K. W. Bridges
Selective Focus with Focus Stacking
kim@hawaii.edu
May 28, 2023
This document is a collection of my ongoing notes on using focus bracketing and stacking using a Sony A7RV camera. I will post drafts of this document as I learn more about the technique of selective focus.
A photos were taken by the author.
Cover: Pisonia brunoniana in Kipuka Puaulu, Hawai`i Volcanoes National Park. Sony A7RV with a Sony FE 90mm f/2.8 Macro G OSS lens on a tripod. Photos @ f/7, 1/25 second, ISO 250; 30 image focus bracket. Stacking done with Helicon Focus 8 and postprocessed with Photoshop 2023 using Topaz DeNoise AI and Sharpen AI.
2023 © K. W. Bridges
Selective Focus with Focus Stacking
Focus stacking is the process of combining a set of photos, all taken from the same spot, so that just the area in focus in each photo is used in the composite image. Any area in the stack at which there is no focus in the stack remain unfocused in the final picture.
Taking the photos uses a process called “focus bracketing.” Processing a set of focus-bracketed images is called “focus stacking.” Bracketing and stacking are the two ends of the workflow that involves many, sometimes a lot, of individual images of the same subject.
There are two basic reasons to do focus bracketing and stacking. The most common application is to create photos that are in focus from near to far; everything is sharp. This is used in landscape photography. Product photographers do it, too. The other application, almost counterintuitively, uses focus stacking to carefully limit the portion of a photo that’s sharp. I call this “selective focus.” Background bluring is one of the goals with this alternative. Here, the focused elements stand out on a soft, non-distracting background.
Let’s begin at the start, with focus bracketing.
Focus bracketing, a standard feature in many DSLR and mirrorless cameras, has long eluded Sony enthusiasts. However, the tide has turned with its introduction in the Sony A7RV, marking a significant improvement from the previous workaround involving a cumbersome arrangement tethered to an Android phone.
The incorporation of this feature, much anticipated by many photographers, has stirred up intriguing discussions on its practical applications. Traditionally, photographers employ focus bracketing to achieve a fully focused scene, be it a sweeping landscape or a detailed product photo. By expanding the range of focus or depth of field beyond what a small aperture (e.g., f/16 or f/22) allows, focus bracketing offers the allure of comprehensive sharpness, a highly sought-after trait in photography.
However, an alternative application of focus bracketing seems to be largely overlooked. This involves obtaining focused results over a short distance while utilizing a wide aperture (e.g., f/3.2 or f/4). Here, the objective is not to capture the entirety of the scene in focus but to create a more expansive area of focus within a specific aperture.
Modern prime lenses are renowned for producing exquisite bokeh, a tool photographers use strategically to accentuate the subject of an image, making it “pop” out against the softly blurred background. This effect is especially pronounced when shooting with a lens “wide open” or nearly so (think: f/3.2 or f/4), such as in portrait photography or nature photography featuring flowers. The theory of isolating the main subject against a blurred background using a shallow depth of field is compelling, yet often challenging to execute.
Objectives of This Document
The first objective is to gain familiarity with Sony’s imprementation of focus bracketing. This includes some testing by taking photos under field conditions. Post-processing is included in this step.
Establishing a tentative workflow comes as the next objective. This involves laying out equipment and procedures for the next set of tests.
A third objective is to test the workflow, again under field conditions.
Objective 1: Establishing a Selective Focus Workflow
Sony’s Focus Bracketing
Sony’s implementation of focus bracketing in the A7RV is not extensively documented. Here are a few notes on the essential basics:
• The system operates using autofocus (not manual focus).
• There are two options for focus bracket order: (1) front to back (0→+), and (2) front, middle, back (0→-→+).
• The front to back order enables you to specify the number of images and a relative scale (1-10) for the focus shift between shots.
• The front, middle, back order captures three images and allows for the same relative scale (1-10) of focus shift.
• Options exist to store the photos in the current folder or to create a new folder for each sequence.
• You can customize the delay before photos are taken and the time between each image in the sequence.
Here’s what we’ve learned so far from exploring the focus bracket feature.
Positive Insights
• Setting a lens on manual focus results in a single photo. On the one hand, you won’t get an image stack until you shift the lens to autofocus. However, with manual focus, you can be set on bracketing and still take one-at-atime images.
• Once the shutter release is pressed, photos are taken automatically, with just one press initiating the entire sequence.
Cautions
Since autofocus is required, the camera focuses before the first shot, necessitating precision in placing your focus point on the nearest area you want in focus. You are likely to need to use a spot focus and carefully place your focus point.
