May 2015 - Digital Group - Focus Stacking and Noise

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May 2015 - Digital Group - Focus Stacking and Noise

Post by spb » Fri May 01, 2015 5:23 pm

Andy Hale gave us the benefit of his experimentation with Focus Stacking in Photoshop CS6;

The problem is that with macro work there is a very shallow depth of field (DOF) – which can be as little as 1 mm. When photographing anything that is three dimensional, focus has to be on the most important spot (eg the eyes, in the case of an insect) and much of remainder of the subject can be extremely soft. The problem gets worse as the sensor size increases and for this reason, macro photographers have found that compact cameras can give better results than SLRs in terms of DOF.

The smaller the aperture, the greater the DOF – this is the mantra that we learn. Unfortunately the high resolution of modern cameras reveals the diffraction effect whereby the smallest apertures create images that are significantly softer! This is an effect of quantum physics where light waves coming through a small aperture bend and interfere with each other.

The solution to this problem is Focus Stacking whereby multiple shots are taken, each focussing at a slightly different subject distance (by adjusting the focus ring on the lens or simply moving the camera). The multiple images are then combined in software to create a single image with much greater DOF.

The software must also deal with another problem which is that as the focal point changes, the size of the subject in the frame also changes. This is called Focus Breathing.

Andy demonstrated the process in CS6;
- File>Scripts>Load Files into Stack (which opens all of the images into layers in a single file whilst keeping them in sequence)
- Select all layers then Edit>Auto Align Layers>Auto (which moves the layers so that their primary features are aligned)
- Edit>Auto Blend Layers (which performs the Focus Stacking magic)
- Crop to remove the bad edges resulting from Focus Breathing

Depending upon the DOF required, it may be necessary to take 10 or even 20 images to fully achieve the required result and this can be quite processor-intensive so it may be tediously slow on a less-well specified machine.

This procedure is possible with versions of Photoshop CS4 onwards. If you have Photoshop Elements there is no specific Focus Stacking capability but there are other solutions. Elements+ which, for the princely sum of $12, restores most of the Photoshop features switched off in Elements – including Focus Stacking! There are also third-party solutions such as Helicon Focus (currently discounted for RPS members) , Zerene Stacker and Combine ZP free.

I have always assumed that digital image noise is an electrical effect arising from over-amplification of a weak signal. However it transpires that this has it’s origins in the randomness of photons hitting the sensor. In the dark area of a scene, very few photons are captured and in this small sample, the randomness becomes more significant, and also more visible as light pixels against a dark background. Noise can be minimised in a number of ways;
- Reduce the ISO to minimise the amount of amplification of the signal
- Use a camera with a larger sensor to increase the amount of light captured
- Increase the amount of light on the scene
- Expose To The Right (ETTR) ie use the highest possible exposure setting without actually over-exposing the highlights (ie the image histogram on the camera should be reach the right edge of the graph without going beyond it.

A certain amount of noise in an image is quite acceptable and will almost certainly be less than the grain that was an inherent part of images on film. However if you want to reduce noise in an image, the controls under the Details tab in Adobe Camera Raw (ACR) are excellent at reducing both colour and luminance noise. Don’t forget that even a JPG can be fed through ACR by using File>Open in Camera Raw (Elements) or Open As>Camera Raw (CS). Alternatively there are excellent third party plug-ins such as Topaz DeNoise

Interestingly, tools such as Topaz DeNoise can also used to clean up a High Pass sharpening layer and prevent any sharpening of image noise which is normally undesirable. See Nov 2013 Notes for more on High Pass Sharpening

Cheers, Steve

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