How To Select The Right Projection Lens

How to select the right parts to suit your requirements

How To Select The Right Projection Lens

hertzblaster
Joined: September 27th, 2004, 5:28 pm
So many builders had made a wrong choice when buying projection lens. So here is the guide ,

Important factors to consider when buying projection lens in this site. This is also the initial step before starting the project. After you decided what projection lens to buy, the rest will come easy.

1. Projection lens (also lcd size) dictates the throw distance and image size
2. Cost factor

To compute for the focal length of PJL ( projection lens) that you need, you have to decide for yourself your target image size,throw distance and lcd size

Use this basic formula

PJL focal length= (throw distance x LCD size)/image size

So look at the set-up in your home and determine the
1. Image diagonal size in inches that you want
2. Decide on the LCD size in inches
3. Determine the throw distance- determine how far from the screen/wall you want to place your projector. In my own experiences, back of the room placement is best for diy projector. This eliminates keystone problem, wont block your views at your sitting position, easy maintenance,cabling and has higher WAF ( wife acceptance factor)

Note: convert the unit in mm after you compute for PJL focal length. (1 inch=25.4mm)

For example:

LCD size: 7 inches
Throw distance: 12ft or 144 inches
Image size: 120 inches diagonal

PJL focal length= (throw distance x LCD size)/image size

PJL focal length= 144 x 7 /120
= 8.4 inches
= 8.4 x25.4
= 213mm

The nearest available focal length for using 7inch lcd is F-210. So buy the F-210. Check the recommeded PJL <B> with F-210

Note: keep in mind that whatever focal length you choose, you can still change image size by
moving the projector away or closer to the screen.

Now if you already know the focal length of PJL and the size of LCD, you can now select the projections lens that is available on this site.

A>For 10.1 inch lcd
1. F-240 OHP lens- 2000 pesos
2. F-345 OHP lens- 1500 pesos
3. F-315 w focus ring- 3000-best for 10.1 inch.

B>For 7 inch lcd
1. F-210 Prime lens- 4000 pesos-The best projection lens here. Cost more but may help you decide to lower the led wattage because this lens produces the brightest image among the lens that is sold here.
2. F-220 triplet w/ focusing ring- 3000 pesos- Cheaper than prime lens, but a little lower brightness. But produces perfect focus on all corners easily.
3. F-240 OHP lens- no focusing ring.- 2000 pesos
4. F-345 OHP lens- no focusing ring.- 1500 pesos

C>For 5.6 inch LCD only
1. F-210 Prime lens- 4000 pesos.
2. F-210 w/ focusing ring- 3000 pesos- Near performance of prime lens
3. F-220 w/focusing ring-3000 pesos. Focus at sides and corners is better than prime lens and F-210.. resolution is better . A little lower brighness than (2) F-210 and prime lens
2. F-240 w/ focusing ring- 3000-pesos-As good as F-210
3. F-240 OHP lens-2000 pesos
4. F-345 OHP lens-1500 pesos

So when you have 10.1 inch lcd, check all the lens at A
For 7 inch lcd, check all the lens at B
For 5.6 inch lcd, check all the lens in B and C

I hope this minimizes confusion especially for newbies.

Another tip For those in a budget. its better to buy a Prime lens than buying a higher wattage led.
Low wattage led is
1. Cheaper
2. Cheaper heatsink, less heat
3. Low power consumption
4. Smaller box
5. box is cooler
6. LCD is cooler,longer life
7. Can use smaller fan, lesser cost, lesser noise and less power usage.
8. led is cooler, longer life.
Pls try to avoid sending PM to me . Send me email instead hertzblaster@hotmail.com .

stayclear
stayclear
Dear Hertz,
Thanks for such a nice guidance. There are some calc from lumenlab available for calculation the Triplet lens focal length,t LCD size & Projection distance.

The main issue is no where there is any formula to calculate the FL of Fresnel lens.
What I request you is please write something more to make us understand the relation of Focal length of Fresnel lens with Triplet lens.

What FL of Fresnel lens to choose in case of different different Triplet lens.
What difference it makes for same Fresnel with various Triplet.

hertzblaster
Joined: September 27th, 2004, 5:28 pm
Its written somewhere . Kindly spend some more time to read and learn how the topic was organized in this site.

Use the search function at the upper right side of the page.

Pls try to avoid sending PM to me . Send me email instead hertzblaster@hotmail.com .

hertzblaster
Joined: September 27th, 2004, 5:28 pm
Re the focal length calculation, Ive seen so many formula.

I chose to use the simplest one.

The simple formula works for me. I really dont know how it work for the others.

Pls try to avoid sending PM to me . Send me email instead hertzblaster@hotmail.com .

hertzblaster
Joined: September 27th, 2004, 5:28 pm
How did I arrived at the right focal length of Field Fresnel to use?

Its a very good question!

I really find it hard to explain it. I hate that you asked this question. Lolz!

This is very long.

I shared so many times this,

Based on experiences, put in other words, my rule of thumb is, for best result, " Absolute maximum difference between Projection lens and Field fresnel lens focal lengths is 20mm".

There is really not much technical basis on this on why is this so. Im not an optics expert.
I questioned myself, why this seems to work?

