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    #1

    ISO and focus points

    I've been trying to find an answer to a comment I saw on a FB page recently regarding high ISO and focus points. It's certainly not something I've heard of before and I can't believe that Canon would spend loads of money on R&D to improve focusing accuracy, only for it to be affected by upping the ISO. Personally I would like to think its a load of 'bull', but I'd be interested to know what others think .... or know.

    ''The reason I ask is the higher the ISO, the grainier (more pixelated) the picture will be. As a consequence of having a higher ISO, the points of the focus differ significantly because they are based, partially, on the pixel area of the digital image as it initially receives, and as a result of this, the focus area points cover a larger area and are not so precise in selecting specific points of the image.''

    Robbie
    From 'The Island Above All Others'
    Tags: None
    #2
    Re: ISO and focus points

    having seen imagines posted by members that have bought the new Canon baby ( 7Dmkii) I think you can safely assume this is "bull" I also own the 5D and taken sharp images at 10k I can also confirm that ......Maybe you were reading a Nikon group ;o)
    :- Ian

    5D Mk III, 24-105 / 70-200 f2.8 L / 100-400 Mk II / 100 macro / 16-35 L / 11-24 L / 1.4 & 2x converters and a bad back carrying it all ;o)

    :- https://www.flickr.com/photos/fotosespana/

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      #3
      Re: ISO and focus points

      Focus is unaffected by the sensor, except in live view. The sensor sees nothing until the shutter opens
      Paul

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        #4
        Re: ISO and focus points

        ISO is part of the "Holy Trinity" of Shutter Speed, Aperture, ISO and all three affect the amount of light that enters through the lens.
        If you change one it has an effect on the others.
        However you may be shooting in Manual and set a high ISO but only have a slow shutter speed which of course may be far too slow for a handheld shot, maybe using a lens with IS, and the result will very likely be a blurry image as you think that the IS will prevent movement.

        In a very roundabout way, bit like driving from say London to Watford via Cornwall, the high ISO may be thought of having affected AF but it hasn't it has affected the shutter speed which has affected the focus.

        Sure hope this makes sense.
        David

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          #5
          Re: ISO and focus points

          Sounds a load of tosh unless an expert on here can say otherwise.
          A few points-
          1) images do not become more pixelated as you alter the ISO - the pixelation is fixed by the physical number of pixels per unit area on the sensor. Turning up the ISO is the equivalent to turning up an amplifier which just electronically makes a weak signal louder ( but unfortunately makes the inherent camera noise louder as well)
          2) the phase focusing system of Canon DSLR uses a different AF sensor to focus and not the main recording sensor.
          In Live a view the contrast focus system uses the main sensor - contrast gets less in darker situations and this can, in some situations, lead to difficulty in locking on focus .
          3) some high ISO images shot in JPEG may appear out of focus but this is due to the smoothing effect of the noise reduction processing ( which can be turned off if you so wish)
          Is this how others on this forum see it ?
          James
          James Boardman Woodend
          www.jameswoodend.com

          Comment

            #6
            Re: ISO and focus points

            Originally posted by jimsphotography View Post
            Sounds a load of tosh unless an expert on here can say otherwise.
            A few points-
            1) images do not become more pixelated as you alter the ISO - the pixelation is fixed by the physical number of pixels per unit area on the sensor. Turning up the ISO is the equivalent to turning up an amplifier which just electronically makes a weak signal louder ( but unfortunately makes the inherent camera noise louder as well)
            2) the phase focusing system of Canon DSLR uses a different AF sensor to focus and not the main recording sensor.
            In Live a view the contrast focus system uses the main sensor - contrast gets less in darker situations and this can, in some situations, lead to difficulty in locking on focus .
            3) some high ISO images shot in JPEG may appear out of focus but this is due to the smoothing effect of the noise reduction processing ( which can be turned off if you so wish)
            Is this how others on this forum see it ?
            James
            That's my understudying of the relationships.
            EOS 7D mk II, Sigma 150-660C, Canon 17-85 EF-S, Tamron 10-24 and a wife who shares my obsession.

            Comment

              #7
              Re: ISO and focus points

              Originally posted by djguk47 View Post
              ISO is part of the "Holy Trinity" of Shutter Speed, Aperture, ISO and all three affect the amount of light that enters through the lens.
              No, the first two affect the amount of light that hits the sensor. The ISO can be seen as affecting how sensitive the sensor is to light but as James says, it's more about amplification than sensitivity.
              If you change one it has an effect on the others.
              This is true but not relevant to the question of autofocus as none of them impact the autofocus of the camera unless the widest aperture of the lens is small enough that AF cannot work at all - f5.6 for most Canons and f8 for the 7DII, 5DIII and most of the newer 1D* range.
              However you may be shooting in Manual and set a high ISO but only have a slow shutter speed which of course may be far too slow for a handheld shot, maybe using a lens with IS, and the result will very likely be a blurry image as you think that the IS will prevent movement.
              I'm not sure this is relevant either but see below.
              In a very roundabout way, bit like driving from say London to Watford via Cornwall, the high ISO may be thought of having affected AF but it hasn't it has affected the shutter speed which has affected the focus.
              I think you may be trying to say that if the shutter speed is too slow, you either get camera shake or movement in the image, either of which can be misread as being poor focus. Is that what you're getting at?
              Sure hope this makes sense.
              Possibly, but then possibly not.
              EOS 7D mk II, Sigma 150-660C, Canon 17-85 EF-S, Tamron 10-24 and a wife who shares my obsession.

              Comment

                #8
                Re: ISO and focus points

                Thanks for the feedback on this. It was a reply by someone to someones query regarding a problem with a 60D/300F4 combo focusing either just in front or just behind the subject (in this case a bird) and it didn't happen with any other lens. I'd suggested getting it checked out as the 60D doesn't have micro adjustment and maybe look at shutter speed/ camera movement during focusing etc etc. After a number of other suggestions, someone came up with the above comment at the start of this thread. I use a 7D2 and frequently use 2000 ISO and above at the moment (Shetland weather and light at this time of year ) with no problem at all using single point focusing.

                Thanks again for the replies.

                Robbie
                From 'The Island Above All Others'

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