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Optics - Lens Selection / Geometry

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  1. Sep 7, 2014 #1
    Hello everyone!

    I recently bought a small raspberryPi board, with its small "camera board".
    (link for camera reference)

    I want to use a CS lens on this camera (CS lens from the security camera industry).
    I have been able to attach the lens, and it focuses very good, yet not "excellent".

    Specifically, the center of the image is focused, yet the edges are blurry.
    I will go down the list of things I have done to provide some background:

    - The raspberry pi camera alone works awesome. Very good image quality.
    - There is a seller in eBay that sells his own "rPi camera board" with a close range CS lens.
    - I ordered this camera board and took it apart.
    - The CS lens mount is made out of metal.

    When I installed a longer range CS lens on this mount, the image was blurry. I could not get it to "focus" with sharpness.
    Trying things out, I removed some material from the top part of the mount - this makes the CS lens sit CLOSER to the image sensor.
    With this setup, I was able to get the long range CS lens to focus very good.
    Yet, as best as I could get it, the "edges" of the image are out of focus.

    Now is where I have doubts:
    What else can I do to make the image sharp?
    I don't think it is any changing the distance between the image sensor of the camera and the CS lens.
    (Is that the "focal point"?)

    As a side-note: I have an Axis 211M camera. Excellent unit.
    In between the CS lens and the image sensor, Axis has setup a small "pane" of transparent material.
    I don't know if it is glass, acrylic, or some "boro-silicate" glass type.
    Its shape is the same exterior dimensions as the image sensor... yet it is thick.

    I was wondering if this somehow helps the image sensor receive a focused image from the CS lens?

    I can upload photos of the components if so desired by someone.
    All help and direction is greately welcomed.

    Thank you!
     
  2. jcsd
  3. Sep 7, 2014 #2

    Mech_Engineer

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    Science Advisor
    Gold Member

    It sounds like a couple of things are going on here, one is that the quality of the lenses you're using aren't all that great and your system's optics probably have some aberration content which makes the image quality suffer at the edges of the FOV. This, or the sensor you're using is larger than the lens was optimized for (which might also cause vignetting/blurring at the corners). The camera you spec has a 1/3" sensor which isn't huge, so that might point to a quality problem in the optics, maybe spherical aberration or field curvature both of which depend on distance from the image center.

    Do you have some sample pictures? What lens are you using, and what imaging distance are you aiming for?

    There's a lot in an optical system to consider, but to put it simply my guess is you need higher quality (and more expensive) optics. You need a lens designed for your sensor size, with appropriately-designed focal range, and minimum aberration within the FOV. Are you using a lens recommended by the camera manufacturer?

    You might call it the "back focal length" or effective focal length of the lens. BFL is the distance from the lens's last surface to the focal point (which is where the focused image will be and therefore where your sensor should be). Typically BFL isn't something given as a spec for CS lenses, but nicer scientific-grade lenses might have it.

    http://en.wikipedia.org/wiki/Focal_length

    It's likely a protective window to prevent dust or particles from getting on the sensor, image sensors are very fragile and can be easily scratched by contamination. Probably simple optical glass.

    The camera manufacturer should have taken into account the "optical thickness" of the glass when determining the axial position of the sensor w.r.t. the lens mount. In any case you should leave it in place, it's strange that you had to modify the mount to make your "long focal length" lens able to focus, but I suspect that's a defect of the lens not the camera.

    Sample pictures would be a big help, maybe of a printed optical target like this or this. Also read through this article on testing photographic camera lenses: http://grayfoximages.com/Pages/LensTesting.html
     
  4. Sep 12, 2014 #3
    Mech_Engineer,
    Thank you for your awesome reply.
    Very, very informative.

    I am in the process of gathering the images for comparisson.

    I am also printing the USAF Resolution chart to test...
    And reading up on that blog post "Testing Camera Lenses".

    Amazing.
    I knew it wasn't easy the problem... yet very interested in figuring this out!

    I *think* the lenses I am using are "ok quality", because used with the Axis 211M unit, they produced very sharp and good images.
    Same lens I used in the rPi camera, and that's where I am getting the bad quality.

    Anyway, I will post up photos and more information ASAP.

    Thank you!
     
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