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Author Topic: Old sensor - broken pixels or noise?  (Read 5181 times)

Rincewind

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Old sensor - broken pixels or noise?
« on: August 15, 2011, 02:24:59 PM »
Hi Canon Rumors Community,

As a long time lurker and first time poster I have to say thank you very much to all of you for sharing so much useful information.  It has helped me spec a future camera system that will blow the pants of what I have right now.  And that brings me nicely onto a shot I took last night with, and don't laugh, a 7 year old 300D with it original 18-55IS kit lens.  The image is a JPEG straight out of the camera and has no post processing.  You can see an over exposed moon in this 30 sec exposure and the lights from a passing plane (continuous and flashing lights creates an interest effect!).

Now onto the question: If you look a bit more closely (click on the image to do this), in amongst the trees and across the sky you will see some random purple/red and blues dots.  One does not have to pixel peep as the pixels are so big already! Does anyone know what is causing these random pixel colours?  Are they broken, permanently-on pixels, like one gets on LCD screens, or is my sensor just very noisy after all this time?  I noticed that some people get very worried about dust creating dark spots in a image - take a white photo to see it.  Well maybe they should be just as worried about this - take a black photo to see it.

dstppy

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Re: Old sensor - broken pixels or noise?
« Reply #1 on: August 15, 2011, 02:44:35 PM »
Well, what does an image of something purely white and purely black look like?
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IWLP

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Re: Old sensor - broken pixels or noise?
« Reply #2 on: August 15, 2011, 02:58:50 PM »
Yarr, they look to be hot pixels, matey.

The first thing to check is if those pixels are brightly colored, or "hot," with a normal shutter speed in good light.  If they disappear with a shorter exposure, no real need to worry (if there was a need to worry in the first place).

Most likely, they are being caused by the senor heating up due to the long exposure, resulting in a longer "on" time.  They are relatively easy to get rid of, of course, being only one pixel in size and easy to spot.  I had a Nikon Coolpix that did this - but only with certain shutter speeds if the camera had been on a while.  Otherwise, they are easy to fix.

It is somewhat frightening to open a photo and see a bright, red (in my case) dot in the middle of the photo, though.
I'm *this* close to switching to Nikon! I swear - I'll do it! Maybe next month, or in a week, or when they release the D ... oh, look a shiny thing!

autochrome

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Re: Old sensor - broken pixels or noise?
« Reply #3 on: August 15, 2011, 03:06:15 PM »
They're almost certainly hot pixels, as a general rule i tend to be careful with exposures over 4s. There are some tools that deal with this automatically, Darktable has a good hot pixels plugin that analyses the pixels neighbourhood to determine what's a hot pixel or not and clean it. You can also shoot a dark frame after the main exposure and subtract the hot pixels from it.
By the way, that's an interesting effect, the blinking lights of the airplane, it never crossed my mind.

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Re: Old sensor - broken pixels or noise?
« Reply #4 on: August 15, 2011, 03:06:49 PM »
That looks like classic "long duration noise" to me. Not sure if the 300D has it, but more recent cameras can remove it if you turn on the long duration noise reduction. What that does is take a 2nd exposure of the same settings but with the shutter closed. It then subtracts that from the 1st one. Bit of a pain as it doubles an already long exposure... You can do a manual dark shot with the lens cap too, and use an image editing program to do a subtraction. Note that way you might instead end up with black spots!
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Kernuak

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Re: Old sensor - broken pixels or noise?
« Reply #5 on: August 15, 2011, 03:23:50 PM »
I don't think it's noise, I would expect that to be more widespread and more visible in the shadow areas, than the brighter areas. At least one of the coloured pixels is in the clouds, which are among the most exposed areas.
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Rincewind

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Re: Old sensor - broken pixels or noise?
« Reply #6 on: August 15, 2011, 04:01:23 PM »
Here is a 10 sec exposure taken in a dark cupboard with the lens cap on (again, click on it to see the hot pixels).  The bright spots correlate.  So yeap, they are hot pixels and nope, the 300D has no fancy features like that described by lol.  Time to sell/scrap the 300D and time to get a 5D mk3...

dstppy, now you see what a black image should NOT look like ;)

Yes autochrome, I was not expecting the lights from the plane to look like this either, although it makes a lot of sense once you see it.  You are all welcome to steal the idea for your photos.  Share and share alike!

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Re: Old sensor - broken pixels or noise?
« Reply #7 on: August 15, 2011, 04:02:56 PM »
Even newer cameras will produce those bright spots on a very long exposure. Just learn to process them out :)
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Rincewind

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Re: Old sensor - broken pixels or noise?
« Reply #8 on: August 15, 2011, 04:08:23 PM »
Yeah, Lightroom is cheaper than a new 5Dmk3 will be, but I want a new camera!  So let's use the "my old 300D has hot pixels that ruin my shots" as an excuse and say nothing to the missus about all this post processing stuff.

Agreed?

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Re: Old sensor - broken pixels or noise?
« Reply #9 on: August 15, 2011, 04:17:42 PM »
Can't argue with that logic :D
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dstppy

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Re: Old sensor - broken pixels or noise?
« Reply #10 on: August 15, 2011, 04:26:14 PM »
Yeah, Lightroom is cheaper than a new 5Dmk3 will be, but I want a new camera!  So let's use the "my old 300D has hot pixels that ruin my shots" as an excuse and say nothing to the missus about all this post processing stuff.

Agreed?

Absolutely nothing can be done about the camera, sir.  You shall be forced to purchase a new one.

You should probably invest in some L glass as well, to prevent further problems  ;D
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danski0224

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Re: Old sensor - broken pixels or noise?
« Reply #11 on: August 16, 2011, 08:50:33 AM »
I see stars....
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K-amps

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Re: Old sensor - broken pixels or noise?
« Reply #12 on: August 16, 2011, 10:33:33 AM »
Yeah, Lightroom is cheaper than a new 5Dmk3 will be, but I want a new camera!  So let's use the "my old 300D has hot pixels that ruin my shots" as an excuse and say nothing to the missus about all this post processing stuff.

Agreed?

I wish my missus was as "enlightened" as that...
« Last Edit: September 21, 2011, 01:24:00 PM by K-amps »
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awinphoto

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Re: Old sensor - broken pixels or noise?
« Reply #13 on: August 16, 2011, 10:37:05 AM »
Yeah, Lightroom is cheaper than a new 5Dmk3 will be, but I want a new camera!  So let's use the "my old 300D has hot pixels that ruin my shots" as an excuse and say nothing to the missus about all this post processing stuff.

Agreed?

I wish my missus was as dumb as that...

aw hell, i shoot professionally and I rarely get away with the "this camera will pay for itself and then some within weeks" with my wife... I might as well drop kick my camera to convince her I need a new one haha. 
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Rincewind

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Re: Old sensor - broken pixels or noise?
« Reply #14 on: August 16, 2011, 10:54:42 AM »
I am very lucky as I got permission to replace it a while ago because I convinced her that a new one with large L glass would take better shots of my 6-year old in her dimly-lit ballet performances.  That was just before the 60D came out.  Then I got carried away with the research (thanks to you lot!) and ended up deciding to wait for the 5Dmk3.  One tsunami later and I am still waiting...