Lens suitability and FOV (cheap lens!)

[justme]

Here you can see the huge reflector from behind

 

[justme]

With both condensors


If you look carefully you can see that the square of light is in the shape of the coils.

 

[justme]

One condenser only


This is an aprolimation of the size I would need to light my lcd.
It seems neater to me.


Thanks in advance for any help!

 

[ace3000_1]

Re: condusing fl with f#

I have a "rule of thumb" question only looking for a general answer if there is one.

I have read that altering the size of a fresnel lense will alter fl.

shorter or longer?

Or is it more complicated than that.

thanks

It doesn't change the focal length when you change the size, because the focal length is the distance to the point where all the parallel light going in the rough side focuses on the smooth side. (Assuming a positive lens.) Even if you block out or remove parts of the fresnel, the light through the rest of it still goes to the same place.

It does change the f number, which is defined as (focal length) / (lens diameter). This is the measure of light gathering capacity commonly used for photographic lenses. Not so important for fresnels used in projectors.

Sorry guys to but in here but i thought id just mention the focal change might have been said by me.

What happend a while back and i mean a good while back was i cut a few page magnifiers up. The focal changed. Why? not sure, but i think its probally due to the page magnifier being cr@p, but it did happen.

Guy's right in this, they focal shouldnt change at all, cut or non cut. What you will change with cutting a frensel smaller is its ability to collect light. The smaller the frensel the less light it will collect. However this doesnt pose a problem as in small projectors, as we concentrate the light to a smaller area (inverse law) and we dont get any losses.

If you had a light engine setup for a 15inch lcd and wer to cut the frensel down without condensing the light into the new size of the frensel, you will not get any gain nor loss. The same amount of light will go through the frensel over a given area so therefore it will be the same brightness.

Anyway i hope that cleared a few things up

Trev🙂

 

[ace3000_1]

It does change the f number, which is defined as (focal length) / (lens diameter). This is the measure of light gathering capacity commonly used for photographic lenses. Not so important for fresnels used in projectors.

Not overly important but it helps to get a brighter image, it helps with eficentcy thats for sure.

Trev🙂