10
\$\begingroup\$

After shooting for one hour with my Rebelt2i + Tamron 17-55 f2.8, I went back home and checked pictures on my computer.

For every single picture, there are a lot of 'dead' pixels at the same location. I searched on Google and it could be dust, dead pixels, hot pixels... Do you have any suggestions on what the problem really is, and how to solve it?

Look between two buildings on the right, a few in the sky too: sample image (Click for large version.)

Question also asked by gsharp:

Every picture that I took today have a red dot on the same place in every photo. It looks like a monitor "pixel error". Is just the lens dirty or did something bad happen to my cam?

Here some samples. Best mode to detect the dot is download the picture in original size (click the (i) then download)

Improve this question
\$\endgroup\$
6
  • \$\begingroup\$ Just to confirm; this happens on regular basis not only this one time and with this particular image? \$\endgroup\$
    – Jakub Sisak GeoGraphics
    Aug 14, 2011 at 11:50
  • \$\begingroup\$ It happends only for long exposure ( +20sec ), that means every pictures from that night have hot pixels at the same location. \$\endgroup\$
    – Eno
    Aug 14, 2011 at 18:16
  • 9
    \$\begingroup\$ Possible Explanation: There Really was a Red Dot in Every Scene The question is: are you following the Red Dot around, or is he (they are usually male) following you around? Red Dots love to be photographed and are known for jumping out on photographers and posing in random shots. I'll grant that this is an unlikely explanation. \$\endgroup\$
    – AJ Finch
    Oct 26, 2011 at 11:22
  • 7
    \$\begingroup\$ @AJFinch - Obligatory Mitch Hedberg quote: "I think Bigfoot is blurry, that's the problem. It's not the photographer's fault. Bigfoot is blurry, and that's extra scary to me. There's a large, out-of-focus monster roaming the countryside." \$\endgroup\$
    – anon
    Oct 26, 2011 at 20:59
  • 2
    \$\begingroup\$ @anon. Perhaps I should try tracking down Bigfoot; I'm very skilled at photographing blurry people, as my photo collection demonstrates \$\endgroup\$
    – AJ Finch
    Oct 28, 2011 at 11:58

5 Answers 5

Reset to default
15
\$\begingroup\$

You're exactly right — it's the same thing as a monitor pixel error, but on your camera's sensor rather than on an LCD screen. *

You can either fix it in post-processing (automatically, with many RAW-processing packages) or have it mapped out with the camera's firmware.

If you're lucky, your camera model includes a built-in feature to do that yourself in the field; failing that, it's almost always covered under warranty service.

* well, not really exactly the same, since it's a photoreceptor rather than an LCD pixel, but it's similar in concept. One of the image elements is malfunctioing.

Improve this answer
\$\endgroup\$
6
  • \$\begingroup\$ oh lord. is this something normal/usual or do I have a defect cam? \$\endgroup\$
    – gsharp
    Oct 25, 2011 at 22:12
  • \$\begingroup\$ It is fairly common. And consider, if you have one dead pixel on a 10megapixel sensor, that's a 0.0001% loss. No big deal, as long as you mask it so it isn't annoyingly bright red. \$\endgroup\$
    – mattdm
    Oct 25, 2011 at 22:16
  • \$\begingroup\$ I guess it affects more that one pixel, because the dot is visible in every photo in original size. \$\endgroup\$
    – gsharp
    Oct 26, 2011 at 9:10
  • 1
    \$\begingroup\$ @gsharp — Can you post an example? A single stuck-on red photosite can really stick out, particularly since it will also affect the surrounding pixels in the Bayer filter interpolation (which reconstructs a full-color image from a bunch of red, green, and blue photosites). If you have the photosite masked out, that won't happen; it'll just be ignored and you won't notice it at all. But it is possible that you have a cluster of dead pixels, or some other problem. Posting an example will certainly help us figure out what's going on. \$\endgroup\$
    – mattdm
    Oct 26, 2011 at 19:36
  • 2
    \$\begingroup\$ @gsharp try shooting a 2 minute long exposure with the lens cap on and watch the resulting christmas tree. It is really common to have cold/hot pixels on a camera sensor. It shouldn't be so apparent on sub-second exposures, but it is not something to worry about. Clone it away, tell the firmware to ignore it, whatever. It's just a single pixel. \$\endgroup\$
    – pau.estalella
    Dec 27, 2011 at 16:37
5
\$\begingroup\$

This effect is a very common one and is caused by dead or damaged pixels.
Pixels may be "stuck on" and a single pixel producing a bright one colour dot or a small group of pixels

In images they are often termed "hot pixels. Most sensors have some. They are a common consequence of aging. Some sensors will have them from new, some won't.

