Focus Stacking to extend Depth of Field (DoF)

19th March 2013
Here's a picture of a small (a few millimeters long) Stenus Rove Beetle, ready to upload to my Flickr photostream. There's a reasonable depth-of-field with respect to the beetle (i.e. the nearest and farthest parts of the subject appear acceptably sharp). This is virtually impossible to achieve though, with a single macro image.





So how has this effect been achieved? Firstly, lets consider some of the issues that affect photography of the small and very small. Macro lenses are designed to allow very short working distances between the end of the lens and the subject. The issue associated with this though, is that DoF is extremely small when focusing on close objects.

Traditionally, to increase DoF (in landscape photography for example), the aperture of the lens would be "stopped down". That is, the aperture would be reduced (say to F18 or F22) and typically, the camera would be on a tripod to cope with the lengthened shutter speed. With macro setups, as the aperture gets smaller, the image quality (IQ) gets poorer (because of light diffraction issues). There's always therefore a "trade-off" between DoF and IQ.

For this focus-stacking exercise, the camera settings were F11, 1/250th second and ISO 100. Lighting was provided by a TwinLite flash. If you click on one of the smaller images, you will see that even at F11, only about 20% of the beetle is on focus. I took six shots in total, with the camera resting on beanbag. Between each shot, I fractionally advanced the camera; starting at the tail-end and working towards the head. The beetle moved before I got to the left antenna and there is an area of the abdomen around the forelegs that is a bit soft. Overall, the outcome was acceptable to me.

So, how can different areas from multiple images be combined? I won't go into details here, but there are several options. With popular image processing packages like Adobe Photoshop, it's possible to "clone" areas from one image onto another. This works best when there are just two or three images. Similarly, images can be placed on different "layers" and blended together.

In this example I used Zerene Stacker software. This is designed specifically for this purpose and very cleverly aligns the images and then stacks them together to produce a composite. It does rely (generally) on the "subject" not moving about but does cope very well with hand-held image stacks. You can see from the three small images that I did introduce some lateral movement when capturing the images. I did have the piece of bark and the camera balanced on the top of a fence though!

Comments

Photo comment By Rachael: Nice job! It's a shame the rules of many/most nature competitions not not allow focus stacked images. Perhaps that explains why so few insect shots seem to get through. Congrats on your BWPA commendation, by the way. Cracking image.

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