Given- Commercially available fresnel lens focal length are F-330 and F-220.
Even if an optics expert here in this site can give us the most accurate focal length of field fresnel to use will likely end up using what is commercially available unless he can spend \$1000 just for a piece of custom made fresnel lens.

For example, If I chose to use F-240 projection lens based on computation, whats the focal length of fresnel to use? Since I have only 2 choices, the F-220 is the right one, if you know about how the light converge when fresnel lens is used as field fresnel.

Where will I place this frenel lens between projection lens and LCD.
Whats the distances?
1. From field fresnel to lcd ?
2. From field fresnel to projection lens?

Factors to consider to determine distance from Field fresnel to lcd
1. Heat- bigger distance will alow heat to escape easily from lcd but too much will soften the projected image.
2. Fresnel grooves projection in the screen- you will see fresnel grooves shadows if fresnel is
placed too close to the lcd. You know this.

Looking at the drawing below, if I will use F-220 fresnel and F-240 PJL, the ideal distance D is 20mm, making sure that all light will be captured by the projection lens back lens and making sure also the light converge at the right spot in the projection lens. Please look at my reference in the projection lens.

How did I arrived at that reference point? Its through experience. This will help determine how long the box size is if you know where to start measuring ( reference) and where to place the projection lens or the distance of projection lens to lcd.

Now next question is, what if I use F-210 or F-200 etc since the only option is to use F-220 fresnel?

I did everything by experiment, since I have a collection of OHP projection lens from one supplier with different focal lengths, i found out that commercially available F-260 and F-180 projection lens will not produce a sharp image compared to F-240,F-210 and F-200 commercially available OHP projection lens when paired with F-220 fresnel.

I have to think about what is commercially available.

And more!

1. Different projection lens are not created equal in quality. Different suppliers have differences in field of view, light transmittance, effective lens diameter, no. of coatings use, contrast, resolution etc etc. So dont expect that all F-240 will perform the same. i remember comparing one supplier from the other. The other supplier F-240 is really very poor in quality .

2. The condenser lens focal length and the distance of light source to back fresnel will change the point where the light will converge in the projection lens. This affects the image size and throw distance, But this still does not affect ( on my visual observation) the image quality as long as the difference in focal length of projection lens and field fresnel is 20mm.

Now i had tried other suppliers of projection lens with different design and that 20mm difference in focal length between projection lens and fresnel lens still is true based on my observation.

I had tried pairing F-195 projection lens with F-220 fresnel. Luckily, F-185 fresnel is available.
The tandem of F-195 and F-185 fresnel produces a sharper image. The 20mm rule of thumb still holds true.

Now what about F-330 fresnel, what Projection lens is ok with this?
Again, think about what is commercially available projection lens which is F-320,F-330 and F-345. They will work nicely with F-330 fresnel. Dont force yourself to look for F-310 or F-300 fresnel, you will never or its very hard to find one.

Now for fresnel lens quality, they also varies in quality from one supplier to another. The F-330 and F-220 is the most matured fresnel design bec its prevalent during ohp days. I thought then that the Chinese have copied already the grooves design of 3M ohp fresnel lens.

Also smaller fresnel lens size is alot easier to manufacture and design as shared to me by one manufacturer. So those fresnel lens for 7inch and 5.6 inch lcd are really good also.

Those fresnel lens F-330/220 that are used for 15.4-16 lcd before are of bad quality, really bad for diy projector! This also I discovered, who is the manufaturer and who is just a seller.
i have them from different supplier. But the only one that is of good quality is the one with size 395x395mm.

The best way to know which projection lens is the best is, to try them all, which I did. Thats why I have several projection lens here that you can choose from . I did this also for LCD controller and that cost me alot. But I am happy that I found the best one. Same thing with led. I spent so many days and nights to find the right supplier before and I am happy , I did find a very good supplier that helped me in the design for the last 3 years. I did evaluated the cheapest led supplier on line and they completely failed in my test. This is also the first site that first evaluated the B101UAN02.1 that I even sent them to several Russian Republic countries. I cant believe it reached them! But I made a big sacrifice on the lvds cable because soon in less than a year, the lvds cable became available on ebay.
Pls try to avoid sending PM to me . Send me email instead hertzblaster@hotmail.com .

kamalnv
Joined: July 17th, 2011, 5:28 pm
Doc,

Thank you for that explaination so detailed and clear, it shows the true knowledge stored and it's a canny not text book knowledge, also making it easy for everyone !.

Your contribution for the DIY PJ Community in invaluable while many sites out there have closed down successfully. The secret of "Continous Improvement" is what energises all of us and most importantly the ability to share knowledge is something i really found only here.

Now did the below work ? (F195 Triplet with F200 Fresnel) >?? dont remember seeing any post on it...or did i miss it ?

had tried pairing F-195 projection lens with F-220 fresnel. Luckily, F-185 fresnel is available.
The tandem of F-195 and F-185 fresnel produces a sharper image. The 20mm rule of thumb still holds true.

hertzblaster
Joined: September 27th, 2004, 5:28 pm
Pls try to avoid sending PM to me . Send me email instead hertzblaster@hotmail.com .

Guest
Guest
Bau to you Guru ! What a complete explanation. Thanks a ton Doc.

I promise, I'll keeps on bugging you..

Have a good day.

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Joined: November 5th, 2014, 5:13 am