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PostPosted: Tue Dec 04, 2007 10:50 pm 
Hi all,

I have spent many hours googling around for topics on hot/stuck dead pixels, and I have found other threads on this site that have gone into such detail but have not gone into what levels of hot/stuck pixels are acceptable, most searches reveal that people have a couple of hot/stuck pixels whereas I have hundreds , is this right ? should I replace the body ?

I have recently purchased a canon 40D (I am very new to DSLR photography!!) with the view of getting into astro photography (BTW I don't have a decent scope yet!!!, so I am very very new/wet behind the ears !!!).

First of all I tested with a 30second exposure with the lens cap on (22 Degrees ambient temperature), I was amazed to see how many hundreds of red/blue/white pixels (and clusters of pixels) there were , especially in the raw image. I then took another shot with a 5 second exposure, again the same pixels were still there , less obvious, which upsets me slightly especially if I am taking normal scenic night time shots (I understand that I can remove them later on in post processing by subtracting them but should I really have to do this for all normal night time shots?

I have cleaned the sensor using the self cleansing mechanism (with the lens cap on) ,this has made no difference.
I have taken an exposure against a pure white background, there is no evidence of any dead pixels.
I have also enabled the “long exposure noise reduction” which does subtract the noise (and does work) however my concern is that if you have a 2 minute exposure then the camera will double the exposure time for a blackout reading, as I want to get into astro photography this could be quite annoying especially for even longer 40 minute exposures with multiple stacks!!!


I don't have a site to post the files I took however I can email them to you if you would like to see the original photos.

Photos taken

1-Lens cap 30sec exp
2-Lens cap 30sec exp + Long exposure noise reduction on.
3-outside shot – 5 second exp
4-outside shot – 5 second exp + Long exposure noise reduction on.

Any help would really be appreciated !

Thanks in advance.

/Stuart


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PostPosted: Tue Dec 04, 2007 10:55 pm 
I forgot to mention files 1 & 2 were done at ISO 800 and 3 & 4 at iso 100.

/Stuart


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 Post subject: Hot pixels
PostPosted: Tue Dec 04, 2007 11:58 pm 
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Posts: 9815
Location: UK
Hi Stuart,

I hope it doesn't appear too insensitive to offer a warm welcome to the CameraLabs forums when you apparently have an issue with hot pixels! It's also nice to welcome another budding astrophotographer.

If you haven't found it yet Christian Buil has a great feature here about using the 40D for astrophotography.

Here's a 2 minute dark frame I exposed a while ago at ISO 400 which you can possibly use to evaluate the performance of your camera compared to mine. It's a 9MB download: Broken link. There will always be sensor noise at such long exposures so in my opinion it is important not to confuse this with hot pixels which will be significantly brighter than the background noise.

You might like to have a look at my post "Comet Holmes". The first image was taken at ISO 1250 with a three minute exposure. You can click it to view/download the 100% image. There was a tiny amount of post-processing done as described in the post but the dark frame was not used. However, I did take one so if you want to download it the ISO 1250 three minute dark frame CR2 file is here: Broken link.

I wouldn't advise using the camera's own “long exposure noise reduction”. Not only does it double the time it takes for each exposure but it doesn't give the sensor time to cool down between the light and dark frames. If you are taking a sequence of exposures then perhaps one minute between each exposure is a starting point. The same would go for the dark frames and I would also suggest that you don't need to expose as many dark frames as lights unless you are really on a mission!

Bob.

EDIT: Downloadable "dark" frames removed from the server.

_________________
Olympus OM-D E-M1 + M.Zuiko Digital ED 12-40mm f/2.8, Lumix 7-14mm f/4, Leica DG Summilux 15mm f/1.7 ASPH, M.Zuiko Digital 45mm 1:1.8, M.Zuiko Digital ED 75mm 1:1.8.
Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


Last edited by Bob Andersson on Thu Dec 06, 2007 7:10 pm, edited 1 time in total.

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PostPosted: Wed Dec 05, 2007 1:15 am 
Hi Bob

Thank you for the warm welcome and responding so fast (at this time of night :) , sorry I was a bit hasty in getting my first post up about the hot pixels i forgot to say hi to everyone first.

I downloaded your file , and there is a remarkable differences, I counted 6 hot pixels on yours compared to the hundred or so on mine

This is posted in my own web server i quickly put up to share this file so I hope it works, let me know if it doesnt.

http://217.41.39.110:8081/photo1.CR2
http://217.41.39.110:8081/photo3.CR2


What do you think ? photo3 is a 4 second exposure, you can see the same pixels that are hot as in photo1. What worrys me the most is that your photos is a 2 min exposure, i dread to think what mine would show up over that period.

trip back to jessops me thinks...

Thanks

Stuart


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PostPosted: Wed Dec 05, 2007 12:57 pm 
Hi Bob, all.

Just wondering if you got the chance to compare my photos with yours. Do you think this is an abnormally high level of pixels?

thanks

Stuart


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 Post subject: Hot pixels
PostPosted: Wed Dec 05, 2007 1:15 pm 
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Joined: Sat Sep 30, 2006 4:30 pm
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Location: UK
Hi Stuart,

Sorry for the delay in responding. I've just had a look at your photos. If t was mine there would be no question in my mind - I would ask for a refund or replacement. I'd probably go armed with a print from the worst area, though you may have to do a significant enlargement to get the problem to show up on a print. Or can you see the problem if you use the camera's LCD screen at maximum magnification?

Bob.

_________________
Olympus OM-D E-M1 + M.Zuiko Digital ED 12-40mm f/2.8, Lumix 7-14mm f/4, Leica DG Summilux 15mm f/1.7 ASPH, M.Zuiko Digital 45mm 1:1.8, M.Zuiko Digital ED 75mm 1:1.8.
Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


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PostPosted: Thu Dec 06, 2007 6:50 pm 
A big thank you to Bob for his advise, I took the camera back today and had the body replaced. I only have 3 small blue dots compared to my old camera which looked like my windows screen saver !!! (it really was quite bad).


I look foward to many hours snapping away !!

/Stuart


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