Most beginners assume that when focus stacking, you should use the smallest aperture possible to maximise depth of field. Professional macro photographers do the opposite. This article explains why, and what aperture you should actually be shooting at.
Contents
- The assumption most beginners make
- What is diffraction and why does it matter?
- Every lens has a sharpness sweet spot
- So why does focus stacking change everything?
- The background advantage
- The light advantage
- So what aperture should you actually use?
- The trade-offs to understand
The assumption most beginners make
When you first get into macro photography, the depth of field problem hits you immediately. You get close to a mushroom, take a shot, and realise that only a sliver of it is actually in focus. The natural instinct is to stop down the aperture as far as it will go — f/16, f/22, maybe even f/32 — to get as much of the subject in focus as possible in a single frame.
This makes intuitive sense. We learn early on that a small aperture (high f-number) means more depth of field, and more depth of field means more of the subject in focus. So for macro photography, where depth of field is frustratingly shallow, you would assume the answer is to just stop down as far as possible.
But look at how experienced macro photographers actually shoot when they are focus stacking, and you will find something surprising. They are often shooting at f/5.6, f/7.1, or f/8. Sometimes even wider. Not f/22. And there are very good reasons for this that completely change how you should think about aperture in macro photography.
What is diffraction and why does it matter?
Diffraction is a physics phenomenon that affects every lens ever made, regardless of how expensive or well-built it is. There is no escaping it. Here is what happens.
When light passes through the aperture blades in your lens, some of it bends slightly as it passes the edges of the blades. This is diffraction. At wide apertures, the hole is large enough that only a small proportion of the light is near the edges, so this bending effect is minimal. But as you stop down to smaller and smaller apertures, the hole gets smaller and a greater proportion of the light is passing near the edges and bending. At very small apertures like f/22 or f/32, this diffraction becomes significant enough to visibly soften the entire image.
This is not a subtle effect. If you take a sharp macro lens and photograph a fine detailed subject at f/5.6 versus f/22, and then look at both images at 100% zoom, the f/22 image will often be noticeably softer throughout, even in areas that are technically within the depth of field. The fine detail that a quality lens is capable of resolving gets smeared and lost to diffraction long before you reach the smallest apertures.
In short: stopping down gains you depth of field but costs you sharpness. And past a certain point, the sharpness loss from diffraction outweighs any benefit from the increased depth of field.
Every lens has a sharpness sweet spot
Every lens produces its sharpest images within a specific aperture range, which photographers call the sweet spot. This is not a myth or a minor effect — it is well documented and consistent across all lens types.
The rule of thumb is that a lens’s sweet spot is usually two to three stops down from its maximum aperture. So for a lens that opens to f/2.8, the sharpest apertures are typically around f/5.6 to f/8. For a lens with a maximum aperture of f/4, the sweet spot is around f/8 to f/11. This is because at very wide apertures, optical aberrations in the glass cause some softness. As you stop down slightly, those aberrations are controlled and sharpness improves. Then, as you continue to stop down past the sweet spot, diffraction starts to take over and sharpness decreases again.
For most dedicated macro lenses, the sweet spot sits somewhere between f/5.6 and f/11. Shooting at f/22 or beyond means you are well past that range, in territory where diffraction is actively degrading the quality of your image.
A practical example The Olympus M.Zuiko 60mm f/2.8 Macro has a maximum aperture of f/2.8. Its sweet spot for sharpness is roughly f/5.6 to f/8. At f/22, diffraction has already softened the image meaningfully. If you are doing a focus stack on a small mushroom and shooting each frame at f/22, you are sacrificing a significant amount of the sharpness that your lens is actually capable of delivering — in every single frame of the stack.
So why does focus stacking change everything?
Here is the key insight that changes the whole equation. When you are focus stacking, you are not relying on a single frame to cover the depth of field of your subject. You are taking many frames, each one sharp at a different focal plane, and combining them. This means you do not need to use a small aperture to get more depth of field into each frame. Instead, you use whatever aperture gives you the sharpest individual frames, and you use more of those frames to cover the subject.
Think about it this way. Would you rather have 30 frames all shot at f/7.1, each one crisp and sharp within its narrow slice of focus, which you then blend into a composite? Or 10 frames all shot at f/22, each one slightly soft throughout due to diffraction? The first approach produces a final stacked image with dramatically more detail and clarity, even though each individual frame has less depth of field.
