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Hot pixels - do you get them and how do you deal with them?

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snowjunkie
28 Nov 2009 - 11:01 AM

Before I start I should say this is not an invitation to a Canon vs Nikon (i.e. which is better) debate!!

With my Canon 5D I notice that on long exposures (e.g. 4-5 minutes) I get quite a lot of hot pixels in the resulting RAW file. These show up as random little red dots in case you don't know what I mean. The reason for my question is that I noticed recently someone shooting next to me one evening and also doing the same sort of exposures didn't seem to get the problem. They were using a Nikon as it happens (- steady guys...!)

Is it just me? I know this isn't a brand-battering thread but do other 5D owners out there get the same thing? That would reassure me that my camera isn't faulty. Do other makes (Nikon, Olympus, Pentax etc.) also get this?

And if you do, how do you process them out?

I understand the theory - sensor heats up because it is being charged for so long - but the fact that someone else next to me doing the same thing didn't experience it set me thinking.

Polite comments welcome. Wink

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User_Removed
28 Nov 2009 - 11:53 AM

Hey,

You know the theory. Smile

You are noticing a difference because of the way the two companies sensor technologies operate, and subsequently deal with the information from the photo sensors.

It just so happens that Nikon do a little more vigorous post-exposure processing from the information from the sensor than canon do: Especially on longer exposure images. Also the current iteration of the Canon CMOS stack is quite sensitive to long exposure noise just because the way it is charged in function.

Anyway; moving away from sensor limitations and companies.

You can easily reduce all noise in long exposure images by thinking about what you are actually doing:

An exposure of 5 minutes total.

To remove the random noise, take six exposures of 30 seconds and then stack them in photoshop.

Problem solved.

This is what star trails photographers do; who sometimes take exposures of 6+ hours.

You can also them employ whatever software methods you like on top of this technique.

Last Modified By User_Removed at 28 Nov 2009 - 11:53 AM Helpful Post! This post was flagged as helpful
MikeA
MikeA  91168 forum posts England
28 Nov 2009 - 12:45 PM

You can also create a hot pixel map or "dark frame" and subtract it from your subject image frame.
To create the map, photograph subject, put lens cap on lens take another image with same time lapse.
Open subject frame and dark frame in photoshop, copy dark frame onto subject, go to layers and alter blending mode to difference.

User_Removed
28 Nov 2009 - 1:03 PM

The 40D has a long exposure mode which is set as a custom function and works for exposures over 1 sec. I have found it deals with this quite well.

The camera processes the image for the same length as the exposure - so it does mean you can't take another shot until it has finished.

I would be surprised if the 5D did not offer this.

cambirder
cambirder  107202 forum posts England
28 Nov 2009 - 1:12 PM

You will get hot pixels on all cameras with long exposures, try enabling long exposure noise reduction.

AnthonyM
AnthonyM  9388 forum posts United States2 Constructive Critique Points
28 Nov 2009 - 2:12 PM

If you're not shooting RAW, the RAW processing software should take care of most of that. Shooting in JPEG mode actually seems to accentuate hot pixels on my camera -- instead of a dot it becomes a star shape. This makes it very difficult to fix well in an automated fashion.

I find with RAW mode, that I generally do not need to use the dark-frame-subtraction noise reduction "feature" of my camera in most situations.

I also simply hate waiting so long for the ark frame capture before I can take my next photo. You can also do dark frame subtraction at home if you have a lot of photos to be taken and you do not want to wait in between. It is a slight post-processing pain in the arse but works about as well as in-camera, and saves up-front time.

User_Removed
28 Nov 2009 - 2:19 PM

Or take multiple shorter exposures and avoid the problem completely...

:/

snowjunkie
28 Nov 2009 - 9:22 PM

Some top tips there folks. That was way more detailed info than I was expecting, so thanks to all. I hope they keep this thread available in the database for anyone doing future searches on the subject.

