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i'm new to optics: question about lens

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: December/16/2008 at 05:02
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Optics GrassHopper
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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
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: December/16/2008 at 05:54
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X = 180 Y = 90 (X+Pyro)+(Y-Pyro) = ?

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: December/16/2008 at 06:04
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isnt this called field of view??
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: December/16/2008 at 06:21
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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: December/16/2008 at 08:57
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Go to the site below for a calculator:
http://www.videologyinc.com/lens%20focal%20length%20calculator.htm

You have to know whether you are optimizing on vertical or horizontal plane...CCD's, generally commercially available, use a 4:3 aspect ratio.  You are, in effect, trying to cover a rectangle with a circle (lens image) resulting in a constant situation of either incomplete image or too much.  Generally, the image is larger than the CCD to try to "fit" everything in.  To optimize you must pick one plane or the other because the focal length to completely fill the CCD vertically or horizontally is different.  

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: December/16/2008 at 16:06
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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: December/17/2008 at 02:51
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Optics GrassHopper
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Thanks Kickboxer, really appreciate.

However, my situation is slightly different. Like formulated in the question, although I might not have put it correctly, I would like to know how to choose a focal length of the lens in order to focus a collimated beam (coming from the objective lens; i.e., this is a two-lens system) onto a CCD to form an image. At this stage I want to make sure that the image fully fill the CCD, or as much as it can as you described.

cheers
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: December/17/2008 at 03:00
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Optics GrassHopper
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One more note: I tried putting in 1000 meters as the distance from lens to object, to mimics infinity (or parallel or collimated beam). The value of focal length I got is absurdly high.


 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: December/17/2008 at 09:39
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I answered your question in the "spotting scopes" forum but, for the sake of continuity I will throw in the bulk of my response here as well since you posted in both places.
 
First, you still haven't defined the abbreviation "NA."  I realize that I'm probably the only person on the planet who doesn't know what it means but, if you could define it anyway, that might be helpful.
 
Second, although I admit to knowing only a very little (almost nothing in fact) about cameras, I am nevertheless unaware of any (digital) camera that allows for the kind of tinkering that it sounds like you are wanting to do - other than trying to find a specific lens for a given DSLR that would maybe offer some specific focal length that would "completely fill the ccd" - the value of which seems rather dubious.  How exactly does "filling the ccd" in the manner you describe help to create good photographic composition?
 
Furthermore, if you already know how to do the necessary calculations to determine that the "spot size is much less than what the lens see[s],"  then you already have the ability to answer your question about "filling the ccd."  


Edited by lucznik - December/17/2008 at 09:40
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