Potential Issues
The front, middle, back order (0→-→+) doesn’t seem to function properly, as no discernible focus change appears in the image sequence. Sometimes, the EXIF data show different focus distances, but the change is negligable.
First Field Testing
Venue
A trip to Hawai`i Volcanoes National Park provided an ideal opportunity to test the photo bracketing system. This is a place where I’ve taken flower photos for many years (and many generations of digital cameras). I know specific plants that have challenged my ability to get good shots.
Gear
My basic toolkit was the following:
• Sony A7RV camera
• Sony 90mm f/2.8 Macro G OSS lens
• Vello Shutter Boss II (cable release)
• Sirui T-025X tripod (with ballhead)
• Spare batteries and memory (CF-Type A)
I also took, but didn’t use:
• Sony Alpha-1 camera
• Sony 100-400mm f4.5-5.6 GM lens
• Lots of articulating arms, a led panel, gripper clamps
The extra gear was in anticipation of two things:
• There might be a chance for bird photography (hence the second camera rig)
• There could be a need to hold plants so that they wouldn’t be blown too much in the wind
Neither happened. The downside is that my gear was heavy. Much too heavy.
Most of the gear was carried in an Ona Astoria messenger bag.
It wasn’t possible to carry a laptop computer. As a result, the only image checking was done by “chimping” the photos on the camera. Obviously, no photo stacking was done until my return to home base.
Taking Photo Brackets
All the images were taken using a tripod. The cable release was used to minimize camera shake.
Photo details are shown on the legend accompanying each image.
Post Processing
Each set of bracketted photos was put in a separate folder on the camera memory.
The Windows app PIE (Picture Information Extractor, Picmeta Systems) was very helpful in viewing both the photo EXIF data and the images.
Helicon Focus 8 was used to do the photo stacking. All that was necessary was to drag and drop the set of images (e.g., all the RAW photos in a folder) into the program and click the Render button. Processing was fast. There are three stacking options. In most cases, they produced nearly identical results.
I had concern for the movement of the subject (e.g., flower) as there was a breeze blowing most of the time. Helicon Focus generally handled this well if there was only a little motion. Only about one-quarter of the photo sets failed to stack without motion artifacts. The movement was obvious when the photos were taken so an additional set or two was shot to help ensure that one set would be sufficiently motionless.
Each stacked photo was run through Adobe Photoshop 2023. The sequence generally involved using the Camera Raw Filter, cropping the image, Topaz DeNoise AI and Topaz Sharpen AI. In a few cases, Nic 6 Color Efex was used.
This workflow was used for nearly 100 stacks of photos.
Objective 2: First Field Testing of Selective Focus
The splendor of flowers often goes unnoticed, as their petite size or the intricate structure of the plants conceals their stunning intricacies. The endeavor here is to use macro or near-macro photography to capture the essence of these blossoms, isolating the flowers from the background.
In simpler terms: We aim to capture floral photographs so that the flowers are clear and they stand out from the background.
Here is an example.
Scaevola kilaueae flower taken at f/20. The background is out of focus but clear enough to present a jumble of visual information. Note also, the flower petal farthest away is out of focus so it might be necessary to further stop down the aperature. This will also increase the clutter of the background.
Multiple Scaevola kilaueae flowers taken at f/8 with focus bracketing using a 20 image stack. Note how the more open aperature makes the background out of focus. Stacking the images makes the flowers stay in focus.
Obtaining a good “stack” of images, particularly for flower photography, had some challenges.
Wind: Quite often, small blossoms were perched on branches that swayed at the slightest hint of a breeze.
Light: Many of the most intriguing flowers were located beneath thick tree canopies, presenting a challenge in terms of limited light. Additionally, cloudy conditions further diminished the illumination.
Large Structures: Frequently, the plants I sought to capture had elaborate branching structures, making the distance between individual floral elements large in comparison to the actual size of the blooms.
Tripod: Carrying a tripod was an added chore. This is particularly true I needed to travel to the photo location (think: airline luggage inconveniences) and walk some distance on trails.
Isachne distichophylla taken at f/9 with a 10 photo stack. The 3D structure needs many more photos in the stack. The background is not sufficiently blurred. The flowers are very tiny and many are enclosed in water drops.
Isachne distichophylla taken at f/4.5 with a 10 photo stack. The background is OK, but there should have been many more photos in the stack.
Isachne distichophylla taken at f/4.5 with a 10 photo stack. Here the flower head is sufficiently compact to be in focus with the relatively small photo stack.
Ceodes umbellifera taken at f/7 with a 30 photo stack. This combination captures both the extesive 3D structure of the flower arrangement and blurs the background.
