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hyperfocal setting with 2x convertor

Discussion in 'Help Team' started by Lusty, Jun 4, 2005.

  1. Lusty

    Lusty Active Member

    I have a cannon A1 with a 50mm lens on which we can read off the depth of field range after setting the aperture and the focus. I now attach this lens on top of a 2x convertor giving me a 100mm lens, essentially. Do you think the depth of field remains the same if I use same aperture/focus? This is a really important question for me as this combo is light and small and I'm using it for portraits.

    Craig
     
  2. huwevans

    huwevans Not Really Here

    Well, yes, and no. The hyperfocal distance (like depth of field in general) is a function of both focal length and aperture (as well as other things not significant here), both of which are changed by the teleconverter. But you can use the original DoF scale for the 50mm lens because happily the changes can be thought of as cancelling each other out (for these purposes anyway). A 2x TC has the effect of knocking two stops off the speed of the lens - so that a marked f/2 becomes an effective f/4, marked f/2.8 becomes an effective f/5.6, and so on. However, the other changes are such that the scale still works correctly if you read it for the marked aperture, and not the converted aperture.
     
  3. El_Sid

    El_Sid Well-Known Member

    The answer is not exactly straightforward and is connected to the effective aperture. When you double the effective focal length then you effectively halve the aperture. That is if the aperture is set at f4 on the lens then adding the teleconverter creates an effective aperture of f8 for the resultant psuedo 100mm lens. This aperture would have a shallower depth of field than the 50mm at f4 so you would need to stop down further. The lens/converter combo can be treated as a 100mm lens though the true depth of field may not be exactly the same as a true 100mm lens as quality of the converter will have some effect depending on how much or little it degrades the image quality.

    Look here for depth of field calculators.
     
  4. LenShepherd

    LenShepherd Well-Known Member

    Re: hyperfocal setting with 2x converter

    Image size has more effect on depth of field than anything else.

    If you change from a 50mm prime to a 100mm prime (keeping the same focus distance)you reduce dof to a quarter.
    Going to a 200mm prime reduces dof to a sixteenth of what you get with a 50mm.
    Going to a 400mm prime reduces dof to a sixty-fourth.
    This is why you can get significantly out of focus backgrounds (compared to a 50mm) with long telephotos.

    ***

    Using a converter adds a complication because an actual aperture of f4 on your prime becomes effective f8 for dof when you use a 2x converter.
    Two stops smaller doubles dof field which offsets the 4 stops lost by going from 50mm to 100mm.

    Overall from the same focus distance you get about half the dof, a noticeably darker aperture and longer exposure times when you use a 2x converter.
    For extra precision divide 50mm dof by 4, and multiply the answer by 1.4.
     
  5. jchrisc

    jchrisc Well-Known Member

    Huw - I always thought that doubling the focal length required stopping down by four stops to restore the same depth of field at the same object distance.

    However, what I mainly wanted to say is that I find the DOF scales on my Canon 35mm lenses pretty useless. The 50mm f/1.4 for example has only one pair of aperture marks (f/22) and they are only 7mm apart so interpolating is very hit and miss. My 100mm macro lens has marks at f/32 and f/16 so they ought to be more useful, because I use f/16 quite a bit for macro, but again, the distance between the marks is less than 2mm and interpolation is all but impossible.

    I do use the stop-down button to try to get an idea of what is happening, but with my poor eyes and all it doesn't help much.

    Digital helps a lot since you can "suck it and see" much more readily, but I depend greatly upon a decent DOF calculator when it is critical.
     

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