Lunch atop a (Springfield) skyscraper

Lunch atop a (Springfield) skyscraper
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On all photos from my last vacation I see this red point. It appears on in-camera JPGs as well as on RAW files. A see a red pixel in the same position on all pictures. How is this problem called? Is it a dirty point on the sensor, dead detector or some other more serious problem? How can I fix it?enter image description here

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OK, so I'll add a link to the solution manual into the comment:…. This links explains what you need to do in order to resolve a "stuck pixel" problem. – Pavlo Dyban Oct 29 '13 at 9:30
up vote 5 down vote accepted

Most consumer digital cameras nowadays do automatic dark frame subtraction, which should hide any stuck pixels. (Actually, just plain subtraction would only make the pixel black instead of red, but pretty much all decent dark frame correction algorithms also detect stuck pixels and interpolate over them.)

In fact, the automatic dark frame subtraction is the reason why such cameras often have a noticeable delay between the shutter closing and the picture actually appearing on the screen, especially with longer shutter times, since the camera needs to take a second exposure for the dark frame after closing the shutter.

It looks, for some reason, your camera isn't doing that. If it's a high-end or "prosumer" model, it may have a configuration menu option to disable dark frame correction, and you may have done that without realizing it. If so, simply re-enabling the feature may be enough to fix the problem.

Alternatively, if you want, you could shoot your own dark frame and use that to correct your photos afterwards. (Most decent digital photo post-processing programs should be able to do that.) The dark frame is simply a picture which is completely dark, except of course for sensor noise and stuck pixels, taken under similar conditions as the actual photos. Simply taking a picture with the lens cap on should give you a pretty good dark frame.

Note that, if you do this, using RAW mode for both your photos and for the dark frame is highly recommended: JPEG compression blurs stuck pixels and makes correcting them properly very difficult.

Some cameras may also have "semi-automatic" dark frame correction feature, where you can tell the camera to take a single dark frame and then save it and use it for any subsequent photos. If yours has that, you may simply need to tell it to retake the dark frame (which you should do always before shooting, anyway, or whenever you change the exposure significantly).

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Many cameras with automatic dark frame subtraction only apply it in certain situations, such as exposures longer than 1 second or when high ISO values are used. When shooting at normal ISOs and exposure times most such cameras will not automatically apply it. With my Canon bodies there is no option to force Long Exposure Noise Reduction (LENR) below a Tv of 1 second. – Michael Clark Oct 24 '13 at 23:47
Using a reference dark frame to apply to multiple photos works best if all are exposed at the same sensor temperature, ISO, and Tv. Stuck pixels will remain constant regardless of those parameters, but hot pixels will vary based on their temperature threshold. – Michael Clark Oct 24 '13 at 23:51
That was a very useful reply! Indeed I could correct dark frame using built-in sensor cleaning menu. – Pavlo Dyban Oct 29 '13 at 9:20

It is called a stuck pixel, because it seems to be stuck all the way on in every image. To be more precise, it a red pixel that seems to be stuck all the way on. When the camera or RAW convertor program converts the information from the sensor, the high value of that one pixel is also causing the surrounding pixels to have artificially high red values assigned by the interpolation of the demosaicing algorithms.

A dead pixel is just the opposite: no matter how much light strikes it, the pixel reports no light. Sometimes a dead pixel will be masked by the interpolation of colors surrounding it being assigned to that pixel.

The only way to fix it is to have your camera manufacturer do what is called pixel mapping to tell the camera to ignore the signal from the stuck pixel. Short of that, you could use a dust delete tool or repair tool to mask the hot pixel in each photo, but that is a tedious way to go.

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They're also sometimes referred to as "hot pixels". – Matt Grum Oct 24 '13 at 8:19
If I understand you correctly, there is no way me solving this problem at home? What could've caused this problem? Is this an aging issue? – Pavlo Dyban Oct 24 '13 at 9:32
Note that some cameras — notably, Pentax DSLRs and most Olympus models (including compacts) — have a user-accessible pixel mapping function in the menu. I really don't understand why everyone doesn't do this. – mattdm Oct 24 '13 at 11:51
@mattdm Too bad, I have a Nikon D90. – Pavlo Dyban Oct 24 '13 at 12:57
@MattGrum From my perspective, a stuck pixel is one that is always on. A hot pixel is one that reads out of range due to the temperature of the sensor, and will produce a normal reading once the sensor is allowed to cool. When doing long exposure work, I often experience hot pixels that last the rest of the session but they are not stuck pixels the next time I use the camera. – Michael Clark Oct 24 '13 at 23:41

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