Blur Detective: Focus vs. Shake
Out-of-focus blur softens every edge uniformly into disc-like smears that ignore motion, while camera-shake blur smears every point in a single direction; inspecting edge shape and smear direction separates the two.
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How can you tell whether a blurry photo was caused by a focus error or by camera shake?
Two photos of the same scene can look equally blurry for completely different reasons — and the fix for one won't work for the other.
Your eyes just see 'blurry,' but a sensor sees whether each point smeared in a circle or in a streak — and that distinction reveals the cause.
Side-by-side synthetic examples of out-of-focus blur (circular bokeh discs) versus motion-blurred shake (directional streaks), plus a transferable checklist for inspecting real photos.
A reliable method to read the shape, edges, and direction of blur in any photo and confidently name its cause.
All blur looks the same to the eye, so it should be impossible to tell the two causes apart from the image alone.
- Lens aberrations and diffraction softness
- Subject motion blur from a moving object
- Sensor noise vs. blur
- Depth-of-field compositional choices
- 01Two blurry photos, one questionslideQuestion
Introduce the driving question by showing two identically-blurry-looking scenes side by side and asking how to tell what caused each.
- Blur is not a single thing — it has shape and direction
- Same-looking softness can come from different physical causes
- The task is to read the blur itself, not guess
- 02What would you look at first?quizPrediction
Force a commitment before any explanation: which feature of the blurry photo should the learner inspect first to separate focus error from camera shake?
- Choose one diagnostic feature to inspect
- Justify the choice in one sentence
- 03Inspect the blur shapeinteractiveEvidence
Let the learner zoom into a high-contrast edge and toggle between three synthetic samples — sharp, out-of-focus, and shaken — to see how each renders a single edge.
- Sharp edge: crisp transition from black to white
- Out-of-focus edge: soft symmetric ramp in all directions
- Shaken edge: directional smear aligned to camera motion
- 04Compare blur across the frameinteractiveEvidence
Reveal that focus blur looks the same everywhere, while shake blur looks identical everywhere but aligned to one axis — slider lets the learner rotate a simulated shake vector.
- Defocus is isotropic — the same in every direction
- Shake is anisotropic — a single dominant smear direction
- Rotating the shake vector rotates every streak in lockstep
- 05Why the blur looks differentslideExplanation
Explain the optics: defocus spreads each point through a circular aperture into a disc; shake spreads each point along the camera's motion path during exposure.
- Defocus = point spread function is a circle (the aperture shape)
- Shake = point spread function is a line segment along the motion vector
- Shape of the smear is the signature of the cause
- 06Diagnose a new photointeractiveTransfer
Give the learner a fresh, unlabeled blurred photo and a checklist (edge symmetry, smear direction, behavior across the frame, highlights) to decide its cause.
- Apply the edge-shape test on a high-contrast border
- Check whether softness has a dominant direction
- Confirm by looking at point highlights, not just edges
- 07When the test breaks downslideBoundary
Show where the rule fails: very long exposures can make shake look like a uniform smear; small apertures can make defocus almost invisible; mixed cases combine both signatures.
- Tiny apertures hide defocus so even slight shake dominates
- Long shutter times blur the shake vector into near-isotropic softness
- Combined focus + shake blur shows both a direction and disc-like halos
- 08Reading blur like a signatureslideResolution
Answer the driving question directly and give a portable decision rule the learner can use on real images.
- Defocus blur is round and direction-independent
- Shake blur is directional and uniform across the frame
- Inspect one high-contrast edge and one point highlight — together they name the cause
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