# I'm new to optics: need help on lens please!

1. Dec 16, 2008

### Almost

Hi there,

I found myself stuck to a simple question really. Trying to find a solution on my own doesn't guarantee a success. Therefore I would appreciate if any of you can offer me helps to solve the following questions.

1. how big a image a lens can see? You see, when you put the image at the focus of the lens, you get a collimated beam behind the lens, which is then focussed onto a ccd by another lens. Normally, I think this is determined by NA. I did some calculation and found that the spot size is much less than what the lens see. So, how can I know how big an area a lens can see. (given its focal length)

2. this is a related questio to question 1: how can I completely fill ccd? how to determine the focal length of the lenses necessary to fill completely the ccd area.

many thanks,
Almost

2. Dec 16, 2008

### mgb_phys

It's bit more complicated than that - you need to calculate the rays for the edge of the field not just the centre.
A simple result for a camera type lens is that FOV = 2 arctan (x / (2 f))
where x=diagonal length of the ccd and f is the focal length.
This gives you the angle in degrees that the lens will image.

If you are trying to build a close-up 1:1 or magnifing imager it's a little different.

3. Jan 16, 2009