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High ISO low noise...technique

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    High ISO low noise...technique

    I've just been experimenting on this dull Wednesday morning and have found that if I set ISO1600 and +2 stops overexposure, and then download the image and reduce the exposure in DPP by -2 stops to bring the exposure back to the correct brightness - then the noise in the final image is massively reduced compared with a normally exposed ISO1600 image - in fact the noise is so low that its virtually indistinguishable from a correctly exposed ISO400 image (not quite as good as ISO100-200 though but you need 100% magnification to tell).

    I believe this is because the sensor is overloaded with light and that improves the signal to noise ratio - I've read this before on this forum but never tried it out as a technique.

    EOS50D 24-105f4L used for the experiment.
    Brian Vickers LRPS

    brianvickersphotography.com

    #2
    Re: High ISO low noise...technique

    Absolutely right Brian and the reason why it works so well, is contained in your first sentence:"On a dull Wednesday morning".

    The reason why it works is the contrast is significantly lower on a dull day and if you look at the histogram, the graph is a lot more compressed to account for this. Therefore, without clipping the highlights, or blocking the shadows, you have more freedom the shift the histogram by over, or under exposing.

    In bright sunshine, the contrast is higher and the sensor has to cope with a higher dynamic range, so the histogram will be spread a lot further across the axis. That leaves you a lot less room to manoeuvre the histogram, by over or under exposing, before clipping the highlights or blocking the shadows.

    The best advice for minimising noise is to "nail the exposure" and what your experiment has successfully shown is that however good the metering system of the camera is, it is set up for an average greyscale reading and while this works for probably 90% of the time, your experience as a photographer will adapt that to varying conditions. The histogram is a valuable tool in determining how much latitude you have and as you have shown, +2 stops in dull conditions, or as little as 1/2 stop in bright contrasty conditions.

    With digital, we were always advised to underexpose by 1/3 or 1/2 stop to preserve highlight detail and camera manufacturers even added the annoying flashing clipped highlights function to keep us in check. Clipped highlights were the big "no no" of digital photography and if the consequences of underexposing was a slight increase in shadow noise, then so be it. As our expectations evolve, we quite rightly want to minimise noise as well and improvements by the manufacturers and experiments such as yours will increase our ability to produce better exposed, cleaner images.


    Well done.

    Colin
    Colin

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      #3
      Re: High ISO low noise...technique

      Interesting experiment Brain, please give some examples how this technique would work in reality.

      Not being negative. What about times when overexposing by two stops means you’re not actually capturing the detail you wanted, specifically in the brighter parts of the scene – you can’t bring back in raw processing what you haven’t captured as we all know.

      Millie

      Comment


        #4
        Re: High ISO low noise...technique

        Absolutely right both - just been out with the dog, considerably brighter now - and yes +2 stops was way over - so I've experimented further - down to 1/3 stop overexposed.
        I now have 127 images to see what I can and cannot rescue.
        I'll post my conclusions.
        Brian Vickers LRPS

        brianvickersphotography.com

        Comment


          #5
          Re: High ISO low noise...technique

          Here is one pic with +2 overexposed and corrected in DPP - as you say the background has just lost all detail.
          I will have to experiment further.
          Attached Files
          Brian Vickers LRPS

          brianvickersphotography.com

          Comment


            #6
            Re: High ISO low noise...technique

            I always did +1/3 on the 50 - reduced the noise to the point of it not being a major problem until iso3200, with some starting to creep in at 1600. On the 5d not seen any noise yet
            ef-r

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              #7
              Re: High ISO low noise...technique

              that's interesting, It makes me wonder why Canon dont set that as a default and then knock the brightness down as the file is saved to memory. I guess it depends on how much contrast there is according to weather conditions. 2 stops were ok this morning, but half a stop was enough this pm.
              Brian Vickers LRPS

              brianvickersphotography.com

              Comment


                #8
                Re: High ISO low noise...technique

                I read an article somewhere and I noticed that noise only appeared when brightening. That was one reason I was trying to get it 'right in the camera'. Noise only seems to appear in the darker areas.
                ef-r

                Comment


                  #9
                  Re: High ISO low noise...technique

                  Having reviewed my results - I think I did myself no favours using spot metering on the dog!
                  Some well lit images at ISO1600 seem ok even without any overexposure - these were probably helped by spot metering.
                  Backgrounds were all overexposed - some unrecoverable.
                  Conclusion is - if there is plenty of light then no overexposure is needed.
                  If dull then up to 2stops overexposure is fine.
                  Moderately bright needs 1/3 stop to 1 stop over exposure.
                  No one setting suits all.
                  Monitoring the histogram is the key I think - just as Colin said earlier.
                  Brian Vickers LRPS

                  brianvickersphotography.com

                  Comment


                    #10
                    Re: High ISO low noise...technique

                    The histogram is the digital photographers best friend, it's just that most of us don't realise it.

                    Colin
                    Colin

                    Comment


                      #11
                      Re: High ISO low noise...technique

                      Hi Brain,

                      Were you shooting in AV mode + spot metering off the dog - then using exposure compensation to dail in 2 stops of postive compensation?

                      Millie

                      Comment


                        #12
                        Re: High ISO low noise...technique

                        Millie,

                        yes , aperture priority, with exp compensation set to +2
                        then spot metering off the dog.

                        I have the function button set-up to change exposure compensation.

                        So its not a question of metering and then dialing in +2.

                        Is there a better method - what would you suggest?

                        Brian
                        Last edited by @imagesBV; 18-03-2010, 19:20.
                        Brian Vickers LRPS

                        brianvickersphotography.com

                        Comment


                          #13
                          Re: High ISO low noise...technique

                          FacebookTweet On my recent workshop in Iceland (July, 2003) I had a fascinating chat with Thomas Knoll about signal to noise ratios in digital photography....


                          Papa.

                          Comment


                            #14
                            Re: High ISO low noise...technique

                            You were metering exclusively for the darkest part of the scene, then over exposing by 2 stops.

                            The inevitable result is the dog over was exposed by two stops, and the rest of the scene is over exposed by 2 stops plus the difference between the dog and the background.

                            My concern is, even though this technique actually reduces noise, the cure is much worse than the disease; figuratively speaking.

                            I'd suggest the same as Colin get the exposure as spot on as you can.

                            Millie
                            Last edited by Millie; 18-03-2010, 19:40.

                            Comment


                              #15
                              Re: High ISO low noise...technique

                              Thanks Papathanassi, thats a good article.
                              Brian Vickers LRPS

                              brianvickersphotography.com

                              Comment

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