Interchangeable lens camera when aperture size remains constant!

Discussion in 'Lo-Fi Cameras' started by Shangheye, Dec 22, 2009.

  1. Shangheye

    Shangheye Member

    Messages:
    1,094
    Joined:
    Jul 26, 2007
    Location:
    Belgium
    Shooter:
    Multi Format
    So I tgot tis Diana F+. Loving it and all my expsoures seem to be spot on for all the settings, given I was using an ISO 400 film (I believe the shutter speed is around 1/60-1/100s) and the apertures on the camera are as follows": Cloudy - f11, Partially Cloudey - f13, Sunny f-19 and a Pinhole setting of f150.

    Now here is my slightly technical question (I know bizzare to say that about a Toy Camera)...

    All the above f numbers are for a lens on the camera of 75mm. On the Diana F+ you can change the lens (e.g. to a 38mm lens) while the Aperture remains a part of the body. Given the focal length has changed, and the aperture diameter has stayed the same for the various settings...does that mean the f number has changed?

    i.e. I now has a 38mm f5.6, f6.8, f9.6 for the same settings? (Using the f numbers claimed by the Lomography group, I have calculated the aperture diameters to be 6.8mm, 5.7mm and 4mm)

    I know this may seem like overthinking it, but I have a project coming up with the camera, and I would rather not come back with massively over-exposed negs (in this case possibley up to 2 stops!!).

    Rgds, Kal
     
  2. Christopher Walrath

    Christopher Walrath Subscriber

    Messages:
    7,129
    Joined:
    Dec 30, 2005
    Location:
    Two inches to the left
    Shooter:
    Multi Format
    The aperture value is a ratio between focal length and aperture diameter. A 75mm lens with a body aperture value of f/11 has an aperture diameter of just under 7mm (75/11=6.81). That same aperture (part of a camera body and not the lens) with a 38mm lens would give an aperture value of about f/5.6 (5.6*7=39.2). If the aperture is part of the lens then the aperture value would remain the value published on the lens.
     
  3. OP
    Shangheye

    Shangheye Member

    Messages:
    1,094
    Joined:
    Jul 26, 2007
    Location:
    Belgium
    Shooter:
    Multi Format
    That's what I was thinking, but that is not referred to anywhere on the web...i.e. that as you change lens on the Diana so the f number changes....two stops is a massive amount Toy camera or not....thanks for confirming my view. Rgds, Kal
     
  4. 2F/2F

    2F/2F Member

    Messages:
    8,003
    Joined:
    Apr 29, 2008
    Location:
    Los Angeles,
    Shooter:
    Multi Format
    One VERY easy way to find out for sure: Take a pic with the attachment off. Then take the same pic with it on. Finish the roll any way you like and process. Compare the negatives.
     
  5. OP
    Shangheye

    Shangheye Member

    Messages:
    1,094
    Joined:
    Jul 26, 2007
    Location:
    Belgium
    Shooter:
    Multi Format
    Thanks 2F...I have switched lenses half way through the test roll and will take a couple of shots at various settings just to be sure. Rgds, Kal
     
  6. ic-racer

    ic-racer Member

    Messages:
    7,809
    Joined:
    Feb 25, 2007
    Location:
    Midwest USA
    Shooter:
    Multi Format
    If you have a way to check the size of the entrance pupil as viewed from the front of the lens at the various settings (like a "B" setting), you can calculate the F numbers from there.
     
  7. OP
    Shangheye

    Shangheye Member

    Messages:
    1,094
    Joined:
    Jul 26, 2007
    Location:
    Belgium
    Shooter:
    Multi Format
    That is what I have done ic-racer..as per the measurement above which is consistent with what I see with my eyes. Need to remember that when using the 38mm, ISO100 film will be better for most uses. K