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Tuesday, November 6, 2012

Extended Depth Of Field

Focus stacking defeats the limits of optics


This Article Features Photo Zoom

How deep would you like your depth of field?
The choice is yours. Today, there are virtually no limits. You can extend depth of field beyond the physical limitations of any lens/camera system with multishot exposure practices and software that composites multiple exposures.

1) Foreground in focus; 2) Background in focus; 3) Two exposures combined to achieve infinite depth of field

To do this, you first need to make a set of focus-bracketed exposures, optimizing focus in different image areas. How many exposures you'll need will depend on how much depth of field a scene contains. At a minimum, make two exposures: one focused on the foreground and another focused on the background. Making three exposures is better, one each for foreground, middle ground and background. When dealing with extreme depth of field, like macro photography, you'll want to make more exposures, at least three, probably six, possibly more. When in doubt, make more exposures than you think you'll need; you don't have to use them all when you stack the separate exposures, but they'll be there if you need them. Unlike bracketing for HDR, it's almost impossible to automate these types of bracketing sequences in-camera as focus needs to be adjusted for each frame. However, for tethered shooting, you can use software such as Helicon Remote to take control of your camera and automate this process and other bracketed sequences like HDR and time-lapse. Whenever possible, use a tripod to make focusing during exposure more precise and registration during postprocessing easier. While using a tripod always delivers more reliable results, don't let this stop you from trying this technique handheld, especially with simpler sequences, like those used in landscape. You may notice that in cases involving extreme depth of field, the relative size of objects may change between individual exposures. These effects will be adjusted automatically during the merging process.


4) Photoshop's auto-masked layer stack
Before you combine a set of focus-bracketed exposures, make all the RAW conversion adjustments you'd like to make to the final file. It's quick and easy to process a focus-bracketed series of files; process one file in the series ideally and then sync the other files to it. Once a RAW file is rendered, you can't reaccess the data in it, such as "recovering" highlights or "filling" shadows, without re-rendering it. While you can adjust lens distortions after stacking with Photoshop's filter Lens Corrections, it's much easier, faster and more robust to apply Lens Corrections during RAW conversion, before focus stacking 16-bit TIFFs. And while you can make adjustments to files in Photoshop that are similar to Adobe Camera Raw's or Lightroom's white balance, noise reduction and Clarity, they're not identical.

Once you've processed a set of focus-bracketed exposures, you can automate the process of stacking and blending them into a single file in Photoshop. Take these steps:

1. Select the exposures to merge— File > Automate > Photomerge.
2. Choose the blending method or let Photoshop choose for you; Auto usually works well.
3. Click OK, and Photoshop will do the rest, making each file a layer in one document, aligning them and masking them to reveal the sharpest information.

 

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