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Primary focus magnification?

  1. Sep 1, 2007 #1
    Primary focus magnification???

    Hello all:

    My name is Miguel, from Buenos Aires.

    I have a question... is it right in formal optics to name "magnification" the below formula or "focal distance equivalent" of a camera attached in primary focus (without telescope ocular and camera objetive, of course) ?

    Magnification = Telescope focal distance / Camera "Focal distance equivalent" ​

    I can find that on my optics books neither Google searching.

    I read this on the web:

    "What is the magnification of my telescope when the NexImage camera is in the eyepeice barrel?

    The NexImage camera is equal to the magnification of about a 5mm eyepiece. To determine the magnification of your image when using NexImage, divide the focal length of your telescope (in mm) by 5."

    on:

    http://www.celestron.com/c2/esupport/index.php?_a=knowledgebase&_j=questiondetails&_i=278&nav=+%26gt%3B+%3Ca+href%3D%27index.php%3F_a%3Dknowledgebase%26_j%3Dsubcat%26_i%3D35%27+class%3D%27lkHdrBlk9%27%3ENexImage+FAQs%3C%2Fa%3E

    Thank you very much in advance

    Miguel
     
  2. jcsd
  3. Sep 1, 2007 #2

    tony873004

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    I'm just guessing, but I don't think that magnification is used for a camera, because unlike the eye, there are other factors involved. With the eye, it is just as simple as saying that the object appears 100 times bigger through the telescope than with the naked eye.

    But with photography, you've formed an image on something other than your eye. When you look at your picture on your monitor, how far are your eyes from the monitor? How large of a monitor did you use? That effects how magnified you perceive the image to be.

    When a camcorder or digital camera claims 7x, they mean 7 times the zoom compared to the widest angle the camera is capable of. But if you take an image at 7x, it will not appear as if you were looking through 7x binoculars.

    With photographs, you want to specify the angular dimensions of the picture. For example, a picture where the Moon fills the frame would be about 0.67 x 0.5 degrees.
     
    Last edited: Sep 1, 2007
  4. Sep 1, 2007 #3
    Thanks for your reply Tony. I think that is no correct the answer from Celestron team, but my knowledge it is not enough for deny it.

    (Sorry, where I wrote "I can find that in my book, must read I can't find that in my book")

    However, I think that that idea could be useful to compare CCD surface coverture with visual filling of the field on an ocular with the image and associates picture CCD image with more common practical concepts about "magnification"

    ¿What do you think?

    Miguel
     
  5. Sep 1, 2007 #4

    tony873004

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    I'd have to agree that's a bad answer from Celestron. I would guess what they meant is that an image taken with their camera, when viewed as large as possible without pixelating the picture, appears about the same size as if you had used a 5mm eyepiece.

    When people sketch surface details on Mars, they often say what magnification they were viewing through. But I've never seen a photograph labeled as 100x. It's just better to give the angular dimensions.
     
  6. Sep 1, 2007 #5

    russ_watters

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    I'd just say that Celestron's answer is simplistic. It is probably based on tony's method of calculating magnification, using some typical assumptions.

    Anyway, tony's method is what I use for estimating magnification of my images. I measure/estimate a viewing distance from my monitor, a dpi and a resolution of the image to come up with an angular diameter of the image. I then divide that by the angular diameter of my subject.

    Magnification is nice to know because lay people ask it all the time, but of much more importance when taking most photos is field of view. It's how you know you can have your subject well framed in the imager. I use Starry Night, which has a fov utility that draws a box around objects in the program to estimate the fov of my equipment.

    [edit] As a matter of fact, this is what my telescope is imaging right now (that's Starry Night's image, not mine):
     

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  7. Sep 3, 2007 #6
    Russ, just a question, how do you change the desired magnification with the Dsi camera? with the Barlow Lens ? I know that as much of the ccd the image occupies more the resolution. But i don't think that choosing between a 1.5x 2x and 3x barlow is very practical or precise for photography. how do you do that?



    Miguel, you are from buenos aires? i am From Avellaneda!! what a small world!, i am sending you a PM!
     
    Last edited: Sep 3, 2007
  8. Sep 3, 2007 #7

    russ_watters

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    You use a barlow or a focal reducer. You can also move your camera towards or away from the focal recuer or barlow to change the amount of reduction or increase in the focal length. But no, there is no way to finely adjust the focal length like with a telephoto camera.
     
  9. Sep 6, 2007 #8
    Si... te respondí a tu casilla privada en este sitio...

    Bye
     
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