Identifying pixel errors

A simulated view of a typical camera electronic viewfinder with a group of dead pixels (circled in red) just above the level display.

Screens are essential components in digital cameras. The technology has been in use since the earliest days of digital photography, although these days the organic light-emitting diode (OLED) screen has largely replaced the liquid crystal display (LCD) in camera electronic viewfinders (EVFs).

Both display screens and digital camera image sensors have one thing in common: they are arrays containing millions of photodiodes arranged in a grid. These photodiodes produce the picture elements (pixels) that make up the image you see on the screen or, in the case of the sensor output, the data that is recorded as an image file.

When everything is working properly, the average photographer gives little thought to those pixel arrays. But what happens when something goes amiss with one or more photodiodes?

In this feature we’ll look at the three most common ‘pixel errors’ – dead pixels, stuck pixels and hot pixels – and explain how they are caused and what can be done to prevent them from affecting your photos.

Dead pixels

A dead pixel represents a photodiode that has stopped receiving any power. It almost always shows up as a black spot on the screen. Dead pixels are easy to spot on LCD monitors; just switch to Live View and point the camera at a bright area such as the sky (but without pointing directly at the sun). Dead pixels will always show up in the same part of the screen when you move the camera. (If there are only a few of them, you’ll need to magnify the displayed view.)

Dead pixels on screens will not affect recorded images, so if you only find a few dead pixels on your LCD screen, it’s best not to worry about them. A typical 3-inch monitor has around a million dots, so a single dead pixel represents a 0.0001% failure rate. Clusters of obvious pixel errors can be annoying and may interfere with your view of subjects. It’s up to you to decide whether it’s worth having the screen replaced.

It’s much more serious when dead pixels occur on image sensors because they can affect the images the sensor produces.  But unless they come in clusters, you probably won’t notice them because the interpolation process applied to data from the colour filter array (Bayer being the most commonly used) will obscure them.

It works like this: interpolation algorithms work on clusters of between four and nine photosites, comparing values and setting colour and brightness values for each image pixel. If one photosite in the cluster isn’t delivering any signal it will be ignored and data from the surrounding eight photosites will be used to fill in the space.

EVF damage

Pointing your camera directly at the sun can burn out pixels in its sensor, inflicting permanent damage. If you’re using a mirrorless camera, the screen in the EVF will also be damaged, again because pixels are destroyed.

Damage to the EVF screen won’t show up in your photos, although damage to the camera’s sensor will – and it may be larger and more visible than dust spots. Dust spots can be removed but sensor damage cannot be repaired; the only solution is an expensive replacement.

This illustration shows the kind of blemish (circled in red) created by burned-out photodiodes (‘pixels’) on the screen of an EVF. It is likely to have been caused by pointing the camera directly at the sun and may have also caused damage to the camera’s image sensor.

Stuck pixels

Unlike dead pixels, stuck pixels always receive power. They occur because one or more photodiodes in the RGB array has become stuck at its maximum value. If only one photodiode is ‘stuck’ it can appear as a bright blue, green, or red point. If all the photodiodes in an RGB cluster are stuck, the result will be a larger dot, which will appear pure white.

Stuck pixels on a monitor or EVF screen should be treated in the same way as dead pixels; if you can ignore them, do so. If they are large enough to be annoying, the screen could require replacement.

Both dead and stuck pixels can appear larger in JPEG images than they actually are because of the way JPEGs are processed. Working with clusters of pixels can produce halos around the defective pixels that, if large enough, can have a visible impact on photos.

Stuck pixels (shown in the magnified oval above the screen) on a computer monitor screen. Because monitors have relatively high resolution it will be difficult to find defective pixels without using a magnifying glass.

Hot pixels

Unlike stuck pixels, hot pixels do not occur in LCD or OLED screens. They only show up when the camera sensor gets hot during long exposures or when the ISO is raised above about 800. They can appear even on brand new cameras, although manufacturers usually map them out as part of the quality control process.

Most hot pixels will vanish over time but the likelihood of their occuring will increase with time, particularly if your camera is often allowed to get hot. You can find out whether your camera has any hot pixels by setting the camera to manual mode and capturing a long exposure (15-30 seconds) with the lens cap on and the ISO at the natural ‘base’ setting of ISO 100 or 200. Then take a second exposure (still with the lens cap on) at the fastest mechanical shutter speed the camera supports with the sensitivity cranked up to at least ISO 800 (or higher).

Analyse both images at high magnification in your favourite image editor, looking for any areas that aren’t pure black. This test can be used to identify all kinds of defective pixels. Nothing that even remotely resembles a coloured or white speck, blob or point of light should appear in a shot taken with a lens cap on. So any spots you see will represent a defective pixel. (You will probably see more hot pixels in images shot with the higher ISO value than in the long exposure with a low ISO setting.)

It’s important to note that pixels which show up as ‘hot’ in a long exposure may never show up in subsequent exposures. Other pixels may appear hot in the next long exposure you take.

Hot pixels can be difficult to identify on camera monitor screens but will show up as coloured or white pixels once images are enlarged.

Fixing defective pixels

Fortunately there are ways to reduce the influence of dead and stuck pixels. One of the best ways is to work with raw files. Photographers who shoot raw files will find that sophisticated converters like Adobe Camera Raw, Capture One and DxO Optics Pro will ‘map’ them out as part of the conversion process.

Some camera manufacturers provide pixel mapping as part of their cameras’ firmware. It’s located in the Custom Menu pages on Olympus cameras. This feature works by closing the shutter and capturing a black reference frame. Any defective pixels detected are mapped and their positions are recorded so they can be eliminated from subsequent frames.

Stuck pixels in most Canon cameras can usually be ‘fixed’ by initiating a Manual Sensor Cleaning procedure. Charge up the batteries, select this function from the menu and wait about a half minute before turning off the camera. This procedure could also work for some Nikon cameras.

Hot pixels are more problematic in video because it’s too time-consuming to attempt to eliminate them with software. Running a sensor cleaning function is the best way to fix hot pixels in video clips. You need to run sensor cleaning for thirty seconds to a minute for it to be effective. In theory, by vibrating the sensor, it can re-set the tiny power lines that supply each photosite, via the effect of gently massaging them.

Hot pixels can be a hit-and-miss problem when it comes to video. In most cases, replacing the sensor will cost as much as a new camera. Sensor cleaning might be a long shot, but it’s also free of charge and worth trying. If it doesn’t work you may need to keep the camera for shooting stills only.

Free software

A web search will reveal plenty of applications for ‘fixing’ dead, stuck and hot pixels. We’ve listed a couple of the more credible free applications below.

PixelHealer is a user-friendly, general-purpose application that can be used to check LCD or TFT screens. It’s designed for use on desktop monitors, laptops or tablets.

JScreenFix is an online app is available for ‘fixing’ stuck pixels. It can be used with both LCD and OLED screens.

Article by Margaret Brown (see Margaret’s photography pocket guides)

Excerpt from Photo Review Issue 86

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