Noise appearing like dead pixels, what is it?

[stigmundfreud]

Bit of a frustrating one this. Noticed on my old body that during long exposures I was getting coloured pixel hot spots that looked like dead pixels, I know they weren't dead because other normal images appeared fine.

Today I took my first long exposure on the new body and still the same - hot spots all over. So I thought it might be specks of dust on the lens. As I dont tend to do any night stuff or long exposure its something I'm not sure of.

Anyway this is a (large) image as an example out of the camera

http://farm4.static.flickr.com/3107/3185377739_25d... (its a bit big)

If you maximize the image in the browser you will see the hot spots. Now I then tried in the studio in pitch black and got the same effects even after making sure the lens was as clean as possible both outer glass and the body side glass. It did improve but still it was suffering. Changed lens and similar issue so I'd imagine it is lens artifact related.

Turning on the Long exposure noise reduction has cured the problem but my question is would these coloured spots be caused by lens dust/artifacts and the more you use the lens over time these things will creep in? The higher the iso speed the worse it gets (Which I'd expect) anyway here are two more examples with and without noise reduction (iso1600)

Noise reduction off: http://farm4.static.flickr.com/3522/3185448741_623...
Noise reduction on: http://farm4.static.flickr.com/3434/3185448801_afc...

I'm happy to turn on the long exposure noise on but would like to understand what causes the coloured spots, on the other test shots I did I did notice a big improvement after a thorough clean of the lens but still getting a few of the spots on iso100 speed. Its just something I never remember happening when I last did long exposures

[Bee_Jay]

Not looked at your image (was taking too long to download).

However I have seen it mentioned on DPReview many times that you can get the camera to map out hot pixels by selecting manual cleaning with a lens still on and just leaving it for 10 mins or so, then turning it on and off again.

There were varied reports of people getting this to work with the 5D, not sure if it works with the MkII yet, some have been trying but not been following too closely.

I can only read DPReview for short periods of time before my piss starts to boil and then I have to have a break for a week or so...

[Bee_Jay]

Just looked at the first of your pair of pictures and that is interesting!!!

Tell me the exact exposure and camera settings and I'll try it on my 5D2 tomorrow and post the results for you to compare.

Do they show up in RAW (I shoot RAW+LJPEG)

[malc350]

Hi I see from the Exif that this was a 30 second exposure which is pretty long. Are these not what's known as "hot pixels" and are the result of the long exposure.

I would try using shorter shutter speeds and compensate by opening up the aperture / using a faster ISO and, starting with an exposure of say, 5 seconds, increase each exposure until you see at what point these pixels start to appear.

I see you're using a 5D Mark II which has a reasonably but not excessively densely packed sensor (according to some real gear-heads it's the same pixel density as a 20D)so I don't know that I would expect excessive hot pixels from that camera.

Becuase of the video capability of the 5D Mark II where the mirror can be up and the sensor on for several minutes I'd expect the sensor to be capable of running quite hot unless I am getting mixed up with what causes hot pixels?

As the NR is helping slightly it odes point to being as a result of the very long exposure...

[Cabinet Enforcer]

Those are indeed "hot" pixels which show up on long exposures, long exposure noise reduction in not like other smeary jpeg-alike malarkey noise reduction, it is known as "dark frame subtraction", basically the cam takes a long exposure with the shutter closed, this measures the hot pixels and the cam then "subtracts" these from the final image.

My camera takes a dark frame on every long exposure (very irritating) but most seem to store the data somehow, and use factory calibration data or just an occasional dark frame to modify images. It does seem like there are rather a lot of hot pixels on your particular sensor (more than on my old P+S), but the dark frame subtraction does rather a nice job of removing them as you can see. I would just turn on the long exposure noise reduction and forget about it

eta: Yes hot pixels do get worse as sensors get older (apparently), you might not have noticed the odd one or two when the camera was new, now they are rather more obvious.

