How CMOS sensors work — and why pixels fail
Modern cameras use CMOS (Complementary Metal-Oxide Semiconductor) sensors. Each photosite is a tiny photodiode that converts incoming photons into an electrical charge. The charge is amplified and read by circuitry adjacent to each photosite.
Photosite failures have several causes. Manufacturing defects leave microscopically damaged photosites that fail immediately or within the first few months of use. Cosmic ray exposure can flip bits in the photosite circuitry — rare at sea level but more common in high-altitude environments. Sensor aging causes gradual photodiode degradation over millions of exposures. High-ISO heat exposure (especially in video mode or long-exposure photography) can permanently damage individual photosites over time.
On a 24MP sensor, there are 24 million photosites. Manufacturers account for defects in their quality control process — most sensors ship with a small number of already-remapped pixels that you never see. What you notice are photosites that fail after the camera leaves the factory.
If you are also testing your camera monitor or laptop display, the dead pixel test tool for your display works on any screen and can help you confirm whether a defect is in the camera sensor or the monitor you are previewing on.
Dead vs hot vs warm pixels — the three types
There are three distinct types of sensor pixel failures, and they look different in your photos:
- Dead pixel — the photosite produces no output regardless of light. Appears as a fixed black or very dark dot in photos, visible most clearly when photographing a bright, evenly lit subject. Dead pixels on a camera sensor do not respond to light at any exposure setting.
- Hot pixel — the photosite reads maximum brightness at all times. Appears as a fixed bright white or coloured dot, most visible at high ISO settings (ISO 1600+) and long exposures. Hot pixels are more common in older sensors and in cameras that have been used extensively for video, which generates heat. A dead pixel photo and a hot pixel photo look very different — one is a dark void, the other a bright dot.
- Warm pixel — the photosite produces a slightly elevated signal compared to its neighbours, resulting in a faintly bright dot. Warm pixels are often only visible at ISO 3200+ or in long exposures of 10 seconds or more. They sit between normal and hot pixel behaviour and may not be noticeable at all in daylight shooting.
Dead pixel in camera sensor defects are permanent and appear in every shot. Hot and warm pixels are more variable — they become more prominent as ISO or exposure time increases.
How to test for sensor dead pixels — the dark frame method
The most reliable method for detecting sensor dead pixels is the dark frame test. This isolates the sensor's noise pattern from any external light source:
- Attach the lens cap. Confirm no light is entering the camera body.
- Set the camera to manual mode, ISO 3200 or higher, and a 30-second exposure.
- Take the shot. The result will look nearly black — that is correct.
- Import the image into Lightroom, Capture One, or any editor that supports 1:1 zoom. Zoom to 100% and slowly scan the entire frame.
- Hot pixels appear as fixed bright white or coloured dots. Dead pixel on camera sensor defects appear as slightly darker areas than the sensor noise floor.
Take three dark frames in sequence and compare. True dead pixels appear in exactly the same position in every frame. Thermal noise (bright speckles) is random and changes between frames — it is normal sensor behaviour, not a defect.
To test for dead pixels in photos (rather than pure dark frames), photograph a plain white wall or clear sky in flat, even light at your base ISO. Dead photosites show as fixed dark dots on the bright background.
Hot pixel mapping — brand by brand
Most modern cameras include a pixel mapping or sensor cleaning function that detects and remaps defective photosites. The camera scans all photosites with the lens cap on, identifies outliers, and compensates for them in firmware — interpolating from neighbouring pixels. This is non-destructive and fully reversible.
Pixel mapping menu paths by brand:
- Canon— Menu > Camera Settings > Sensor Cleaning > Clean Manually. Run with lens cap on and battery fully charged.
- Nikon— Menu > Setup Menu > Pixel Mapping. Available on most DSLRs and mirrorless bodies. Not present on all entry-level models.
- Sony— Menu > Setup > Pixel Mapping. Present on most Sony Alpha mirrorless bodies.
- Fujifilm— Menu > Set Up > Pixel Mapping. Run after the camera has been at room temperature for at least 30 minutes.
Run pixel mapping with no light entering the sensor. The process takes 30–90 seconds. After remapping, test again with the dark frame method to confirm the defect has been resolved. A dead pixel no sensor function can address — one that was present from manufacturing — may require a warranty claim if pixel mapping does not cover it.
When to claim warranty
Camera manufacturers apply pixel defect tolerance policies that differ from consumer display manufacturers. Most camera brands (Canon, Nikon, Sony, Fujifilm) allow a small number of defective photosites before a warranty repair is triggered — a single hot pixel on a 45MP sensor is often within specification.
The factors that strengthen a warranty claim for a dead pixel kamera defect are:
- Multiple defects (3 or more dead or hot pixels)
- Defects in the central 20% of the frame where they appear in every shot
- A dead pixel that pixel mapping did not resolve
- Defect appeared within the first few months of use
Submit sample images to your manufacturer's support — dark frame shots at high ISO and a bright-background shot showing the defect clearly. If pixel mapping resolved the issue, the defect may return over time; keep the test frames as documentation.
Post-processing fixes — DaVinci, After Effects, and Lightroom
If pixel remapping does not resolve the issue, or you have archival footage already captured with sensor defects, post-processing tools can remove them from photos and video:
- Lightroom / Camera Raw — Use the Spot Removal (Healing) tool. Select a circular area slightly larger than the dead pixel, then set the source to a nearby clean area. For photos, this is a one-time fix per image. Lightroom Sync can apply the same spot fix to all photos in a batch.
- DaVinci Resolve — Use the Dust Busting node (Resolve 18+) or the Fusion tracker for automatic frame-by-frame removal. See the full dead pixel fix in DaVinci Resolve guide for step-by-step instructions.
- After Effects — Use the Clone Stamp tool on an adjustment layer or the Smart Defect Fixer plugin for automatic removal. See the dead pixel fix in After Effects guide for the manual and plugin methods.
Post-processing fixes are non-destructive to the sensor but add time to your editing workflow. For active cameras, in-camera pixel remapping is the better first step before committing to a per-clip post workflow.