Switch to English Language Passer en langue française Omschakelen naar Nederlandse Taal Wechseln Sie zu deutschen Sprache Passa alla lingua italiana
Members: 70,593   Posts: 1,546,089   Online: 903
      
Results 1 to 7 of 7

Thread: Extention Tubes

  1. #1
    hortense's Avatar
    Join Date
    Jun 2004
    Location
    Riverside, CA
    Shooter
    Large Format
    Posts
    612
    Images
    5

    Extention Tubes

    To use a 150mm lens on a MF camera to avoid getting on top of the image that is being photographed, what size extention tubes should be considered?
    [FONT=Times New Roman]MAC[/FONT]

  2. #2

    Join Date
    Dec 2002
    Shooter
    Large Format
    Posts
    6,242
    A 21 mm extension will get me about 18 inches out from the object with a 150 mm lens.

  3. #3
    hortense's Avatar
    Join Date
    Jun 2004
    Location
    Riverside, CA
    Shooter
    Large Format
    Posts
    612
    Images
    5
    Donald, with your 21 mm extension that puts you out 18 inches out from the object with a 150 mm lens, am I correct in assuming that your focus would be set at infinity? And that the area of coverage would be about 14 sq. in. (therefore, assuming a square format, the image size would be approximatly 3 3/4 in. wide amd
    3 3/4 in. high?
    [FONT=Times New Roman]MAC[/FONT]

  4. #4

    Join Date
    Jan 2005
    Location
    Austin, TX
    Shooter
    4x5 Format
    Posts
    46
    hortense,

    It depends on the close focus distance of your lens. You can use these two formulas:
    1/f = 1/rd+1/od, and magn = f/rd-f; where f is focal length, rd is rear distance (to film plane) and od is the distance to the object.
    I'll use my own example. I have a (Bronica) 150mm lens that focuses to 1.5 meters (1:9 magnification). I can get the rear distance by rearranging the first formula as:
    rd = 1/(1/f-1/od), which with f = 150mm and od = 1500mm gives me 167mm rear distance.
    Now, if I want to determine the magnification an 18mm extension tube will get me, I can just add 18 to my rear distance (which is 150mm at infinity and 167mm at the closest distance) and calculate the magnification using the second formula. This gives me about 1:8.3 at infinity and 1:4.3 at the closest focus setting.
    Note that you can also recalculate the focus distances by manipulating the first equation.

  5. #5

    Join Date
    Jan 2005
    Location
    Austin, TX
    Shooter
    4x5 Format
    Posts
    46
    The second equation should actually read magn = f/(rd-f) . I forgot the parentheses. sorry for any misunderstanding.

  6. #6
    hortense's Avatar
    Join Date
    Jun 2004
    Location
    Riverside, CA
    Shooter
    Large Format
    Posts
    612
    Images
    5
    Thank Michel. I'll run the numbers.
    MAC
    [FONT=Times New Roman]MAC[/FONT]

  7. #7
    bobfowler's Avatar
    Join Date
    Sep 2003
    Location
    New Jersey, Land of the Living Dead
    Shooter
    Multi Format
    Posts
    1,440
    Images
    19
    Quote Originally Posted by hortense
    To use a 150mm lens on a MF camera to avoid getting on top of the image that is being photographed, what size extention tubes should be considered?
    Well, a lot depends on which system you're using because THAT will determine what size tubes are available. For the 6X6 Bronica SQ series, there are only two, an 18mm and a 36mm. I don't know what's available for the Hassey or others. I bought both of the available tubes for the Bronica. The 18mm gets the most use.
    Bob Fowler
    fowler@verizon.net
    Some people are like Slinkies. They're really good for nothing, but they still bring a smile to your face when you push them down a flight of stairs.



 

APUG PARTNERS EQUALLY FUNDING OUR COMMUNITY:



Contact Us  |  Support Us!  |  Advertise  |  Site Terms  |  Archive  —   Search  |  Mobile Device Access  |  RSS  |  Facebook  |  Linkedin