This is exactly why professional macro photographers who focus stack often shoot at relatively wide apertures. The stack compensates for the shallow depth of field. The wider aperture delivers the sharpness. You are using the technique to solve the depth of field problem rather than trying to solve it with the aperture setting.
The background advantage
There is another significant benefit to using a wider aperture when stacking that is easy to overlook: background blur.
When you stop down to f/22 for a macro shot, everything that is further away from your subject also becomes sharper, including the background. In a forest setting, this means moss, leaves, twigs, and soil can all become visible distractions in the background. The cluttered, detailed background competes with your subject and can make the image feel messy and unfocused.
When you shoot at f/7.1 and focus stack through your subject, the background stays beautifully blurred. Only the subject itself is sharp in the final image, and the background falls off into a soft, clean bokeh that isolates the mushroom or slime mold exactly the way you want. The aesthetic of the image is completely different, and in most cases much more pleasing.
This is actually one of the things that makes professionally focus-stacked macro images look so distinctive. That combination of complete sharpness through the subject and a beautifully blurred background is only really achievable through focus stacking at a moderate aperture. You cannot replicate it by just stopping down a lens.
The light advantage
Shooting at a wider aperture also means your lens is letting in significantly more light. Each stop wider you go doubles the amount of light reaching the sensor. Going from f/22 to f/8 is roughly three stops — meaning about eight times more light.
This matters in macro photography for a few practical reasons. More light means you can use faster shutter speeds, which reduces the risk of motion blur from wind moving your subject between frames. It means you can keep ISO lower, which reduces noise and preserves fine detail. And it means your images will be more consistently exposed across the stack, making the blending process cleaner in software.
In dark forest conditions, under leaf canopy where light is already limited, shooting at f/22 often forces you into very slow shutter speeds or high ISO values that add noise to every frame. A wider aperture removes much of that pressure.
So what aperture should you actually use?
As a starting point, aim for two to three stops down from your lens’s maximum aperture. For most dedicated macro lenses this puts you in the f/5.6 to f/11 range. Within that range, f/7.1 or f/8 is a very common choice among experienced macro photographers as a good balance of sharpness, light, and background blur.
The exact aperture worth experimenting with will depend on your specific lens, since every lens has slightly different characteristics. The best way to find your lens’s sweet spot is to photograph a fine-detailed flat subject (like a printed page or a ruler) at several different apertures from wide open through to f/22, and compare the results at 100% zoom. The aperture where detail looks cleanest is your sweet spot.
Once you know your lens’s sweet spot, shoot your focus stacks there. You will need more frames to cover the subject than you would at a small aperture, but the final image will be sharper throughout.
Caveat: At very high magnification levels — 2:1 or beyond, especially when using extension tubes or the Raynox DCR-250 — diffraction becomes even more severe at small apertures. At these magnifications, you may find that f/5.6 or even wider gives you the best per-frame sharpness. The same principle applies, just more so.
The trade-offs to understand
Shooting at a wider aperture for focus stacking is not without costs. It is worth going in with a clear understanding of the full picture.
More frames required
At f/7.1 on a macro lens, the depth of field per frame is very shallow. To cover a full mushroom from front to back you will need more frames than you would at f/16. This means longer shooting sessions, larger file volumes, and more processing time in Helicon Focus or Zerene. For very small slime mold subjects at high magnification this can mean dozens or even hundreds of frames per stack. That is just the reality of doing it properly.
Wind becomes more critical
More frames means more time for wind to cause problems. If your subject moves between frames, those frames become unusable. This is one of the main reasons serious macro photographers often shoot in early morning when the air tends to be still. It is also one of the arguments for in-camera automated focus bracketing, which fires all frames rapidly in sequence and minimises the window during which wind can ruin a stack.
You still cannot fix a bad stack in software
Wider aperture improves per-frame quality, but the frames still need to be clean. A stack of 40 sharp frames at f/7.1 will produce a better result than a stack of 10 soft frames at f/22, but a stack of 40 blurred, wind-damaged, or poorly exposed frames at f/7.1 will not produce anything useful. Technique in the field still matters enormously.
If you are still getting to grips with the fundamentals of focus stacking, the article on cameras with focus stacking capability covers which camera systems support automated bracketing. And if you want to go deeper on the accessories that help push magnification further, the guides on extension tubes and the Raynox DCR-250 and focusing rails are good follow-on reads. The macro lenses guide also covers which lenses tend to have the best sweet spots for this kind of work.
Best of luck out there.
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