A&P Better understanding?

Discussion in 'Lifestyle' started by tenplanescrashing, Jun 15, 2006.

  1. tenplanescrashing

    tenplanescrashing Active Member

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    I know enough about cameras and lenses to do the job but I can't really answer questions when people ask me. Mine may sound simple but no one has given me a simple answer.

    1. if you have a 16mm lens, what exactly is the 16mm measuring?
    Some have said that the smaller the number the more you get in frame, which theoretically is true, but doesn't truly tell you everything.

    2. What is bokeh and what causes it?

    3. What causes vignetting?
    This is what happens with my Tokina when at 12mm. Not a big deal, but curious.

    4. What is aspherical glass and SLD glass and what is the difference/similarities?
     
  2. Tedrzz

    Tedrzz New Member

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    bokeh is how the areas that are out of focus are defined.

    "[FONT=Arial, Helvetica, sans-serif]Bokeh describes the appearance or "feel" of out-of-focus backgrounds and foregrounds."

    it is caused by how the lens is designed
    [/FONT]
     
  3. g33k

    g33k OT Supporter

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    16mm is the focal length, if the lens is only one lens then it's the distance from the lens to the film or sensor. with multiple lens lenses it's not the measured distance. if the focal length is long, the angle of view is small (picture the single lens far away from the sensor, the field of view will be small)
     
  4. Derrict

    Derrict No, I am not Amish OT Supporter

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    bokeh derives from the japanese word 'boke' which means blur.
     
  5. tenplanescrashing

    tenplanescrashing Active Member

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    I understand the last part you put but that still doesn't truly answer how the number is derived. thats the info im after.

    after reading the wikipedia site, im guessing its the distance from the front piece of glass to the sensor/film, is that worded better?
     
  6. g33k

    g33k OT Supporter

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    if you take let's say a magnifying glass, and tried to focus the sun's light to one point to burn ants, then the focal length length of that lens is the height of the lens from the ground, assuming the sun is directly overhead.

    1. multiple element lenses can be represented by a single lens.
    2. the focal length of a single lens is the measured distance from the film to the center of the lens when the lens is focused at infinity.

    so if you want to find out the focal length of a photo lens, you cannot measure the distance from the front element to the film (if you can, then all 200mm zooms would be 200mm long, which is not true). but if you approximate the behavior of the multi element lens by a single lens, then it is the measured distance...
     
  7. tenplanescrashing

    tenplanescrashing Active Member

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    now that makes more sense!
     

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