Selective focus is a technique used to draw the viewer's attention to a specific subject in an image. While commonly employed by macro photographers, it is also effectively applied to other genres.
A subject in sharp focus surrounded by a blur is considered selective focus. This can be achieved in-camera by shooting with a lens that offers wide apertures which yield a shallow depth of field (DOF). I often shoot with an aperture, or f-stop, of 3.5 on my Canon 100mm 2.8L IS macro and 4.0 with my Canon 180mm f3.5L macro. However, one can achieve selective focus on much smaller apertures (see the Orchid image below). The circumstances of a given setting ultimately dictate the f-stop that I choose and many times, I will shoot at a few variations so I can compare the images later.
Other contributing factors include the photographer's distance to the subject and the distance between the subject and the background and/or foreground. Each situation is unique and, thus, it's important not to look for a recipe, but rather to take into consideration all of the elements at play before adjusting your settings. Give it your best shot and then evaluate the results and analyze what needs to be changed to achieve your vision. It's an iterative process with much to be learned through experimentation.
Some examples of selective focus:
This Meconopsis, Himalayan Blue Poppy, shot at f/3.5 with my 100mm macro, was made by gently moving one stem out of the way and another flower in front of my lens. What the scene looked like before:
The white Phalaenopsis Orchid, above, was shot at f/7.1 with my 180 mm. I was very close to it as were the other flowers in the front and rear. Both the Blue Poppy and the White Orchid are examples of what I refer to as extreme selective focus in which both the foreground and background are used to sandwich the subject in a "floral aura." The take-away is that different lenses and apertures, paired with varying distances to the subjects and their surrounds, can yield similar results.
Using a narrow plane of focus can tell a story. It felt like these Forget-Me-Nots were peeking out to watch critters in the garden from behind the Iris leaves.
Not everything has to always be completely out of focus. Sometimes it offers context to have other flowers and objects in the image especially when they add to the story.
Occasionally, when shooting at wide apertures, it's hard to get the whole subject in focus. At times like this, I'll create several images in which the focus is set at different places from the front to the back of the subject. To get the nice creamy back and foreground surrounding this tiny mushroom, I shot at f/3.5 with my 180mm macro, and made at least three images with focus on the front of the cap, the stem, and the back of the cap. In post-production, these were blended in Adobe Photoshop.
Shallow depth of field, or plane of focus, can result in a pleasing aesthetic also known as bokeh, the style of which can vary dramatically depending also on optics, lighting and vantage point. This is a fodder for another discourse and begs comparison between different lenses so stay tuned for a future blog on the subject of bokeh styles!