In Episode 398: Is off-grid mooring the last freedom? (Life on the riverbank), we featured a number of macro photos, specifically of wild flowers. Many of these were ‘stacked’ photos, a composite of multiple images. In this post we take a look at why we do this and how we did it, but to understand the process we need to get our heads around depth of field (DoF).
What is depth of field?
Quite simply, it’s the area of acceptable focus in an image. The aperture blades on a camera’s lens are used to increase or decrease the depth of an image that is in focus (changing the aperture is actually controlling the amount of light entering the camera, but we’ll come onto that in a moment). Often, when we take a portrait of a person or a specific object like a flower, we like to blur the background to help isolate the subject and draw our attention to it. Busy backgrounds are a distraction.
As a side note, most phones have a ‘portrait’ setting on the camera app to achieve the same effect, but the blurring is done computationally (some quite badly). On a ‘proper’ camera, we change this bluring effect by moving the aperture blades, which changes the f-stop.
What is an f-stop?
Aperture is that strange set of numbers on your lens. They may go from 2.8 to 22, are expressed as a fraction, and are written as f/2.8 or f/8, for example. It’s known as an f-stop and is just a way of standardising the measurement of the aperture’s effect on different lenses. You don’t need to know the mathematics behind this fraction (it’s the ratio of the lens focal length divided by the diameter of the entrance pupil of the aperture)…
What happens when you change the f-stop?
…you just need to remember that the lower the number, the wider the aperture, the more light entering the camera and the shallower depth of field. Confusingly, the smaller the whole number, the more open the aperture is. 2.8 opens up the aperture blades, while 16 or 22 closes down the blades. Of course you can increase the light entering the camera by also lowering the shutter speed or increasing the ISO, but we’re concentrating on aperture only so assume your shutter speed and ISO remain constant. If all this sounds confusing, Digital Camera World has produced a great crib sheet to help simplify things.
So for macro, you just open up the aperture to blur the background?
Yes and no! There is a new problem introduced when taking macro photos. The closer you are to your subject, the shallower the apparent depth of field gets. If you take a landscape photo of, say, a horse in the middle distance in a field with a tree in the background and set the aperture to f/16, much of the image would be in focus. You focus on the horse, the focal plane, and the area around the horse falls into the area of acceptable focus. If you use exactly the same settings on an object close up, f/16 won’t get the entire object in focus.
In the following examples, using the same settings, we fill our subject in the viewfinder. What we see (field of view) is represented by the lower horizontal line, but only a part of that will fall into the area of acceptable focus.
We are trying to fit the whole flower into our field of view, but the area of acceptable focus won’t allow us to get the entire flower in focus. The stamen and a petal will be out of focus.
Focus Stacking
To get round this problem, one solution is to ‘focus stack’. This means taking a series of photos and shifting the focal plane slightly in each shot. We start by focusing on the closest part of the subject to the lens and work backwards.
To the eye the differences maybe subtle (especially on small, optimised images), but note the change in focus on the lower-left petal between the first and the last images. Interestingly the stamen remains in the area of acceptable focus throughout, though this would eventually fall out of focus as we continued to shift the focal plane backwards. When stacked, we end up with the entire subject in focus.
How do you stack multiple images?
There are many apps out there that do this, but Liz and I use Helicon Soft’s Helicon Focus. It works by building a 3D map of all areas in focus, specifically the edges, before aligning and stacking them for the final image (click the Helicon Focus link to see a quick video of this in action). The paid version handles RAW file input and will output in RAW too. Note: we are not affiliated with Helicon, it’s just the app we ended up using.
I mentioned that Helicon Focus aligns the images. This is useful when there is a little movement in the subject, especially if there is a breeze, but there are a number of things to consider when taking the images.
Things to consider when taking a stacked photo
Tripod
You will definitely want a tripod! I have attempted handheld macro and it can work, but that inevitable forwards and backwards motion introduced when hand-held is not fixable in post.
Remote Control
I would recommend using an app to trigger the shutter. Any movement from the camera can ruin the first couple of frames and, with macro photography, that movement is magnified. I have many stacked images ruined by pressing the shutter button manually, even on a tripod.
Open up the aperture
To keep your ISO down so as not to introduce noise, and your shutter speed fast enough to evade blur, you will need to open up the aperture to let enough light in. This may mean opening right up to f/2.8 or f/4. This means less of our subject will be in focus on each shot, and therefore will require more images, but if we have the right conditions and are using a tripod, this shouldn’t be an issue. Don’t forget your lens will have a sweet spot when it comes to aperture (e.g. better edge-to-edge sharpness), often a couple of stops down from its widest setting. A 2.8 lens may produce the best results at f/5.6.
Auto focus stacking
Check to see if your camera has focus stacking as a feature. It means we don’t have to touch the camera and can get a stack done in seconds. Our Lumix cameras, the GX85 (GX80) and GH5, have focus stacking. We tend to use the GX85 as it is smaller and pair it with the Olympus 30mm 3.5 macro lens. The lower pixel count of the GX85 isn’t an issue, as you can see with some of Liz’s superb shell images.
Shoot in a studio
If you can take your macro shots in studio conditions it is preferable. You’ll have better control over light and won’t have issues with wind. All of my flower images were taken in the field and I was often contending with a breeze. Also, I didn’t want to pick the flowers unnecessarily. You don’t need a full-blown photography studio, just a table and a couple of defused lights. We built a soft-box for Liz, had a light mounted on a tripod and a further two strip lights to create a ‘triangle’ of light.
Focus stacking is a lot of fun
The fact Liz spent many hours taking and stacking her shell photos says much about the process: it’s a lot of fun! Also, it is satisfying to get an image where the entire subject is in focus where a single shot would have failed.
If you like our content and would like to support us, we will give you ad-free access to our videos before they go live to the public, discounts in our shop, access to Jamie’s iconic full-res photographs, and supporter-only blog posts. Click our ugly mugs for more info!
