Focus stacking in macro

Learning goal

You will be able to apply focus stacking to macro to show an insect sharp from its eye to the tip of its wing. By measuring the subject's front-to-back depth, you will find with a simple calculation how many frames are needed at the magnification and aperture you chose. In macro you will shift focus not with the ring but with a rail that slides the camera forward millimeter by millimeter, or with the camera's own focus-shift feature. You will know the difference between a long, flawless sequence set up on a table and a short sequence shot in the field on an insect that may move, and choose which one works when. In the combined frame you will look for traces such as doubled legs and edge halos, and not hide those you can't fix. When combining a hand decorated with henna from several frames, you will get permission and keep the skin's color, whatever its tone, steady across the sequence.

The question

After the rain, your friend finds a metallic green beetle on a wet leaf in the garden. In the frame shot close to 1:1, at f/11, the beetle's eyes and head are sharp, but its back starts to soften and the tips of its wings are completely blurred. They know they can't stop the aperture down any further. At the club someone shows them a rail that slides the camera forward millimeter by millimeter with a screw, and a program that combines dozens of frames into a single frame; on the screen, an insect sharp from its eye to the tip of its wing. How do you build sharpness from an insect's eye to the tip of its wing?

Context

In the focus stacking lesson you learned to combine frames focused at different depths, the focus step between frames and the traces combining leaves (Lesson 22.5). In this track's depth of field lesson you calculated that in macro the slice of sharpness thins quickly with magnification and that the effective aperture starts diffraction early (Lesson K.3). In the insects lesson you also saw that insects are slow on a cool morning, and photographing them in place, without touching (Lesson K.5).

Macro is the field where focus stacking helps most; but it is also where it is hardest. When the slice of sharpness drops below a millimeter, showing an insect sharp from front to back asks for fifteen, twenty, sometimes more frames; in every frame the insect, the wind and the camera must stay put. This lesson takes up building this combination with tools specific to macro, with the right step and with an honest result.

Simple explanation

How many frames are needed?

Stack depth is the total depth a focus stack must show sharp from front to back: the distance from an insect's eye to the tip of its wing, from the leaf in front of a flower to the petal behind. The number of frames is found from this depth and the step: the depth is divided by the step and one is added. The step is chosen a little smaller than each frame's sharp slice, so that neighboring frames overlap in sharpness; roughly between half and two thirds of the slice is a good start.

At one-to-one magnification, with f/8 written on the lens, the depth covered by sharpness is around one millimeter; you need to choose the step between half a millimeter and two thirds of a millimeter. For an insect with a depth of eight millimeters from eye to wing tip, this means between thirteen and seventeen frames. If you photograph the same insect at 1:2, the slice rises to roughly three millimeters and four or five frames are enough; the frame is a little wider but the sequence is much shorter. Lowering the magnification is the most effective way to make focus stacking easier in macro.

How is focus shifted?

A focus rail is a slide that you seat the camera on and that moves it toward the subject and back, millimeter by millimeter, with a screw or a motor. In macro, turning the focus ring changes the magnification too; the rail keeps the magnification fixed and moves only the plane of focus. On rails turned by hand, each turn of the screw corresponds to a certain distance; motorized rails apply the desired step and number of frames by themselves. The camera's own focus-shift feature, too, shoots a good sequence without a rail, especially at 1:2 and lower magnifications.

The camera must stand on a solid tripod, free of vibration; pressing the shutter remotely and raising the mirror in advance on mirror cameras prevent small shakes. The exposure, the white balance and the flash power are kept fixed across the sequence; if you use a flash, check that it fully recharges for every frame, otherwise the brightness fluctuates between frames. The frames are given to a stacking program; the program picks the sharpest region of each frame and builds a single image.

On the table or in the field?

A field stack is a focus stack made of a small number of frames shot in nature, on an insect staying where it is, in a short time, handheld or with a light support. Long sequences set up on a table, in a studio arrangement, can be flawless, but because they require taking the insect from its place, they have no place in nature photography; if it was done, it is said clearly. In the field, on the other hand, on an insect that isn't moving on a cool morning, a series of five to ten frames shot with the camera's focus-shift feature, or by rocking the camera very slightly forward in the hand, is often enough to show the eye and the back sharp together.

In the combined frame, traces are looked for: if the insect moved during the sequence, a leg appears in two places; at the edge of a twig in front, the blur of the background leaves a halo. These traces can be fixed, or the frame not chosen; but covering a trace by adding a piece from another frame is an intervention beyond stacking, and it is said. A person's hand can be combined too: when a hand decorated with henna rests on a table and stays still, five or six frames are enough. Keeping the white balance and the exposure fixed across the sequence keeps, in every frame, the difference between the orange of the henna and the skin on light skin, and the deep red of the henna and the warmth of the skin on dark skin, the same.

