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technical question about using a tc

Discussion in 'Lens Lust' started by kevin pyle, Aug 26, 2008.

  1. I've always wondered that if you shoot a tele lens thats a 300 2.8 and you put a 2x tc on it and shoot wide open which will give you 600mm 5.6 being two stops.
    Is your DOF like shooting the lens at 5.6? Something I have often wondered
    about. Thank you for the info.
    Kevin P.
  2. rvink


    Mar 21, 2006
    New Zealand
    OK, first let's be clear by what we mean by DOF: it is the zone in front and behind the plane of focus which is sharp enough to be called "in focus". What is "sharp enough" is defined by the circle of confusion (CoC) which basically says how much a point can be defocused while still being small enough to be "in focus". For 35mm film photography, the CoC used for DOF tables was usually 0.03mm (?) but the value really depends on the image format (DX will be smaller), how large you make your prints and how critical you are about "sharpness".

    Also, by definition, anything outside the DOF zone is out of focus. The nature of the out of focus part is where bokeh comes in - how blurred is the fore/background, what shape are the blurs and how smooth they are.

    With all that in mind, a 300/2.8 with a 2x TC is effectively a 600/5.6 lens, so the DOF is the same as a 600/4 stopped down to f5.6. Compared to the naked 300/2.8, the DOF is halved. That makes sense if you think about it - blurs on the edge of the DOF zone are magnified 2x so you need to go back halfway towards the plane of focus before the blurs get down to the CoC. So yes, the DOF of a 300/2.8 with 2x TC is more or less the same as the 300/2.8 at f5.6.
  3. Aleksey


    Mar 10, 2008
    New York
    You mean 600/4 at 5.6?
  4. Nikkor AIS

    Nikkor AIS

    Jun 5, 2008
    Yes it's the same. A Nikkor 300 2.8 ED-IF AIS with a TC 301 converter gives the same depth of focus(feild) as a 600 5.6. However, the nikkor 600 5.6 ED-IF AIS will give better IQ than a 300 with a 2x.

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