Focus Stacking – Something New for Me

Some of you may know that I love photographing the unique wild flowers of Western Australia, especially in the pristine bushland on our farm near Wandering, south east of Perth. We have about 400 different varieties, just on a couple of hundred hectares.

On my late mother’s birthday, 19th August, I casually glanced through the book Wildflower Country by William and Kaisa Breedon, a magnificent volume featuring one of my favourite wildflowers on the cover. I was struck by the stunning quality of the photographs in the book. They were not so forthcoming on the details of their methods, so I did a bit of online research and guessed they were using focus stacking. I resolved to try the technique for myself. [some results]

Normally, photographing a flower in the wild requires some compromises. For many years, I have resorted to flash photography, using a hand-held flash to simulate low-angle lighting that creates interesting light effects for flowers. Flash is especially useful in fading light, darker cloudy weather, or when the wind is blowing and the flower is moving, even slowly, ruling out anything but a short exposure. Here’s an example above, a rare form of patersonia on our farm. I first saw one of these about 15 years ago, and I have been looking out for another. I found this one close to sunset so flash was the only option.

Yet there limitations. To get significant depth of field, to see at least some of the flower in sharp detail, I had to use a small aperture setting with my 100 mmm macro lens, around f22. However, diffraction limits the resolution of even the best lenses at this aperture to about 5 pixels on my 3000 x 4000 pixel image sensor. My camera is a Canon EOS 80D, so the image sensor is smaller than in a full frame SLR camera.

Also, the background is often black or very dark. It’s difficult to obtain natural light in the background.

The principle behind focus stacking is to capture anywhere from 5 to 50 images of the subject, each with relatively wide aperture and rather shallow depth of field. Special image processing software combines all the images to generate a single image with far more in razor sharp focus. Typically lenses produce their best resolution at an aperture setting around f8. With a wider aperture, compromises in the lens design start to become significant and the resolution degrades. At this aperture, my 100 mm macro lens theoretically produces a resolution of about 1.5 pixels, several times better than at f22.

Here’s one result.

Below is an image of Thryptomene australis, a small tree that flowers profusely through September and October. On the left side is a single image at f8 and the blurring of much of the interesting parts of the flower is obvious. On the right is the result of merging 33 images. I could have captured more images and retained sharp focus for the entire scene had I wanted to. So in some ways, the improvement is subtle, but the enlargement beneath shows the level of detail this technique can capture.

Thryptomene Australis – left is single image, right the result of focus stacking.
Enlarged section of focus stacking result

I use two apps. Zerene Stacker is a superb purpose-built app for focus stacking and runs on my laptop. The user interface is not fancy, but it works far better than Adobe photoshop and includes a retouching studio. It automatically aligns the images before starting, matching the slight change of scale as the lens focus changes.

Bees, flies and ants love the flowers too. If they are blurred, the stacker will ignore them. But sometimes, a fly or ant will appear in sufficient focus in different parts of the flower for perhaps three or four of the images. Retouching enables me to decide whether to include her or not. Also, a slight puff of wind can distort part of the flower in a couple of images, and I can correct for that.

The other app is Camera Connect & Control (CCC Pro) on my phone with a really useful feature. Its focus control changes the lens focus by one of three fixed increments: small, medium or large. I can set up my camera where there is no way to see its built-in viewfinder, yet see the image on my phone while standing in a comfortable position, shaded from the sunlight. I set up the focus so that the nearest part of the flower is in sharp focus. The rest is blurry. I wait till the light is just right, then I capture an image and then tap the focus shift button. I repeat that 20 – 40 times, shifting the focus each time, till the rear of the flower is in focus.

Recording RAW images makes for fine results, but it takes 10 – 30 Gb of disk space after processing the RAW images to TIFF format for the stacker to work.

One other tip. I use a length of wire to stabilise the main part of the flower, minimizing any movements in the gentle puffs of wind now and then. It helps to be out just after dawn when the wind is almost dead calm, or just before sunset. Good insect repellent is essential as you need to stay anchored to the spot for 10 – 20 minutes. And a water bottle of course. I think the results make it all worthwhile. I feel so blessed to have access to such wonders of nature and sharing the images with others is part of the reward. Take a look for yourself. I still have a way to go before I can match Kaisa’s work, but the improvement on narrow aperture flash photography is remarkable. A nice birthday gift. Are you interested enough to try it for yourself?