Edited by Cabinet Enforcer on Sunday 11th January 01:06

[stigmundfreud]

if it were hot pixels I'd expect on long exposures but on one of my shots it was on 6 seconds, they also map per lens so on my 17-40 each test image had the same colour spots but on the 24-105 they were different areas but in each set, side by side images on same lens gave spots in same location. I suspected it was hot pixels but as 2 shots from lens 1 gav e the same hot pixels then on 2 shots on lens 2 gave to identical hot pixels (but different areas to lens 1) I'd expect it was dust on lens.. also it seemed to reduce after giving the lens a proper good claen

Either way I found that long exposure reduction set to auto doesnt work or didnt work but on it did. Bejay if you just go into a solid black room you'll get them from 10 seconds or 30 secongs try on 100 then 1600 and see

[malc350]

Bee_Jay said:

I can only read DPReview for short periods of time before my piss starts to boil and then I have to have a break for a week or so...

Aint that the truth, it's full of loons: "I've heard this camera is better than that one, blah, blah!!"

Meanwhile back on topic: It will be interesting to see if Bee_Jay's camera does the same thing. At 6 seconds I'd say it's much less likely to be hot pixels though I'm intrigued here as I have never known dust to have such an effect.

I know I have had dust in some of my lenses because I always check them now. And that's only down to Ebay where people have kittens if you try and sell a lens that has actually been used, everyone wants a pro camera and gret lenses for no money but which are "boxed, as new, and have no signs of use".

Will be interesting to see what the outcome of this "problem" is?

[Bee_Jay]

Just off out, I'll give it a try this evening.

[stigmundfreud]

cheers. Definitely improved on the lens clean if it comes to it I'll just send it back for investigation

[stigmundfreud]

you know, I never thought of just using hte lens cap :smakshandonface:

[Bee_Jay]

Or even better, the body cap - take some pics with just the body cap or lens cap on, tell me what you see/do then I'll reproduce it here and see what I get.

[stigmundfreud]

will do it later, got a shoot this evening so as soon as thats done will do it

[malc350]

Stigmund, you may be interested in this link from the forums on DPr*vi*w. The guy describes a very similar phenomenon to what you are seeing. After his first post or so you'll have to ignore some of the that are trying to sg him off and you'll find some sensible replies, e.g. from orbitalpunk that says he's also experienced it and certain software packages like Lightroom mask them out automatically. There are also some other worthwhile comments but, as usual, you have to scroll through all the imbeciles' comments too...

Hope this helps, here's the link: http://forums.dpreview.com/forums/read.asp?forum=1...

[stigmundfreud]

malc350 said:

Stigmund, you may be interested in this link from the forums on DPr*vi*w. The guy describes a very similar phenomenon to what you are seeing. After his first post or so you'll have to ignore some of the that are trying to sg him off and you'll find some sensible replies, e.g. from orbitalpunk that says he's also experienced it and certain software packages like Lightroom mask them out automatically. There are also some other worthwhile comments but, as usual, you have to scroll through all the imbeciles' comments too...

Hope this helps, here's the link: http://forums.dpreview.com/forums/read.asp?forum=1...

thanks for the link and taking the time and pain to search through it I'll read later

[L4MBOLUV3R]

very strange, there pure white so not normally what you would get from dust on the image processor and the red ones look like burn marks? the centre is white and the edging is red. how uniform is this? are they roughly in the same place every pic?

[stigmundfreud]

lens 1 = same place for all lens 1 pics
lens 2 = same place for all lens 2 pics

will double check, not too fussed as long noise reduction really does remove them. Just processing tonights shoot so wont get a chance to test until the next day or so

[malc350]

Did the cause of this ever get sorted?

[stigmundfreud]

will go and do a body cap test now and find out

[Bee_Jay]

Cool - tell me how you get on and what you do and I will try and replicate it for you.

[slartibartfast]

i get hot pixels with my canon 300D when doing long exposure astroimaging through my telescope, i've heard the way to get rid of them from the shot is to take another shot of the same length of exposure and stack the dark image over your normal image.

have never tried it though.

Paul