With some cameras you may find a strange spot on your pictures that is not a form of digital noise—the noise you see when you use higher ISO sensitivity settings. The spot can be colored (often red), dark, or white. Often, the spot will stay in the same place on all pictures the camera makes and it cannot be removed by cleaning the sensor.
A few years back, I had a Nikon D300 that had a small group of about 4 pixels that stayed red all the time. It was that way when I bought it new. I learned to look for the tiny red spot and remove it with the clone tool in Photoshop on important images.
You may see the same problem with one of your Nikons. A sensor has millions of pixels, so it is not really all that uncommon to have one or two that do not work correctly. Most camera sellers will replace a camera with a bad pixel, after you can prove that the problem is there. However, many digital photographers do not worry about a few pixel problems out of millions of good pixels.
There are three main types of pixel problems: stuck pixels, hot pixels, and dead pixels. Let’s briefly discuss each one of the types. What’s the difference between a stuck pixel, a hot pixel, and a dead pixel?
This type of pixel is receiving power from the camera, so it is not a dead pixel. However, the pixel has lost its mind, in sense, and ignores the actual shades it should be recording. The pixel will display in one color that does not change from picture to picture. The most common color seems to be red, although you might see another color. The key is that a stuck pixel is stuck on one color all the time, and that color does not match the surrounding pixels that are reporting correct color shades. It is always present in the image in the same spot and does not disappear.
A dead pixel is a defective pixel on the camera’s sensor. It is not receiving power, or it is no longer able to function when power is applied. It could appear as a tiny dark spot in your image that does not change position from picture to picture. Or, it could appear as an off-colored pixel due to Bayer-filter demosaicing errors. Generally, cameras come from the factory with all dead pixels mapped out. However, if a pixel dies over time, you will see evidence of the dead pixel once it is completely dead. Once a pixel dies, it usually stays nonfunctional.
This pixel problem is very common. Even the most expensive cameras suffer from them. Even brand-new cameras! They come and go from various places and can have almost any color, although, again, the ones I see are usually red (maybe because that’s the most noticeable color). These off-color pixels are caused by heat in the sensor and sometimes even from high ISO settings (above ISO 800). Camera manufacturers try to map out pixels that tend to become hot pixels before the camera leaves the factory. However, due to the variability of where they are located at different levels of heat and ISO settings, it is impossible to map them out fully. This type of pixel is often removed by the camera’s two noise reduction functions: Long exposure NR and High ISO NR, with Long Exposure NR being the most effective.
Mapping Out Bad Pixels
If your camera suffers from the first two types, stuck or dead pixels, you can send it to Nikon to have the pixel mapped out (for a fee unless the camera is in warranty), or you can tolerate the problem and remove the spot(s) yourself with software.
If you are seeing hot pixels, and you are shooting longer exposures, you should really be using Long Exposure Noise Reduction (Photo Shooting Menu > Long exposure NR), which will help remove the worst of the problem through a method called dark-frame subtraction. This method causes the camera to take two picture for each image stored—for images having exposures of over one second with current Nikons and eight seconds with older Nikons. The first exposure is the actual image and the second is an equivalent exposure with the shutter blades closed. The second image will contain only the hot pixels and fog (amp noise) from a hot sensor and can then be automatically subtracted from the first picture by the camera. This dark-frame subtraction method is quite effective, although it does double the time required for each final image.
Film photographers had to deal with graininess in their images from the silver grains used to form the image. Digital photographers have to deal with digital noise and often with one of the three pixel problems we have discussed, not to mention dust on the sensor.
However, it is relatively easy to deal with pixel problems these days. Especially for those who postprocess their images and know how to use today’s digital darkroom software.
Keep on capturing time…
Darrell Young is an active member of the Nikonians User Community, Nikon Professional Services (NPS), Professional Photographers of America (PPA), North American Nature Photography Association (NANPA), and the author of 18 photography books from NikoniansPress and Picture and Pen Press, through Rocky Nook. You may review a few of Darrell’s books here. He has been an avid photographer since 1968 when his mother gave him a Brownie Hawkeye camera. Here is one of Darrell’s nature image portfolios.
His website, www.PictureAndPen.com, was created to support the readers of his educational books, photography students, and clients. Visitors to his website will find articles and reviews designed to inform, teach, and help you enjoy your photographic journey.