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Thread: How and when do you apply noise reduction?

  1. #1
    The Blue Boy's Avatar
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    How and when do you apply noise reduction?

    I'm currently reading Real World Image Sharpening with Adobe Photoshop CS2: Industrial-Strength Production Techniques by Bruce Fraser as suggested by a certain person here. I'm about two thirds through and it's been revelation but it's thrown up some questions.

    Now as most of you know I'm relatively new to digital (Two & half years) so when I started using Photoshop it was quite a steep learning curve. I read some bad online tutorials (not here!) and I realise now that I've developed a few bad habits regarding "workflow" that I think need to be reviewed.

    I'd just like to know how and when everybody else here applies noise reduction, what program do you use, plugin filter?, and do you do so on a certain channel.
    Last edited by The Blue Boy; 30th January 2009 at 09:22 PM.

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    Moderator Dave Humphries's Avatar
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    Re: Noise Reduction

    Hi Mark,

    I use the Neat Image plug in for PS Elements 6 (I gather it also works standalone), cost about $30 from memory.

    Not sure if I'm doing it right either, but I do it fairly soon after the import from ACR.
    It's fairly obvious you don't want to sharpen the noise, so it must come before that, but I also do it before any levels or cloning etc. probably after cropping (if I didn't do that in ACR).

    I hope the more experienced PP'ers will correct me if I'm wrong.

    Cheers,

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    Re: Noise Reduction

    Quote Originally Posted by Chelseablue View Post
    I'd just like to know how and when everybody else here applies noise reduction, what program do you use, plugin filter?, and do you do so on a certain channel.
    Hi Mark,

    When I recommended the book I should also have mentioned that it's probably just as relevant to noise reduction as it is to sharpening.

    In my opinion, the key to noise reduction is avoiding the noise that requires reduction in the first place - and the best way to do that is to get the exposure right.

    In a digial environment, correct exposure is a real can of worms. Many say "expose to the right" (which can be good advice), but it doesn't tell the full picture - ultimately, where to put things on the histogram depends on how dynamic the scene is and whether or not you've got things like shadow detail you want to retain or complications like specular reflections.

    I wrote a wee bit about this in a couple of posts here:
    Choosing an Exposure Setting: Dynamic Range vs. ISO Speed

    At the end of the day, minimising noise is all about getting the levels of the detail you want to retain as far above the noise floor as possible - which inturn means not wasting the sensor's dynamic range - which (in practice occurs very easily, and very often) For anyone still awake at this point, it's worthwhile doing a quick test to illustrate it ...

    ... Keep in mind that sensors are linear in operation - so as I mentioned in the above referenced thread, for a 12 bit A/D converter (most common), 2048 levels of the available 4096 are used on the brightest stop, and 1024 of the remaining 2048 are used in the 2nd brightest stop. Here's the test - whack on your favourite lens - stop down the aperture by 2 stops (eg if it's an F2.8 lens then stop down to F5.6) and whilst looking through the viewfinder press your depth of field preview button in and out a few times - you'll see the image through the viewfinder go noticable dimmer, but not by a heck of a lot. A full THREE QUARTERS of all the information captured is contained within the difference in highlight levels that you see - which by the time you get down to the shadow areas (where the noise is also lurking) doesn't leave many levels - and those that do remain are closer and closer to the noise floor. So - if under-expose an image by 2 stops (easy to do as it's hardly even noticeable) (remembering that the sensor is working in a linear fashion, not on the gamma adjusted response curve that we see) than already your back is up against the wall in that when you raise your highlights by two stops - you're also raising your shadow data, and dragging up your noise floor in the process (remembering that the camera really can't tell the difference).

    So - get exposures right - reduce noise - and eliminate the need for noise reduction :)

    Hope this helps (someone ... ANYONE!) (my fingers hurt!)

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    Re: Noise Reduction

    Quote Originally Posted by Dave Humphries View Post
    Not sure if I'm doing it right either, but I do it fairly soon after the import from ACR.
    It's fairly obvious you don't want to sharpen the noise, so it must come before that, but I also do it before any levels or cloning etc. probably after cropping (if I didn't do that in ACR).

    I hope the more experienced PP'ers will correct me if I'm wrong.
    I think you're pretty on to it Dave. I tend to do dust bunny removal first - then capture sharpening - and then levels so that I can better see just what else needs doing. Sharpening is the enemy of noise so noise reduction before sharpening is one option, and masking off noisy areas prior to sharpening is another. Sometimes you can even reverse the mask and apply a blur or even (ab)use the dust and scratches filter to clean it up.

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    The Blue Boy's Avatar
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    Re: Noise Reduction

    Thanks Colin,

    I actually understand all of the above. However as we all know we have to raise the ISO from time to time and I was (am!) curious as to how everyone else deals with noise in post.

    Colin, Im not suprised your fingers hurt as you've replied again as I've been writing this!

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    Re: Noise Reduction

    Quote Originally Posted by Chelseablue View Post
    Thanks Colin,

    I actually understand all of the above. However as we all know we have to raise the ISO from time to time and I was (am!) curious as to how everyone else deals with noise in post.

    Colin, Im not suprised your fingers hurt as you've replied again as I've been writing this!
    Typing with my tongue now ...

    The other thought that springs to mind with high-ISO noise is "just don't worry about it" - at extreme ISOs it may look ghastly, but if it's only ever destined for a 6 x 4 print then often it's hardly noticeable, it's usually only us photographers (working at 100% view of course!) who are really disturbed by it (I'm convinced there's a little bit of Obsessive/Compulsive Disorder in every photographer - certainly is for me anyway).

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