Lens spec, what do the numbers mean ?

[RimBlock]

Hi,

Looking at lenses for projectors they have numbers on them. Not being into photography or knowledgeable about projectors could someone just give me a quick explaination as to what the numbers mean.

i.e.

A projector lens with the number below on;

1:2,8/100

Many thanks
Rb

[severnsource]

1:2,8 means that it has an aperture of F2.8.

100 means that the focal length is 100mm.

Manufacturers use the throw ratio of their lenses to define to image required to the projection distance. So for a lens of 1.2:1 the projector needs to be about 1.2m back for a screen size of 1m WIDE. This number is not usually given on the sites like projector central, you have to search for this on the manufacturers website or technical manuals. Beware of the zoom factor number that is given though..this is NOT the throw ratio.

Also bare in mind the percentage of lens offset as this will effect the layout a huge amount. Most single chips have around a 1.7-2.4:1 lens and around 100% lens shift, these are optimised for viewing so you don't need to have them angled and inducing huge amounts of keystone.

Just a few pointers.

[RimBlock]

Thanks for the info guy's.

The lens is a Wetzlar Maginon 100mm f2.8 projector lens made in Germany, that was used on a Leica P150 projector. Picked it up on ebay for £4.50 or there abouts.

Building a projector (LCD) and will be looking to experiment, for £4.50 I can't go too far wrong.

If the lens has a focal length of 100mm then presumably the LCD panel needs to be 100mm away from the rear of the lens for it to work correctly ?.

Is there any way to work out the distance needed from the screen to produce a 4ftx3ft image given the lens details in the first post (as this is all I have at the moment).

Cheers
RB

[severnsource]

Originally posted by RimBlock
If the lens has a focal length of 100mm then presumably the LCD panel needs to be 100mm away from the rear of the lens for it to work correctly ?.
RB

Basically yes; actually it would be 100mm away from the panel if your screen was at infinity, if your screen is closer the lens has to be a bit further away than 100mm. You focus the image by moving the lens backwards and forwards. Also the 100mm is from the plane of focus to the optical centre of the lens which is usually in front of the back of the lens, if you see what I mean. Just try hold the lens between an image source and a screen and moving it around and you will find the sort of distances involved.

Originally posted by RimBlock
Is there any way to work out the distance needed from the screen to produce a 4ftx3ft image given the lens details in the first post (as this is all I have at the moment).
RB

The size of the image is simple proportion. If your screen is 1M from the lens the projected image will be 1M/100mm times the size of the LCD. So if your LCD was 25mm wide the image would be 1000/100 x 25mm, i.e.250mm wide. This is not exactly right because the lens will be slightly more than 100mm in front of the lcd but its close enough.

Incidentally, where are you going to get the difficult bits from, LCD panels, driving circuitry etc.?
Bill

[RimBlock]

The LCD is a Nec flat panel so is 15" (1024x768 native resolution with 16mil colours). I understand that the Nec displays are easy to take apart if you are gentle with the ribbon cables connecting the circuit boards.

The light and optics are from an OHP and will be upgraded as money allows.

With good deals from ebay it is running up at around £200 for the setup + screen price which isnt bad for a low end LCD projector.

I believe the cost difference is due to the smaller high resolution panels they use so if you are willing to have a bigger projector then you can build one quite easily and save a lot of money.

Have a browse here.

Cheers
Rb

[Mr_Belowski]

Remember the focal length will be significantly different at the corners of the image cos the screen is much wider than the lens. The lens will (probably) only focus the centre of the image properly.

Nope, the lens should be designed in a way that the whole lens FOV will be in focus. be careful though as the lens might invert your picture therefore your optics will have to be inverted before getting to the lens. It is not just the bit of glass bolted on the front that dictate the projection. A whole load of lenses make up a projection device..good luck though.

[RimBlock]

Have the optics from the OHP to focus the light from the buld onto the LCD and by inverting the LCD I should overcome the upside down image.

Unfortunatly I hav just found out that the monitors PSU (which wasn't part of the auction) will cost £125 from Nec. I can get a 12v 400Ma psu from Maplins for £8, only problem is the Nec monitor uses a din type power plug and if more than two wires are connected internally then I may have a problem.

Cheers
Rb

[severnsource]

An interesting idea. I don't quite see were the 35mm projector lens fits into the scheme tho'. Surely the OHP optics include the lens? Also a 35mm projector lens will only have a field of view of about 50mm, so you could only use a small part of a 14" LCD panel.

It sounds a good project and it will be interesting to hear what results you get.

Bill

[RimBlock]

Hehe,

Well as I said, for £4.50 it is just a learning experience.

The ultimate idea is to dismantle the OHP and rebox the whole thing to make it more visually appealing and more robust.

I will update about the LCD when it arrives tomorrow (hopefully).

Cheers
Rb