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Extenders, stops and pixels

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    Extenders, stops and pixels

    Hello all
    I'd like to take up the question of whether an extender reduces the effective image stabilisation. This appeared in the latest EOS magazine (Q&A, Oct-Dec 2015). There are two considerations to the effective image stabilisation which raises another question so far not asked. The questioner asked whether his 2x extender reduced the image stabilisation by 2 stops. EOS Magazine gave the answer "no" but then qualified it.
    The suggestion that the image stabilisation effectiveness is reduced derives from a consideration of the focal length of the lens you have, not the extended focal length. If a lens has 4 stops of stabilisation, then the maximum exposure speed is generally reckoned to be numerically 1/4f where f is the focal length in mm. Doubling the effective focal length would give either 1/4f' where f' is the new focal length, or 1/8f where f is the original focal length.
    So EOS Magazine is correct when taking the extended focal length into account, but applying the image stabilisation to the lens in your hand, the exposure time is halved. This could be considered as reducing the image stabilisation by one stop.

    The question I hinted at is that the effectiveness of the image stabilisation also depends on the sensor or pixel size to be precise. If Canon rate their lenses to 4 stops of image stabilisation, this presumably means that they can correct for a certain velocity or speed of wobble. On a full-frame sensor with around 20 megapixels, the pixel size is (let's take the 5D Mk III as an example) 6.2 microns. So the anti-vibration wobble may limit the "pixel drift" to 6.2 microns. On an APS-C sensor the pixel size is about 4.6 microns, which means that it has a linear sensitivity of 50% greater than the FF. This means that you would drop another effective stop (with or without an extender) - or, in other words, the APS-C is more difficult to achieve a sharper picture than on FF, but does not mean this is impossible!

    Perhaps this explains the apparent variation in effective image stabilisation. It also raises the question that with the new 5Ds sensor, these bodies will have pixel sizes around 4.1 microns, making it a bigger challenge than on the APS_C format!

    And the third unasked question now is that because the pixel sizes on the 5Ds and APS-C are similar, does this mean that APS-C users have to use the recommended lenses for the 5Ds to realise the sensor resolution?

    John

    #2
    Re: Extenders, stops and pixels

    The in body IS of Olympus cameras will take account of the extended focal length and adjust the sensitivity accordingly. For non native lenses the focal length can be dialled in manually to adjust the IS sensitivity. Unfortunately these functions are not available on Canon DSLR cameras.

    David
    PBase Galleries:-http://www.pbase.com/davidmorisonimages


    Canon 7D II, Sigma 150-600mm Sport, Sigma 18-300mm, Sigma 8-16mm, National Geographic Expedition Carbon, Lensmaster RH1 Gimbal.


    "It is better to light a single candle than curse the darkness" - Confucius (551–479 BC)

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      #3
      Re: Extenders, stops and pixels

      Originally posted by neonlamp View Post
      Hello all
      I'd like to take up the question of whether an extender reduces the effective image stabilisation. This appeared in the latest EOS magazine (Q&A, Oct-Dec 2015). John
      Hi John,

      You raise some interesting points. I will delve deeper into the issue and see if I need to update the reply in the current issue in the next issue of EOS magazine.
      Robert
      robert@eos-magazine.com

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