Swwils - excellent suggestion about multiple exposures. It may seem obvious to many but I'm still new to the digital manipulation stuff so this just never occurred to me. I discovered recently that some Nikon cameras have a multiple exposure feature built in; I saw it in action and it's really impressive. (Oh no, the brand debate again...)

Thanks for the suggestion about subtraction in PS. That's something I think I'd have to work up to. Having seen the Nikon in-camera exposure layering though it seems to me that multiple exposures is the way to go.

Thanks to all who replied. Knowledge sharing is what drives the site.

Cheers.

Tony.

uggyy
uggyy  82104 forum posts Scotland9 Constructive Critique Points
28 Nov 2009 - 9:52 PM

Just to make you feel better, the Nikon D200 isnt too hot for hot pixels on long exposures.. Wink not hard to fix but just a pain.

Hugeknot
Hugeknot  91212 forum posts Iceland2 Constructive Critique Points
28 Nov 2009 - 10:18 PM

I only get them on my 5D2 when I underexpose. Not much of a problem in cold climates anyway.

User_Removed
28 Nov 2009 - 10:50 PM

Nikon's sensor technology doesn't have individual active amplifiers for every pixel.

This is a feature of Canon CMOS technology and so noise is generally a little more exaggerated under longer exposures.

http://apod.nasa.gov/apod/image/0609/startrails11h_hambsch_f1.jpg

Images like that are taken with 11 hour exposures, compromised of lots of shorter exposures. Its easier to stack noise free images together rather than software the noise out. - Think back to film, multiple exposure loveliness. Grin

Yes Nikons have that feature. - But Canon have the LC and highlights features. So its a 50/50 swing for the application.

Helpful Post! This post was flagged as helpful
tomcat
tomcat e2 Member 85912 forum poststomcat vcard United Kingdom15 Constructive Critique Points
29 Nov 2009 - 2:54 PM


Quote: Hot pixels

What a name for the EPZ staff if they ever decided to take part in a charity calendar shoot.

Starring Peter Bargh & the Hot Pixels

Adrian

Metalhead
Metalhead  61866 forum posts England2 Constructive Critique Points
29 Nov 2009 - 4:12 PM


Quote: I discovered recently that some Nikon cameras have a multiple exposure feature built in

Your Canon can do that too Smile

There was a similar thread on here a few months ago when someone was asking for advice about star trail photography.

One wise EPZ member (apologies for not remembering who you are!) suggested you set your camera to 30 seconds on Tv mode, set the Shot mode to Continuous Shooting, and use a remote cord.

Click the remote cord button to On and sit back and relax while your camera takes 30 second exposures continuously until you run out of battery or memory card space!!

Helpful Post! This post was flagged as helpful
snowjunkie
30 Nov 2009 - 9:54 PM


Quote: Your Canon can do that too


WHAAAAT? Well that made me sit up.

I've read the method you give several times and I'll be out this week to try it. The mental image of sitting in a garden chair wrapped in a duvet sipping hot tea while the camera clicks away is too good not to try! It snowed all day yesterday and it's a full moon so maybe tomorrow night, once I dig my thermals out.

Cheers for that Chris.

Metalhead
Metalhead  61866 forum posts England2 Constructive Critique Points
1 Dec 2009 - 9:30 AM

Just make sure you don't have the Long Exposure mode set in Customs Functions otherwise once the first 30-second shot has taken, the camera will take another 30-second dark frame with the shutter closed...

I think my instructions were correct... Have a play around with the method and make sure a single 30-second exposure is giving you enough of an initial image to work with. Should do.

I'd hate to think you've sat there while your camera snaps away and not get some good results at the end.

I've only tried the method once with a shot totalling about 5 minutes, and ocne I tried to stack the images in CS2 I had little gaps between the stars between exposures. I'm using a 400D though, and maybe it wasn't quite quick enough to write the image to the card before starting the next subsequent shot. I would have thought this would be minimal, so it could well be something I'd done wrong when stacking the shots!

Don't be disheartened by that though - go out and give it a try!

Look forward to seeing your results!

Chris

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