Forked Blue Curls (Trichostema dichotomum) and an Explanation of Focus Stacking

I know I posted some similar pics last year, but I can’t get enough of these flowers. Although we literally had thousands of these flowers blooming in the yard this year from seed I collected last fall, I didn’t get around to photographing them until on a WGNSS Nature Photo Group trip to Don Robinson State Park in early September.

Forked Blue Curls (Trichostema dichotomum)

These flowers are both tiny and deep in multidimensions. Because of this, a narrow aperture is typically required to photograph with enough depth of field to get all parts of the flower in reasonably sharp focus. However, stopping down the aperture needed for this greater DOF comes with a couple of problems. First, adjusting the aperture too much above f/14 or so begins to dramatically lower sharpness due to the diffraction of the incoming light. Second, and probably more importantly, a small aperture will also bring more of your background into focus. Depending on the closeness and business of the background, this can simply ruin a nice composition.

So, what’s another alternative to stopping down? This flower is a perfect example of when it is a good idea to use focus stacking. In focus stacking, the photographer takes several images at a lower aperture to get “slices” of the subject in focus. Depending on the size of the subject, the focal length of the lens you are using and the magnification you are shooting it at will determine how many of these slices are required to get the entire subject covered. Then, you combine the individual images, or slices, in the computer to hopefully get a perfectly sharp subject with the creamy out-of-focus background that makes a nice image.

Forked Blue Curls (Trichostema dichotomum)

For my macro focus stacking, I typically use a 180mm macro lens and shoot at f/8. Depending on the criteria mentioned above, I will typically need 10-50 images to cover a subject. There are a few ways you can go about taking the images needed for a focus stack. You can shoot them manually, typically taken on a tripod and moving the focus ring a little at a time, or by using a macro focusing rail, which you move your rig closer to the subject for each image. If you are using an autofocusing lens, there are also automated ways to collect the images needed for a focus stack. The one I use is a specialized extension tube that has a computer chip inside. I let the extension tube know what the focal length is of the lens and the aperture I have the camera set to, make sure my focus is just before the first part of the subject I want to focus on and then hit the shutter release. The camera will then take image after image, changing to a deeper focus with each one until either I feel I have covered the entire subject or the lens hits infinity. Finally, newer cameras allow you to focus stack using controls built into the camera’s software. These typically provide a wide range of options for the photographer to control. I imagine using this has somewhat of a learning curve. I have not used this in my Canon R5, partly because I like the simplicity of what I use and partly because you cannot use flash when using this feature in Canon cameras to date.

If you’re having troubles getting the types of images you want of small subjects under high magnification, give focus stacking a try. But, remember, your subjects need to be stationary!


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