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Canon 2x + 1.4x teleconverter bonus?

Discussion in 'Digital Photography' started by Grimace, Nov 7, 2008.

  1. macrumors 68040


    Hey gang,

    I have the 2x and 1.4x teleconverters, and I noticed a very strange bonus feature when coupling them together. The 1.4x loses 1 full stop of light. The 2x loses 2 stops of light. So, per the chart below, an f/2.8 lens will be an f/5.6 lens with the 2x attached.


    However, when adding in the 1.4x tc to the 2x, the f/2.8 lens STILL shows f/5.6 and has use of ALL of the autofocus points. This doesn't make ANY sense, but I'll take it!! :)
  2. macrumors 6502a

    Be careful, you could be vignetting... not to say anything about the image quality, either.
  3. macrumors 68040


    There is no vignetting at all - and the image quality is actually quite good. I wouldn't try this on anything but a sharp prime lens.

    I am still curious about why I can still use f/5.6 with all AF points.
  4. macrumors G4

    You can't. The electronics likely is not reporting the aperture correctly. As we all know f-stop is the ratio of the diameter to the focal length. If you double the focal length then you loose two stops. It is just basic geometry and you can't get around it. Yes I can believe the AF still works but it's working past it's specs at f/8
  5. macrumors 6502a


    what lens are using?
    i know the stacking works fairly well with the 300 2.8IS and alot of surfer shooters use it.
  6. macrumors 68040


    I'm using the 300mm f/2.8L IS. I tried out some shots today, and the EXIF show 600mm, even when I have both the 2x and 1.4x attached. The camera is most certainly confused into thinking that there is only one TC there (as I still have all AF points) - but the image is 840mm.
  7. -hh
    macrumors 68020


    Ugh! I hate Time Zone changes...its 3AM EST and I'm wide awake.

    It simply sounds like you're using the "Canon Teleconverter Tape Trick" without somehow knowing to invoke it.

    The Canon EOS "Teleconverter Tape Trick" consists of putting a thin strip of insulating tape over three of the pins on the TC.

    Step 1: look at the side of the converter that connects to the camera.
    Step 2: rotate it so that the contacts are at the top.
    Step 3: place a small piece of tape (transparent OK) on the 3 leftmost pins.

    Question #1: does your 1.4x have some tape covering its pins? (or it might otherwise simply be a bad electrical connection?)

    Question #2: is your 1.4x a 3rd Party TC or Canon OEM? I seem to recall that some 3rd Party TCs have fewer connection pins or something that doesn't report the +1stop, so this is also worth checking out.

    Question #3: Does using the 1.4x alone report in properly as f/4? This might also help figure out the 'why' behind what you're seeing.

  8. macrumors member

    When you stack TCs only the TC mounted on the camera is sending data to the camera.

    Also, the loss of light is calculated by multiplying the power of the teleconverters together (2.0 x 1.4) and then multipying that to the max aperture of the lens. This is also how you calculate the effective length.

    Optical quality will be affected of course, but there should not be any issues with vignetting.

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