What a shaken frame looks like up close
A hand that moved during the exposure drags every point of the scene along the same short path. Up close that has a signature nothing else has: straight edges across the path stay sharp-ish, edges along it vanish, and small bright points turn into short dashes that all point the same way. Those dashes are the kernel, drawn by the scene itself — which is why the dial on this page is drawn the same way.
Missed focus is different. It spreads every point into a disc, softens edges in all directions equally, and has no preferred angle. When the measurement cannot find a direction it reports defocus, and this page lets you switch to that kernel in one click rather than forcing a streak onto a round blur.
Four moves with the kernel
- Let the dial show what it measured. The frame opens straight into the motion kernel. The streak on the dial is the smear the arithmetic will undo, one dot per tap; the dashed ghost is what the measurement suggested, kept in view after you move the knob.
- Turn it until straight edges stop doubling. Put the probe on a lamp post, a window frame or a line of type, and turn the knob a few degrees either way. A kernel that is off by more than about ten degrees leaves a faint second edge beside every straight one; the loupe shows it at source resolution.
- Lengthen it until the halo appears, then back off. Too short and the smear is only partly walked back. Too long and every edge grows a bright rim on one side. The right length is just before that rim — the strength pair beside the loupe is the quickest way to see the difference.
- Keep the reconstruction where the shake was. Camera shake moves the whole frame, but a subject who moved during the exposure does not match it. Paint only the part that shared the camera's motion, or lower strength, and leave the rest as it arrived.
Reading the dial
| Angle | Degrees anticlockwise from horizontal, 0 to 179. A smear has no front or back, so 10° and 190° are the same kernel. |
|---|---|
| Length | How far the camera travelled during the exposure, in pixels of the full-size frame. Longer than about a twentieth of the frame is past what this arithmetic can walk back. |
| Ghost line | The measured kernel, kept on the dial after you change it, so a hand adjustment can always be compared with what the frame suggested. |
| Taps | The dots along the streak. The preview runs up to 41 of them on the working copy; the saved file runs the full length on the full frame. |
Where camera-shake reconstruction goes wrong
- A curved or zig-zag shake is not a straight line. One kernel can match one leg of it; the rest comes back with a doubled edge that no angle removes.
- Two subjects moving differently — a passing car and a still building — need two different kernels. Paint the region that matches the dial and leave the other untouched.
- Rolling-shutter skew from a phone panning quickly leans verticals as well as smearing them. The smear can be walked back; the lean cannot.
- Night shots carry noise, and every round amplifies it. Raise the artifact guard and lower the rounds before reaching for more strength.
- Text is the risky case: a reconstructed stroke can close a letter into the wrong one. Read numbers and names against another source.
Related pages
- The passOne photograph in, one pass over it, one photograph back.
- Blurred writingWhen the question is whether four characters can be read.
- What can be reconstructedLearned priors, the GoPro corpus, and reading a result sceptically.
- Blur, pixelation and mosaic are not the same damageTell a reversible-looking smear from block replacement and deliberate mosaic censorship.