Noise Reduction with Adobe Camera Raw (ACR)

[Gidders]

KCs thread about using high ISOs prompted me to run a few experiments and then put digit to keyboard. A while ago when I was looking for noise reduction software I settled for Noise Ninja having read a number of articles and in particular Michael Almonds very helpful comparison of number of programmes. At the time I was shooting in .jpg not having a camera with a raw mode which has now changed my thinking.

Like most things there are plusses and minuses to noise reduction. Getting rid of noise means getting rid of pixels, which means loosing detail. Noise comes in two forms – colour noise – which shows up and random coloured speckles and luminance noise, which looks more like the grain of fast film.

Now a commonly held wisdom for digital photography is to under expose slightly so you don’t blow out the highlights. However noise is exacerbated by under exposure so it’s important to get the exposure right. I’ve discovered (by using an 18% grey card) that my new 20D underexposes by between 1/3 & 1/2 a stop, so particularly when shooting at high ISOs, I can compensate for that.

Since acquiring a camera that can shoot RAW I do all my noise reduction through ACR converter (although I expect other RAW converters have similar options). In ACR, when you open an image there are a number of tabs. The one that deals with noise reduction is the detail tab. On this there are three sliders

• “sharpness” which I set to preview only which means that the image is not sharpened on conversion – I leave my sharpening until I’ve finished editing

• luminance smoothing

• colour noise reduction

To make life easy, I have developed a range of “standard” setting for my camera. To do this I have created a test target. test target.jpg If you want to understand how I created this, or download the printable one I’ve created from my website, see the following posts. I then printed this and photographed it at ISO 100, 200 etc – 3200 ensuring that I optimised the exposure.

The next step is to open the images one at a time in ACR, select the details tab and adjust the sliders to remove as much noise as possible without removing detail. The steps I followed are:

• With the image open in ACR select the details tab ACR detail tab1.jpg

• First set the sharpening to preview only (Edit >Camera Raw Preferences >Apply sharpening to – preview images only). I believe that sharpening is better carried out after all outer post processing is complete so this setting lets you judge roughly how the image will look and then more controlled sharpening can be applied in Photoshop with unsharp mask or smart sharpen (CS2)

• I then set a sharpening amount of 25, which, to me, gives a reasonable balance between sharpness and artefacts.

• Set the luminance & colour noise sliders to 0

• Looking at the grey scale patches in the centre of the target in the 8-12 range, adjust the colour noise slider until the coloured speckles disappear. Check this setting buy looking at some of the saturated coloured patches. Check the ISO resolution chart mid right for detail loss.

• Repeat with the luminance tab. In my experience this needs a higher setting than the colour noise slider and is more destructive of detail so check your ISO resolution chart carefully. It’s a trade off between removing noise and destroying detail.

• When you are happy with these setting click the arrow to the right of the settings box (circled in red) ACR detail tab2.jpgand select “save setting subset” from the drop down ACR save settings subset.jpg

• Select “details” from the subset dialogue box and save the setting with a descriptive name like Noise ISO 0100 ACR settings details.jpg(I add the preceding 0 so that when I get to ISO 0200 & ISO 0400 they appear in menus before ISO 1600 & ISO 3200)

• Repeat for each of your ISO test targets.

You now have a standard set of noise reduction setting for each of you camera's ISO setting FOR CORRECTLY EXPOSED images.

...continued in next post

[Gidders]

There are two ways of using your standard settings:

1. Select the appropriate setting from the setting dialogue box ACR Noise settings.jpgwhen you open the image in ACR and then make any other changes to the adjust tab as usual
2. Apply directly from Bridge. Select the image(s) to apply the setting to, right click and them select the required setting from the pop up. Bridge Noise setting.jpgThis is the most efficient way because all images in a file at the same ISO can have the same setting applies at once.

The setting I ended up with for my 20D are
IS0 _ Luminance _Colour Noise
100 _____10 _________0
200 _____10 _________5
400 _____15 _________5
800 _____25 _________10
1600 ____35 _________15
3200 ____55 _________20


I find that, unless I have to apply more that ½ stop exposure correction when I open the image, I can usually leave these setting alone. Over ½ stop I either readjust the colour noise & luminance manually or, apply the standard setting for the next ISO up. Eg an image shot at ISO 200 but one stop under exposed, I use the standard noise setting for ISO 400

I have compared the results of using ACR with Noise Ninja on the target shot as ISO 3200 and both do a pretty good job. See 100% crops of selected parts of the test target attached, with no noise reduction and the ACR & Noise Ninja each with then sharpened with smartsharpen 150% radius 0.5. There is not much to choose between them on colour noise removal, but to my eye Noise Ninja apples a little more luminance smoothing, but at the expense of a little detail looking where the line merge on at the resolution chart. I could have increased the luminance smoothing in ACR but my preference is to retain the detail – yours may be different. The BIG advantage of ACR is that it is a one shot process.

Hope this all makes sense – if not let me know.

[Gidders]

1. First I downloading an image from the test images posted on the Imaging Resource taken with a Nikon Dx2 (4.8Mb).
2. Then because I was not sure of the colour accuracy of the Macbeth Color patches in this image I downloaded the Macbeth Color Checker (ProPhoto RGB) from Colour Remedies. This file is in ProPhoto colour space and has the RGB values of each patch on it IN THIS COLOUR SPACE

ProPhoto is a very wide gamut colour space. Adobe RGB is also wide gamut but not as wide as ProPhoto and the sRGB is narrower again. As I always work in Adobe RGB colour space because I’ve been lead to believe that inkjet printers can not take advantage of the ProPhoto colour space but it is a benefit over sRGB. If you are preparing work that is only going to be viewed on a colour monitor, eg for web use, then sRGB is fine

3. I converted the Color Checker to Adobe RGB colour and then sampled and re-encoded the patches with the RGB values in Adobe RGB.
4. I then resized and superimposed this onto the DX2 image I had downloaded.
5. Apply a little smartsharpening and I have my test target ready for printing.

Now you might say (with some justification) that all that messing about with the colours is a waste of time because my printer won’t reproduce the colours accurately anyway. To some extent that’s true although I have had my printer profiled for each paper I use so it should be pretty close. However I did it more for my own benefit to understand the differences between the different colour spaces.

If you don’t want to go to the trouble of creating you own target you can download mine by clicking here. NB this is in Adobe RGB colour space so the colours will look a bit flat in a not colour managed environment – ie when you open it in your web browser, but when you open it in Photoshop it will look right (the Macbeth Color Checker is fairly low in contrast anyway)

[pxl8]

One thing to remember with working with different colour spaces is to use the one that best fits your image. Whilst ProPhoto has a wide gamut the data is still represented by an 8 or 16bit image, this means you lose tonality for the sake of the extra colour space. If your image doesn't use the extra colours in the larger space you have effectively reduced the number of colours you have available so you're worse off than working in sRGB, esp. if your device driver only support 8bit data.

To illustrate although it's not a true example:

Let's say you work in a colour space that has 512 shades of grey, the 8bit image is now trying to represent 512 shades of grey using 256 values. If your image has black to mid-grey gradients you will likely see banding because you've lost the tonality