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
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:
- 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.
- 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