Visual evidence

The next morning your friend finds the metallic green beetle on the same leaf; in the cool air it isn't moving. They estimate it is about eight millimeters from eye to wing tip. Instead of 1:1 they switch to 1:2 and bring the written aperture to f/8; sharpness covers about three millimeters of depth. With the camera's focus-shift feature they shoot five frames in steps of two millimeters.

At home they combine the frames: the eyes, the back and the wing tip are sharp. At the tip of one antenna they see a doubled line; the beetle moved its antenna a little in the middle of the sequence. They leave that frame out of the sequence and combine again with four frames; the antenna is single and clean.

The same week, the day after the henna night, their aunt, a dark-skinned woman, wants to show the henna pattern in her palm. They ask permission, rest her hand on a table and shoot a short sequence of six frames; white balance and exposure fixed. In the combined frame, the fine lines of the henna are sharp from the line near the wrist to the fingertips, and the skin is in its warm brown.

Common mistakes

Summary

Sharpness from an insect's eye to the tip of its wing is built by dividing the subject's stack depth by the step to find the number of frames, keeping the step between half and two thirds of the sharp slice, and shifting focus not with the ring but with a focus rail or the camera's focus-shift feature; lowering the magnification shortens the sequence. In the field, a short field stack is enough for an insect staying in place; traces are looked for and those that can't be fixed aren't hidden; a hand decorated with henna is combined in the skin's real tone, with permission and with fixed white balance and exposure.

Mini assignment

A closing project ties up the Macro track: you will build a five-frame macro selection from the small world near you.

1. With what you learned on this track, shoot five frames: a detail at a magnification you measured with a ruler, a wing or a leaf shot parallel to its surface, a subject lit with a flash or a reflector, an insect photographed in place on a cool morning, and an abstract frame set up in the kitchen.
2. Shoot at least one of these frames as a focus stack, measuring its depth and calculating the number of frames; look for traces in the combined frame.
3. Under each frame write a short note: magnification, light, stacking and, if it was set up, the scene; if someone's hand or face appears in the selection, check that you got their permission and that their skin appears in its real tone.

Check yourself

• Did the number of frames you calculated match the number actually needed?
• Did a doubled line or a halo remain in the combined frame?
• Does each frame's note clearly say how it was shot?

Glossary

Mini quiz

1. For an insect eight millimeters deep, how many frames are needed with a two-millimeter step?

• A) Two
• B) Five
• C) Twenty
• D) One

2. Why is focus shifted with a rail instead of the ring in macro?

• A) Because the ring changes the magnification too, while the rail moves only the plane of focus
• B) Because the rail is cheaper
• C) Because the ring doesn't turn at all in macro
• D) Because the rail also stops down the aperture

3. Which is the most effective way to make focus stacking easier in macro?

• A) Going down to the narrowest aperture value
• B) Enlarging the step
• C) Lowering the magnification a little
• D) Turning off the flash

4. If an antenna appears in two places in the combined frame, what is done?

• A) A piece is added from another frame and not mentioned
• B) Nothing, it won't be noticed
• C) The frame is deleted in every case
• D) The frame with the trace is taken out of the sequence, or the intervention is declared

Answers

1. B — Eight millimeters divided by the two-millimeter step, together with the starting frame, makes five frames.
2. A — The rail keeps the magnification fixed and the frames line up in the stack.
3. C — As magnification drops, the sharp slice thickens and the number of frames needed falls quickly.
4. D — A movement trace is removed with a clean sequence, while an added piece is told to the viewer.

Summary card

• Number of frames = stack depth ÷ step + 1.
• Step: between half and two thirds of the sharp slice.
• Shift focus with a rail or the camera's focus-shift feature.
• Lower the magnification, shorten the sequence; in the field a short sequence is often enough.
• Look for traces, don't hide them; combine a hand with permission, fixed white balance and exposure.

Bridge to the next lesson

The Macro track closes here: from what 1:1 means to equipment, from millimeter sharpness to the light of a small world, from the slow insects of the morning to the abstract drops of the kitchen and to focus stacking. One evening your friend puts macro aside and joins the club's night walk. On a dark hill their eye can barely pick out the stars, but someone shoots a thirty-second frame on a tripod and on the screen thousands of stars the eye didn't see and a band of light appear. Does the camera see what you can't see in the dark? The next lesson opens the Night and Astrophotography track and takes up the tripod, long exposure and focusing by hand in the dark.