How to focus stack photos online for front-to-back sharpness

Focus stack in five steps

1. 1

   Shoot the frames

   Use a tripod and keep everything fixed except the focus: same aperture, shutter speed, ISO and zoom. Take the first shot focused on the nearest detail, then move the focus a little farther for each shot until the most distant part is sharp. For a landscape with a close foreground, three to five frames are usual; a close-up of an insect or a ring can need ten or more. The subject must keep still.

   To follow along, download the four sample frames from the top of this page. They were made to match frames focused at four distances: one sharp photo was split into depth layers (the ground row by row, the bin as a separate object, the sky, mountains and reflections at infinity), and each layer was blurred by how far it is from the focus and composited back to front, so an out-of-focus object spreads softly over what is behind it. Each frame is also very slightly larger than the last, as real lenses do when you refocus.

   

2. 2

   Put the frames in one document and select them

   Open all the frames with File› then Open… (Ctrl+O or ⌘O); you can select them together. Click the second tab and choose Layer› then Duplicate Into Other Document, then do the same from the third and fourth tabs. Each copy goes into the tab furthest left, so all four frames end up as layers in the first document.

   Click the first tab. In its Layers panel, click the top layer, then hold Shift or ⇧ and click the bottom one, so all four are selected.

   

3. 3

   Line them up with Edit › Auto-Align

   Choose Edit› then Auto-Align. Even on a tripod, refocusing changes the image size slightly, so a detail at the edge drifts by a few pixels from frame to frame. Auto-Align scales and shifts the layers until they line up closely. On the sample the canvas grows a few pixels to hold the slightly larger frames.

   

4. 4

   Blend with Stack Images

   With the layers still selected, choose Edit› then Auto-Blend… and click Stack Images. The Blend method setting appears; Seamless tones and colors sits below it in both modes:

   • Blend method: Adaptive Detail (the default) keeps the most sharpness, with transitions that follow edges; it is the right choice for a well-aligned stack. Smooth feathers the transitions more widely, which hides small alignment errors or movement, such as leaves in a breeze, but can soften fine detail.
   • Seamless tones and colors (on by default) adjusts each frame’s brightness to match. A focus stack shot at one exposure has no brightness differences to even out, so untick it.

   Click Blend. On the sample it takes a second or two. Each layer now has a mask that is white where that frame is the sharpest.

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5. 5

   Check the details, crop and save

   Zoom to 100% with View› then Actual Size (100%) (Ctrl+1 or ⌘1) and look at the nearest and the farthest details, and at the edges where a near object stands in front of a far one. On the sample, measured in 40-pixel bands from the planks to the peaks, the stack keeps 77–96% of the fine-detail contrast of the sharp photo the frames were made from: it is sharp from front to back. The edge of the bin, which is blurred and spread over the grass in the nearest and the farthest frames, stays clean in the stack.

   The frames are slightly different sizes, so the edges of the canvas come from fewer frames. Crop inside the smallest one: draw a rectangle with Marquee Select (M) just inside the canvas edges and choose Image› then Crop to Selection. The sample ends up about 1524 × 1016 px.

   Export with File› then Export› then JPEG…, or keep the layers and masks with File› then Save As› then Lil Cactus Project… (Ctrl+Shift+S or ⇧⌘S). File› then Save on this document saves the stack, flattened, back into the first frame’s JPEG; the first time, the Save Image dialog asks for its quality.

   

   

   

   

Why not just close the aperture?

Stopping down from f/4 to f/16 does deepen the zone of sharpness, but light passing a very small opening spreads out (diffraction), so every detail in the photo gets a little softer, even at the focus distance. Close-ups make it worse: at short distances the depth of field can be a few millimeters even at f/16. A stack lets you shoot at the lens’s sharpest aperture, often around f/5.6 to f/8, and still get front-to-back sharpness.

Fix common problems

Doubled edges

The frames don’t line up exactly, or something moved between them. Check that Auto-Align ran on all the layers; for small movement such as leaves, undo the blend and try Smooth as the Blend method.

A soft rim where near meets far

In real stacks, an out-of-focus near object spreads over the background in the frames focused behind it, and a little of that can remain along the edge. Paint on the masks to choose which frame shows there.

A band stays soft in the middle distance

The focus steps were too far apart, so no frame was sharp there. Shoot another frame focused on that distance and add it to the stack.

Auto-Align or Auto-Blend is grayed out

Select at least two visible layers; text, shape and Smart Object layers must be rasterized first. Auto-Blend also refuses a layer locked with the padlock button (Lock layer) in the Layers panel.

What to do next

• Join frames side by side instead: how to stitch a panorama.
• Add a final touch of crispness: how to sharpen a photo.
• Fix a transition by hand: layer masks.
• Resize the result for the web.

Coming from another editor

Photoshop

Edit › Auto-Align Layers… and Edit › Auto-Blend Layers… with Stack Images. In Lil Cactus the same job is Edit › Auto-Align and Edit › Auto-Blend…

Focus stacking: questions and answers