Where To Measure Lens Efl From?

discussion about projection lens, fresnel lens etc.

Where To Measure Lens Efl From?

no_-_name
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Joined: October 5th, 2014, 8:46 pm

March 7th, 2017, 7:43 pm #1

Hello,

where is the effective focal lenght of the OHP lens measured from? Is it like I show in the picture?
Assuming my lens is f=315 mm, the flange should be "+-" 315mm from LCD?

Also for the "+-" value - how much focus travel do you recommend? Would 50 mm be enough?

Thank you very much for your replies :-)

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jthedd
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jthedd
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Joined: June 27th, 2012, 6:49 pm

March 8th, 2017, 8:32 am #2

Yes the efl of this lens is at the flange more or less.

We need to know the specs of your optics

the size of your lcd

the distance of throw

and the image size you want

with that we can give you the optic spacing.



Jeff
jthedd@yahoo.com
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no_-_name
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Joined: October 5th, 2014, 8:46 pm

March 8th, 2017, 1:31 pm #3

Hello Jeff,

I design my box according to these dimensions:
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jthedd
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jthedd
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Joined: June 27th, 2012, 6:49 pm

March 8th, 2017, 7:24 pm #4

Please try a few changes to your calculations.

set led at 120 mm from back Fresnel
15mm between rear Fresnel and lcd
15mm between lcd and front Fresnel
337mm from lcd to flange of triplet.

This will give you a 85 inch image.

20mm + and - (total of 40mm) adjustment is plenty for the triplet.

Hope this helps.

Jeff
jthedd@yahoo.com
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bevo77
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bevo77
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Joined: October 29th, 2011, 4:49 pm

March 9th, 2017, 6:14 pm #5

Because FL of fresnels and lenses can vary, I usually leave the placement of the triplet for last. Hold it in place with my hand to get the approximate placement and allow for fine tuning adjustments, usually with some sliding mechanism.

The implications to the build process are that you have to leave the base and sides longer and then trim to fit when the triplet placement is determined. Then lastly the top is cut to fit the corners. I rarely get perfectly plumb and square corners for the lid, so tracing the outline of the box onto the lid materials gives a good fit minimizing light leakage.
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no_-_name
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Joined: October 5th, 2014, 8:46 pm

March 12th, 2017, 8:53 pm #6

Hi guys,

@ Jeff: thank you, I'll try :-) I will design the box in such a way that I'm able to move the components few mm back and forth. I have acces to sheet metal laser cutter & bend press and also luxmeter, so I hope I'll find the best solution for me.

@bevo 77: yes, that's what I've thought - I'll make the first "base board" for my optics as trial and error and to find out the right position of the triplet.

The design goes approx like this (I do not have ventilation holes & stuff yet, it is just a basic layout of my PJ that I've thrown together in Solidworks)...

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ega
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ega
Newbie
Joined: May 12th, 2017, 7:59 am

May 12th, 2017, 12:24 pm #7

Hi,
Building diy pj but have no knowledges about focal lenghts.

-I will need spherical reflector IR-pass, and condenser lens.
•Have mh bulb 10mm arc gap
•Have lcd 10.6" 16:9 FHD
(Need some space around for keystone)
-Which fl of back, front fresnel and triplet I will need?
I want to build it with best parts I can get.
Priority is image quality and brightness.
Projector will be for outdoor use.
Box size and distance from screen doesnt matter for now.
Priority is brightness and image quality FHD, thank you.

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