LCD screens also have them. Software is available to map them and to adjust the image to greatly remove their effect. As a simplistic example, if you had a single "hot pixel" if you averaged the light values in the adjacent pixels and replaced the "stuck" value wit the average value it would be exceeding unlikely to be noticed in the very large majority of images. The algorithms used for correcting them are liable to be more complex than this but this explains the concept well enough.

Some cameras have a hot pixel correction function - a long exposure shot is made in 'total darkness' (eg - with lens cap on) and the camera then adjusts for anything it finds in the dark frame.

Hot pixels will usually be most obvious in areas where they stand out against the background, and in longer exposure images.

"Stuck" pixels are most obvious if they are stuck on - but pixels that are always off will also be encountered. Here's an image showing an always off green pixel.

enter image description here

Here's a comment on them by Ken Rockwell

Wikipedia - defective pixels

Also informative

DPReview user discussion

Think yourself lucky

Related

Improve this answer
\$\endgroup\$
7
  • \$\begingroup\$ What is this caused by? How does the damage occur? Also, why are there several bright pixels in one dot instead of a single pixel? Are there actually several adjacent damaged pixels on the sensor, or is this effect due to demosaicing? \$\endgroup\$
    – Szabolcs
    Aug 8, 2012 at 11:24
  • \$\begingroup\$ Read the references. Look at the large amounts that Gargoyle says on the topic and then ask further if desired. \$\endgroup\$
    – Russell McMahon
    Aug 8, 2012 at 11:36
  • \$\begingroup\$ Hey Russell, could you move this answer to photo.stackexchange.com/questions/14889/…? \$\endgroup\$
    – mattdm
    Aug 8, 2012 at 11:56
  • \$\begingroup\$ @mattdm - I can just reenter the answer details under the other question but that doesn't "move" it per se. Is there a formal means of moving it? Points from upvotes are not especially important wrt their affect on my reputation BUT the upvote count against the answer is an indication of the value that others thought the answer has, and I do value that. \$\endgroup\$
    – Russell McMahon
    Aug 8, 2012 at 18:13
  • \$\begingroup\$ A mod can move it, but after it's accepted as an answer the only way is by doing merge of the questions. I'm not saying it's a bad answer — it is — but it's really better for people looking for information later if we can consolidate all basically-identical questions in a canonical place, with all the answers there. And this one we get a lot! \$\endgroup\$
    – mattdm
    Aug 8, 2012 at 19:57
1
\$\begingroup\$

After moaning about the supposed pixel mapping myth in the comments I thought I'd provide one possible solution. Digital Photo Professional provides a dust mapping option for dirty sensors that removes dust data from images. Info about it can be found here http://support-th.canon-asia.com/contents/TH/EN/8201038200.html

I 'think' you can use this method with test images of the hot pixels to automatically remove those locations from the image, but I haven't tried it myself and wouldn't exactly be sure how to do this, so this is only really a half answer. Maybe one worth exploring though.

Edit: Also, depending on how long your exposures were at that time, if you use Long Exposure Noise Reduction, the camera will take a hot pixel map after your photo for the same length as the exposure to remove hot pixels. This may not be convenient though with the extra time it takes, especially with very long exposures. Also I'm not sure the minimum exposure lenth it needs to be to kick in.

Improve this answer
\$\endgroup\$
2
  • 1
    \$\begingroup\$ Adobe Lightroom does (effective) automatic hot-pixel removal. DXO Opticsworks does it too, though it's significantly less effective. Neither needs a dark-frame. \$\endgroup\$
    – Fake Name
    Aug 15, 2011 at 5:36
  • \$\begingroup\$ The dust data method is another mathematical model than is needed for hot/dead pixels, so that part of the answer is wrong, but the long exposure noise method is a nice bit of information. \$\endgroup\$
    – Michael Nielsen
    Dec 20, 2012 at 10:04
1
\$\begingroup\$

Those are probably hot pixels, to test this just take a picture with the lens cap on - this picture will come out all black except for hot pixels (dust or lens problems wouldn't show up because there's no light to see them with).

