Questions about small distances from the object.

Discussion in 'Beyond the Basics' started by kostas578, Nov 9, 2004.

  1. kostas578

    kostas578 TPF Noob!

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    Hello everyone ,
    I would like to know what happens if the distance from the object is too short.I mean shorter than 2 times the focal length.From what i have read, if the distance is between 1 and 2 times the focal length , there is a zoom in the foto.And if the distance from the object if shorter than the focal length the lens cannot focus correctly.I would like to know if all these are correct and if you can give me some more information or a link about very short distances.

    As this is my first post here , i would like to congratulate you for your great site.

    Thank you in advance.
     
  2. Hertz van Rental

    Hertz van Rental TPF Noob!

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    If you have a good macro lens you shouldn't have much of a problem. If you are using a prime then there are a number of problems.
    If the distance from the lens is less than the focal length the image is bigger than the object so you get a magnification effect. This is calculated with M=v/u (where M: magnification v: image/lens distance u: object/lens distance)
    Most lenses can't really focus much below their focal length unaided anyway. You need to use extension tubes or belows to allow the lens to move further away from the film plane in order to focus. This brings in the Inverse Square Law and exposure has to be calculated as marked f-stops no longer work. The exposure has to be multiplied by 'Bellows factor' obtained from: M+1 squared OR v squared divided by focal length of lens squared OR u/u-F squared.
    If you have TTL metering it should take care of all that for you but bracket anyway.
    You can use supplementary lenses in combination with your lens instead (like wearing glasses) but they often degrade the image.
    Hope this helps. Can simplify at length if you find this confusing. It is a tricky one to get your head around.
     
  3. kostas578

    kostas578 TPF Noob!

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    Thank you for your answer , I found it realy helpful.
    Allow me another question, I am interested in board cameras , such as this one : http://www.rfconcepts.co.uk/board_camera_bw.htm . Is there any limitation of the focal length and the minimum/maximun distance? The distance have not been decided yet, but it will be from 3cm to 50cm.I am asking this because I have found a formula that shows the distance form the lens were the eidolon is created depends from the distance of the object.The f-stop value will be about 11.
     
  4. Hertz van Rental

    Hertz van Rental TPF Noob!

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    The same rules of optics apply to digital if it uses an optical lens. Lenses always work in the same way so my first post applies I think*.
    Question: Eidolon? Have never come across this word used in conjunction with optics. Do you mean virtual image?

    *Have just re-checked things and need to correct myself: The image and object are the same size at 2 focal lengths. Between 2 and 1 focal length the image is magnified. If the object is less than 1 focal length from the lens a real image cannot be formed without a supplementary lens because the image is formed at infinity. The nearer you move one thing towards the lens the further away you move the other - if that makes sense. ( 1/v+1/u=1/f where f= focal length)
    Sorry for the duff first info. Brain a bit rusty. Calculations are right though.
     
  5. kostas578

    kostas578 TPF Noob!

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    Thank you for your information Hertz van Rental ,
    When I was refering to the eidolon , I meant the virtual image, I found the world eidolon in the dictionary and I guessed it was the right one.
     

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