big, and you need a 72inch image. so that math is that your focal lenght is about 48% too short. so take your 140mm/5.5inch focal lenght lense and replace it with about a 290mm focal lenght lense.

#### xblocker

You can use the lens formula here:

1/f=1/d1+1/d2

f=focal lenght, d1=distance image from lens, d2= distance object from lens

xblocker

#### SushiMasterX

Definitely,

Try the formula, a URL was posted on "DIY Video Projector" today!

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