Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

  1. Dec 16, 2008 #1
    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,
  2. jcsd
  3. Dec 16, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    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.
  4. Jan 16, 2009 #3
    I have a more or less related question. Can one focus an object on the slit of a spectrometer by positionning the object close to the lens (<focal length) and slit at infinity (~150m)?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook