Focus Stacking

I believe our development as photographers goes in cycles of continuous increasing (bootstrapping) our skills both on the technical and artistry side of our craft. Today, thanks to a winter storm where we live, I spent the day at home concentrating on increasing my technical skills; and in particular how to achieve tack sharp images over the depth of focus that I require for my capture.

All photographers are conscious of the limitations that our lenses and cameras bring to our craft, namely: depth of focus and aberrations inherently linked to the ‘physics’ of our equipment. When we first develop as photographers we soon become aware that you can only be in focus in one plane that is orthogonal to the lens axis (assuming you are not using a tilt shift lens). All other image slices are, by definition, out of focus. However, we accept this ‘out of focusness’ as long as it looks acceptable when we print or project it (on paper or our computer screen).

Typically we define a print to be tack sharp by line pairs per mm, eg 10 lp/mm for a 250mm print size is usually considered ‘good’, and we talk of circles of confusion for our sensors, eg for my Canon 5DIII a CoC of about 30 microns is typically quoted as the number to use for depth of focus calculations. 

There are many depth of focus calculators on line and you can download Apps to your iPhone or IPad as well. The majority of these calculators, however, suffer in one key area, namely they don’t account for diffraction. 

After comparing several Apps and looking into some math, I have now settled on three complementary Apps from the same author: http://goo.gl/qU792m. If nothing else I encourage you to read the articles that are on the home page.

I have all three photography Apps (TrueDOF-Pro, OptimumCS-Pro and FocusStacker), and can recommend all three as money well spent. Having all three, and reading the author’s articles, will greatly increase your depth of focus understanding and your tack sharp image capture.

Rather than repeat what you can read, here is an example of what can be achieved. The attached is a test shoot I took using FocusStacker with my 14mm lens (on the 5DIII). To get the best tack sharp image, rather than use a blur spot of 30 microns (blur spot = the RMSQ (CoC and the diffraction spot)) I used one of 15 microns.

Using TrueDoF-Pro I knew that I would need to shoot at just under F8 and focus at about 9ft. These numbers would give me an acceptable focus from about 4.5ft to infinity. But I knew I could do better by focus stacking. 

Turning to FocusStacker, I knew I needed to set my aperture to F7.1 and take 5 images at 1.1ft, 1.4ft, 2.0ft, 3.3ft and 10ft. Taking these images would increase my (tack sharp, 15 micron class) depth of focus from infinity down to 1ft. About as good as it gets!

To make things interesting I also decided to do this in a high contrast environment (internal lights and an outside scene) that needed bracketing, so at each focus point I took five brackets.

Having captured the data, it was ‘simply’ a matter of following this workflow:

• Ingest the images into LR
• Carry out basic corrections, eg white balance
• Export from LR each set of five brackets to Photomatix 5
• Use Fusion in Photomatix rather than Tone Mapping (more photo realistic)
• Auto import back into LR
• Export the five fused brackets into Photoshop-CC as layers
• Align the layers
• Auto Blend the layers
• Auto import back into LR
• Finish off in LR

Bottom line: as we develop our photography skills, we become ever more critical of our efforts. Composition, colour balance and other artistic skills sets are all well and good, however, if the data captured is too soft, or just out of focus, then we will not achieve the desired result: personal satisfaction and, hopefully, praise. Single focus image capture may not always be sufficient. If you wish to capture large depths of focus, eg 1ft to infinity, then focus stacking is a must, and so are some calculations. I thoroughly recommend the three Apps I have mentioned above at http://goo.gl/qU792m