Techbytes: In Search of Unlimited Depth of Field
I’ve been fighting the limitations of a camera to cover focus a good number of years. Recently there has been an awareness of digital photography's ability to combine multiple images and extend the limitations of image capture. While multiple exposures extend dynamic range (HDR), recently released software allows multiple exposures to emulate scanning light photomacrography.
Historically, several techniques have been used to increase depth of field (DOF). The simplest was to stop the aperture down. It also increased diffraction, though, and had limits, which couldn’t fully encompass the range needed for most macro photographs.
I’ve looked at other techniques over the years to deal with this challenge. View cameras don’t generally come to mind when thinking of macro photography, but they do allow for use of the Scheimpflug principle. This rule, developed by Theodor Scheimpflug, defines the orientation of the focus plane in a view camera, or small and medium format cameras, with tilt focus lenses. The lens plane is moved to a point that is not parallel to the image plane. This has the effect of re-orientating the focus plane to coincide with the direction of the desired area of the subject. This gives the appearance of greater depth of field. Since this is a reorientation of DOF rather an actual increase, application of the Scheimpflug principle works best when one is trying to increase DOF along a single axis. Unfortunately this is not the case in most photomacrographic situations.
My favorite solution is scanning light photomacrography. This technique passes the subject through a sheet of light that is thinner than the DOF of the camera. The camera is focused so that it only sees the area of the subject being illuminated. The result is a set of seamless contiguous exposures that build a sharply focused image of the entire subject. It is this digital analog (no pun intended) of scanning light photomacrography that I would like to explore.
Currently the two most often used software applications for this purpose are Photoshop CS4™ and Helicon Focus™. Photoshop can be used to blend layers with different points of focus to give the appearance of increased depth of focus. Helicon focus has taken this concept of blending multiple images together, to create extended depth focus to its logical conclusion.
One begins by taking a series of photographs in which the point of focus is moved in every shot (Figure 1). The software then auto-aligns and seamlessly blends the “stack of images,” after first analyzing the images in the layer stack and determining the sharpest area on each layer (Figure 2). It then auto-masks them to create an overall sharp image (Figure 3). The user interface is very intuitive and allows for retouching of the final rendered image from the individual source files (Figure 4). This allows the user override any choice made by the software.
Helicon Focus can also be used to achieve panorama blends, but I’d like to concentrate on the extended focus aspects of the program.
Working with the program in a macro mode, I found that a geared focusing rail was extremely helpful. Helicon Focus does best when there is an overlap of the focused areas in the sequence. My first efforts were disappointing, but I soon learned that more is better. I found that typically shooting a dozen or so source files per rendered image was enough to give me about a 25% overlap of the focused areas. The use of strobe illumination also helped. Helicon Focus needs to see contrast to judge the areas it will mask. Any softness due to motion in the subject defeats its ability to do this. Once one gets used to workflow, the software is fun to use. I found myself watching the rendered file appear with the same anticipation I used to feel when I watched a print come up in a tray of developer.
There are a number of different parameters that can be adjusted to control the rendering. Once the image is rendered, it’s possible to enter a retouching mode. Cloning from the original image to the rendered image can correct a multitude of sins. When finished, the image can be saved in a number of formats.
I found Helicon Focus to be a stable and wonderful addition to any biomedical photographer’s tool chest. A fully functional trial version is available for download from www.heliconsoft.com. There are several options for registering different packages, ranging from $30 to $300.
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Table of Contents for VOLUME 35, NUMBER 1