I have no idea what's the acceptable number of hot pixels and when you should send the camera to be fixed but I can tell you how to map out those pixels, this worked for my 550D (copied from my answer on another question)

http://www.slashgear.com/how-to-remove-stuck-pixel-on-your-digital-slr-2227392/

The relevant part:

Test for dead pixels : Left lens-cap on, set camera to exposure 30 seconds black-out image at various ISO settings.

Fix for dead Pixels : Set camera to perform sensor cleaning mode manually (not auto) for one minute. You only execute the function, not exactly performing the whole sensor cleaning procedure. Magically, the sensor cleaning task should shake the stuck pixels off, if not, repeat couple more times until it does. Otherwise, time to call the technical support.

I've tested for hot pixels using only ISO 100 (I'm lazy), the image clearly showed the 2 pixels that caused me to look for this + another red pixel.

After enabling the manual sensor cleaning function (without even taking the lens off) and waiting for a minute the first two pixels disappeared, after 2 more "cleanings" the third also disappeared.

Improve this answer
\$\endgroup\$
15
  • \$\begingroup\$ Note that the sensor cleaning task does not actually "shake the stuck pixels off", as the linked text suggest. It apparently is doing the separate operation of mapping out the hot pixels. On many cameras (Olympus and Pentax, at least), this is a separate menu option. It is a little bit odd, though, that the suggestion is to repeat several times. That should not be necessary. \$\endgroup\$
    – mattdm
    Aug 14, 2011 at 13:41
  • \$\begingroup\$ @mattdm - I agree that in theory repeating several times should not be necessary - but in reality, when I did it with with my actual camera, it took 3 times until all the hot pixels were mapped out. \$\endgroup\$
    – Nir
    Aug 14, 2011 at 14:56
  • 3
    \$\begingroup\$ The sensor cleaning mode is purely a myth. As far as I know it is not linked to any pixel mapping on any Canon cameras. Any higher end cameras that have have a hot pixel mapping option have it separately in a menu option, and any lower end cameras that don't I think you can send them off to have it done. There may be a way to do it with software that I don't know of. All the sensor cleaning option will do after seeing 'Hot pixels' is open up the sensor to the air, which will cool it down slightly quicker, meaning the next test shot may have less pixels visable. \$\endgroup\$
    – Dreamager
    Aug 14, 2011 at 17:42
  • 1
    \$\begingroup\$ If after the 'sensor cleaning' the software was to map the pixels as you suggest, as the cleaning left the sensor off for 30s (or however long you left it open), would the camera not then have to spend 30s for the sensor to expose itself so that it could see the pixels it needs to map? \$\endgroup\$
    – Dreamager
    Aug 14, 2011 at 21:26
  • 1
    \$\begingroup\$ @Fake Name Yes I gave that a mention in my answer, it's very effective I hear although 'cos of the double time I've not used it myself. This extra time is something that definitely doesn't happen after the 'sensor cleaning' myth method however, so when the pixel mapping is supposedly supposed to take place I don't know. \$\endgroup\$
    – Dreamager
    Aug 15, 2011 at 9:25
0
\$\begingroup\$

When shooting raw, hot pixels are often handled by telling the raw converter to ignore signal from pixels listed in dead pixels file or similar. When shooting JPEG it's get harder, because it's camera, not PC software that does the interpolation step.

In normal digital cameras the raw conversion needs to interpolate pixels even if every sensor pixel was working, because you get only one color component per pixel.

Improve this answer
\$\endgroup\$
2
  • \$\begingroup\$ The camera can be made to ignore the stuck pixels in its JPEG conversion. In most high-end cameras (and low-end ones from at least Olympus and Pentax; maybe others) you can do this from a "map out hot pixels" menu item. For others, it can be done at the factory, almost always as free warranty service. \$\endgroup\$
    – mattdm
    Aug 15, 2011 at 12:54
  • \$\begingroup\$ Yeah, that's why I said "harder", because you need manufacturer support to get it done, so the easiness varies. \$\endgroup\$
    – Zds
    Aug 15, 2011 at 13:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.