Ceodes umbellifera taken at f/20 with a 30 photo stack. This aperature shows too much of the background.
Above: Hibiscadelphus giffardianus taken at
with a 20 photo stack. Note the ants, mostly at the base of the flower. Fortunately, the ants were not moving much. In another stack with ants, the insect’s motion produced “ghosts” when the stacking was done.
f/7
Right: Hibiscadelphus giffardianus taken at f/7 with a 20 photo stack.
Hibiscadelphus giffardianus taken at f/4 with a 20 photo stack.
Left: Pipturus albidus taken at f/7 with a 30 photo stack. This is a small crop from the original photo so the details of the flowers can be seen. This is very 3D and the image would likely benefit from even more photos in the stack.
Above: Pipturus albidus taken at f/7 with a 30 photo stack. The linear character of this subject works well with this size stack. However, moving the front focus to the leaf in the center of the image and increasing the stack size might improve the overall composition.
Pipturus albidus taken at f/4 with a 20 photo stack. This combination works (barely). The composition could be improved with more front focus and some additional photos in the stack.
Alyxia stellata taken a f/4 with a 20 photo stack. This combination captures the key elements of the subject. Slightly closer front focus and a few more images in the stack would be helpful.
Left: Alyxia stellata taken at f/4 with a 20 photo stack. The fruit is slightly out of focus, demonstrating the difficulty of getting the near-focus point placed correctly.
Above: Alyxia stellata taken at f/7 with a 30 photo stack. The front focus point works here but there needs to be more photos in the stack to capture the flower at the rear.
Metrosideros polymorpha taken at f/8 with a 20 photo stack. This works well although it might be improved with a more open aperature and more images in the stack.
Lessons from the Field Testing
Overall, the field tests were moderately successful. The stacked photos are better than previous pictures of the same species taken under nearly identical conditions.
The goal now is to develop an end-to-end workflow to guide future photography that involves static subjects (such as flowers) where focus isolation improves the quality of the final photo.
The following comments are thoughts about Objective 3, upcoming Second Test of Selective Focus
Equipment
Future Goal: Have essential gear while minimizing the weight & mass, keeping in mind cost, security, safety, and reliability.
To a certain extent, some of the ultralight backpacker philosophy fits here.
• Carrying a minimal load means that you can go farther, faster, and with less fatigue. In the context of photography this means that you can take more photos of more things and be better able to handle the work.
• Each item should serve as many functions as possible. This aspect will be discussed below.
• Expert knowledge of how to use key gear substitutes for extra (often unused) gear. For this type of photography, this means that practice is very important and that it should be done in many contexts.
The gear list for the upcoming Second Field Test includes the following items:
• Sony A7R5 Camera Body
• Sony 100-400mm f/4-5-5.6 GM lens
• Sony 2X teleconverter
• Surui T-025X Carbon Fiber Tripod (Leofoto G2 head)
• Vello Shutter Boss II (cable release)
• Batteries & Media (CF-Type A cards)
• Sony CF-Type A card reader, cable
• Sony Battery Charger
• LG Gram 16” Laptop Computer
Much of the photography in the upcoming Second Test is for birds, specifi-
cally hummingbirds. This adds a challenge as small wildlife subjects require the use of a long (i.e., telephoto) lens. Mine is the Sony 100-400m model. At 400mm this provides the “reach” necessary to get bird pictures. At 100mm it is similar to the 90mm lens used in the First Test.
Workflow
Having a computer will shorten the turnaround between taking the bracketed photos and viewing the stacked results. That was a missing element in the first field test but unavoidable due to circumstances. Laptop software will include photo organization, stacking, and photo editing apps. The computer is necessary to offload images from the camera media to large storage disks (SSD).
The basic camera settings are expected to be the following.
Zoom: 100mm (although experiments might be done up to 400mm).
Aperature: f/4-5 to f/7
Speed: As slow as necessary as a tripod and shutter release are used. There may need to be some adjustment if there is a breeze.
ISO: Low as possible.
Bracket Width: 4
Number of Exposures: 40 (this should be adequate for an 8” depth of field).
Distance to Subject: At lest 3.2 feet (minimum focus distance).
Photos will be examined each evening after the photo shoots, including stacking representative sets of images.
One item to be noted will be the Bracket Width, if it is changed from the anticipated value.
Venues
The following Southern California botanical gardens will be used for testing.
• Huntington Library, Art Museum and Botanical Gardens
• Los Angeles County Arboretum & Botanic Garden
• The California Botanic Garden
• Descanso Gardens
• South Coast Botanic Garden
Objective 3: Second Field Testing of Selective Focus
