Identifying Pixel Errors – Photo Review
How to identify dead, stuck, and hot pixels on camera screens, and what you can do about them.
A simulated view of a typical electronic camera viewfinder with a group of dead pixels (circled in red) just above the level display.
Screens are essential components of digital cameras. The technology has been used since the early days of digital photography, although nowadays the organic light-emitting diode (OLED) display has largely replaced the liquid crystal display (LCD) in electronic viewfinders (EVFs) in cameras. Photo.
The displays and image sensors of digital cameras 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 sensor output, the data that is saved as an image file.
When everything is working properly, the average photographer places little importance on these pixel arrays. But what happens when something goes wrong 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’re caused and what can be done to prevent them from affecting your photos.
A dead pixel represents a photodiode that has stopped receiving all power. It almost always appears 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 not 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, you’ll need to enlarge the view displayed.)
Dead pixels on screens not affect recorded images, so if you only find a few dead pixels on your LCD screen, you better not worry about it. A typical 3-inch monitor has around a million dots, so a single dead pixel represents a failure rate of 0.0001%. Clusters of obvious pixel errors can be distracting and can interfere with your view of subjects. Whether it’s worth having the screen replaced is up to you.
It’s much more serious when dead pixels occur on image sensors, as they can affect the images produced by the sensor. But unless they come in clusters, you probably won’t notice them because the interpolation process applied to the color filter network data (Bayer being the most commonly used) will hide them.
It works like this: the interpolation algorithms work on groups of four to nine photosites, comparing the values and setting the color and brightness values for each pixel in the image. If a photosite in the cluster delivers no signal, it will be ignored and data from the surrounding eight photosites will be used to fill the gap.
Pointing your camera directly at the sun can burn pixels into its sensor, inflicting permanent damage. If you use a mirrorless camera, the EVF screen will also be damaged, again because pixels are destroyed.
EVF screen damage won’t show up in your photos, although camera sensor damage will – and it can be larger and more noticeable 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 type of defect (circled in red) created by burnt-out photodiodes (“pixels”) on an EVF screen. This was most likely caused by pointing the camera directly at the sun and may also have damaged the camera’s image sensor.
Unlike dead pixels, stuck pixels are still powered. They occur because one or more photodiodes in the RGB array are stuck at their maximum value. If a single photodiode is “stuck”, it may appear as a bright blue, green, or red dot. If all photodiodes in an RGB cluster are blocked, the result will be a larger dot, which will appear pure white.
Stuck pixels on a monitor or EVF should be treated the same as dead pixels; if you can ignore them, do so. If they are large enough to be in the way, the screen might need replacing.
Dead and stuck pixels can appear larger in JPEG images than they actually are due to the way JPEGs are processed. Working with clusters of pixels can produce halos around defective pixels which, if large enough, can have a visible impact on photos.
Stuck pixels (shown in the magnified oval above the screen) on a computer screen. Since monitors have a relatively high resolution, it will be difficult to find defective pixels without using a magnifying glass.
Unlike stuck pixels, hot pixels do not occur on LCD or OLED screens. They only appear when the camera sensor gets hot during long exposures or when the ISO exceeds around 800. They can appear even on new cameras, although manufacturers usually map them out as part of the process of quality control.
Most hot pixels will disappear over time, but the likelihood of them occurring will increase over time, especially if your camera heats up frequently. You can tell if your camera has 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 “base” setting. natural at 100 or 200 ISO. Then take a second exposure (still with the lens cap on) at the fastest mechanical shutter speed supported by the camera with the sensitivity increased to at least ISO 800 (or higher).
Analyze the two high magnification images in your favorite image editor, looking for areas that are not pure black. This test can be used to identify all kinds of defective pixels. Nothing even remotely resembling a bright colored or white speck, speck or spot should appear in a photo taken with a lens cap. So any dots you see will represent a defective pixel. (You’ll likely see more hot pixels in images taken at the highest ISO than in the long exposure at a low ISO setting.)
It is important to note that pixels that appear “hot” on long exposure may never appear on subsequent exposures. Other pixels may look hot on the next long exposure you take.
Hot pixels can be difficult to identify on camera screens, but will show up as colored or white pixels when images are zoomed in.
Repair 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 shooting raw files will find that sophisticated converters like Adobe Camera Raw, Capture One, and DxO Optics Pro will “map” them as part of the conversion process.
Some camera manufacturers provide pixel mapping as part of their camera firmware. It is found in the custom menu pages of Olympus cameras. This feature works by closing the shutter and capturing a black frame of reference. All detected defective pixels are mapped and their positions are recorded so that they can be eliminated from subsequent frames.
Stuck pixels in most Canon cameras can usually be “fixed” by running a manual sensor cleaning procedure. Recharge the batteries, select this function in the menu and wait about half a minute before turning off the camera. This procedure may also work for some Nikon cameras.
Hot pixels are more of a problem in video because it takes too long to attempt to remove them with software. Running a sensor cleaning function is the best way to fix hot pixels in video clips. You must run the sensor cleaning for thirty seconds to a minute for it to be effective. In theory, by vibrating the sensor, it can reset the tiny electric lines that supply each photosite, by gently massaging it.
Hot pixels can be a random problem when it comes to video. In most cases, replacing the sensor will cost as much as a new camera. Cleaning the sensors can be time-consuming, but it’s also free and worth a try. If that doesn’t work, you may need to keep the camera on just to take pictures.
A web search will reveal many applications to “fix” dead, stuck, and hot pixels. We have listed some of the most credible free apps below.
PixelHealer is a user-friendly and versatile application that can be used to check LCD or TFT screens. It is designed for use on desktop monitors, laptops or tablets.
JScreenFix is an online application available to “fix” stuck pixels. It can be used with LCD and OLED screens.
Excerpt from Photo Review Issue 86
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