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Tuesday, November 16, 2010

Focus Stacking: a technique for increasing depth of field

Focus stacking. A technique which is used to increase the depth of field of a macro photograph.

As the focussing distance decreases and the image magnification increases, the depth of field of an image decreases. Add this effect to the phenomena that a larger sensor size will always have a shallower depth of field than a smaller one, makes shooting macros with a medium format camera a significant challenge.

The Depth of Field Challenge

To overcome thedepth of field challenge, one can select a smaller aperture. The Hasselblad HC 4/120 macro I use is optimized at f/11, though still gives good results at f/16. You can see this from the MTF curves that Hasselblad publishes. Apertures smaller than f/22 usually will result in a softer overall picture, as diffraction will set in. I usually shoot watches at either f/11 or f/16.

At f/11, on my 48mmx36mm sensor in the Hasselblad H3d-39, the depth of focus is about half a milimeter at 1.94X magnification. This is the highest magnification possible using the HC 4/120 and the two extension rings I own - the H26 and H52, making a total of 78mm extension. The HC 4/120 on its own can do life size 1:1 magnification. This miniscule depth of focus will mean that when I take the system to the extreme magnification, and selecting f/11 which is sharpest, I can only get one part of the movement in focus at any one shot.

To increase the depth of field, one methodology, which is rather technical, is focus stacking. This is a special technique not available during the film era, and is afforded only by digital manipulation.

What the technique entails is to combine several images, each taken of the same subject, same camera and lens settings, but focussed at slightly different points of the image. The idea is to then take all those images (a stack, as it is refered to), and combine them digitally, selecting only the parts in focus to blend into an image which has significant depth of field.

My technique is to mount the camera on a Manfrotto macro focussing rail. This rail allows me to move the entire camera system in small amounts using the micrometer screw on the rail. I mount the rail on top of my Photoclam Multiflex head.

I begin by focussing for at the nearest point I want to have sharp focus. And in small steps, run through the system to focus on important parts I want to be in sharp focus till the farest point I need to be in focus. I do this by moving the helicoid focus mechanism on the lens or by moving the rail to its appropriate position. If I want the maximum magnification, I just set the lens helicoid at min focus distance, and hence max magnification, and move the entire camera assembly using the rails.

I roughly work this out in my mind how many shots intermediate I would need to render everything between the near and far points in focus. Instead of using equal steps, I usually make sure that the intermediate points between min and max focus distance are at the points on the movement (subject) that I want to render sharp, though I ensure that there is sufficient focus overlap between points. This helps me visualise the photograph, and also work out what micrometer movement steps I need.

I then back the rail to the start position which is the point of nearest focus.

I then make the exposure reading to correctly expose the subject at f/11 and usually a shutter speed of 1/250s or 1/160s. Typically I shoot using one or two Profoto Compact 600W-s lights on their own light stands, shooting into white reflecting umbrellas. I have toyed with the idea of using soft boxes, and I will probably get them later, but for most purposes, the umbrellas throw a light which is large and uniform enough to evenly light the watch.

Then I start shooting. Usually locking the mirror, and taking care only to advance the micrometer by a small amount so that I capture a series of pictures, each with the sharpest point of focus on one of the critical parts of the watch that I cant in sharp focus. I almost always shoot tethered to the computer if possible, so I can immediately view the image at full resolution. In situations where a tethered computer is not convenient, I will then shoot untethered.

In Phocus, I then export the stack into TIFF files. I usually work on 8 bit TIFF files for web and small prints, but will use 16 bit TIFFs for larger prints. And use either Photoshop CS4 to blend the images together or an external program called CombineZF (freeware).

Sometimes CS4 is able to do a good job, sometimes not. And the same for CombineZF.

Here is a stack I took of Laurent Ferrier's journeyman piece. This is an extreme macro, at 1.94X magnification, and is a result of a stack of 5 images.

From Photoshop

And using CombineZF. Usually the results from CombineZF are better. The algorithm used makes intense use of the CPU cycles, and is rather slow. And the result is rendered as jpeg files downsized to 120dpi density, while Photoshop keeps the original resolution. This may be a problem when stacking for larger prints.

For this reason, I usually try and work with CS4 on large prints. CS4 is also somewhat faster, but sometimes it makes masking errors which might require a lot of retouching up work.

Apologies that both images are adjusted differently, but it is immediately clear that either image has more depth of field than a regular one shot image.
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