What is this problem?

CaveDweller

CaveDweller

Suspended / Banned
Messages
1,946
Name
Paul
Edit My Images
Yes
The pictures down below are number 1 out of 150 shots for some star trails and there's some strange blue and red dots everywhere. I'd guess they were got pixels if I saw them towards the end of the 150 photos, but these coloured spots are visible on the first shot I took of the night and in exactly the same place for the rest of the 150o_O

I've circled all the coloured spots in the first image and the the other two is when I zoom in on them. I've uploaded the original size but I don't know if you can see it that well. You can see it better when you click on the pictures. Any help would be appreciated, cheers.

All shot with Canon 550D with kit 18-55mm lens.

IMG_1444 by Peanut651, on Flickr

Untitled-1 by Peanut651, on Flickr

Untitledfgdfd-1 by Peanut651, on Flickr
 
Last edited:
Oh lol stupid me:rolleyes:. I wasn't really concerned about noise because I wanted it high to pick up the fainter stars and at the end I was stacking them all together which reduces noise.
 
These are hot pixels.. which is not quite the same as noise. Noise is a result of high ISO and puling detail from the toe end of the sensors response curve and will appear uniformly over the entire image.


Stacking will only reduce hot pixels if you have a dark reference file usually.

If you're not doing star trails, then using the camera's long exposure NR will remove these. You take the first shot, then the camera makes a second exposure with the shutter closed of the same length. As the two frames it generates will both have the same hot pixels, it removes anything that appears on both frames.. so in effect, removes the hot pixels only. The drawback is that it takes twice as long. A 30 second exposure will take 60 seconds, plus processing time... so around 1min 20 secs to complete. So if you're doing star trails, you can't do this, as the resulting gaps in the trails will be shown on your final stack.

To take a reference dark file, just take an exposure of the same length with the lens cap on. It is important that all settings are the same, and even the temperature is the same. Most stacking programs will have the option to load a dark frame while stacking.


daugirdas said:
camera noise? Nothing unusual. Make sure you have high ISO NR on and do another pass in LR if you are that concerned.
Click to expand...


These are hot pixels, and high ISO NR will do nothing to remove them. High ISO NR doesn't do anything if you shoot RAW anyway.

What you need if long exposure NR... not high ISO NR.
 
Last edited:
Pookeyhead said:
These are hot pixels.. which is not quite the same as noise. Noise is a result of high ISO and puling detail from the toe end of the sensors response curve and will appear uniformly over the entire image.


Stacking will only reduce hot pixels if you have a dark reference file usually.

If you're not doing star trails, then using the camera's long exposure NR will remove these. You take the first shot, then the camera makes a second exposure with the shutter closed of the same length. As the two frames it generates will both have the same hot pixels, it removes anything that appears on both frames.. so in effect, removes the hot pixels only. The drawback is that it takes twice as long. A 30 second exposure will take 60 seconds, plus processing time... so around 1min 20 secs to complete. So if you're doing star trails, you can't do this, as the resulting gaps in the trails will be shown on your final stack.

To take a reference dark file, just take an exposure of the same length with the lens cap on. It is important that all settings are the same, and even the temperature is the same. Most stacking programs will have the option to load a dark frame while stacking.





These are hot pixels, and high ISO NR will do nothing to remove them. High ISO NR doesn't do anything if you shoot RAW anyway.

What you need if long exposure NR... not high ISO NR.
Click to expand...

Cheers. Because these were visible from the first shot does that mean they are permanent and I could have gotten them from a previous long exposure?

I'll have to have a look through my pictures and aee if I can see any more. I don't mind editing them out. I only do photography as a hobby so it's no as if the tiny bit of extra time processing or doing in camer NR will get in the way of anything.
 
Your camera will exhibit exactly the same hot pixel pattern if you take another shot with the same shutter speed at the same temperature and ISO, yes. The shorter the shutter speed, the less you will get. You will only get it on long shutter speeds. You'll only notice it on dark images most likely, but essentially, your camera's sensor will produce the same hot pixels if you shoot at the same speed and ISO as these images. Long exposure NR will remove it in camera, or you can just retouch out. It's easy to retouch out though.
 
Paul, yes they are permanent but are usually a "characteristic" of any sensor from new so it's unlikely that anything you've done has caused them. They're not so much faults as imperfections and can be disposed of in PP with a little time - having a reference photo so you know where to look for them can help.
 
Ok thanks both. Looking back on some more long exposure shots I can notice them in the same places. Not really noticed them before because I normally try star trails when there's a bit of moon light to light up the scene, but that night with the pictures above there was no ambient light at all.
 
I thought a hot pixel was a single pixel. You can see there are more than one pixel involved in each location. Does that mean some processing has already taken place? Is there just a single affected pixel at each location in the raw file?
 
ianp5a said:
I thought a hot pixel was a single pixel. You can see there are more than one pixel involved in each location. Does that mean some processing has already taken place? Is there just a single affected pixel at each location in the raw file?
Click to expand...
That's what I was unsure about. It is a jpeg format though because I couldn't be bothered converting the raw to jpeg to put it on flickr lol. I'll have a look at the raw file and see if it's the same. I shoot raw+jpeg at the same time.
 
Last edited:
As has been pointed out its nothing you've done it's just a few defective pixels, just a case of cleaning them up in PP as far as I'm concerned, you can do a dark frame and use that for mapping the defective pixels

daugirdas said:
camera noise? Nothing unusual. Make sure you have high ISO NR on and do another pass in LR if you are that concerned.
Click to expand...

You cannot have LENR (long exposire noise reduction) on if your doing star trails as you'll end up with hundreds of gaps in your trails from the dark exposures after each normal exposure...and personally unless your going to do no PP on your images I see no reason to ever have LNER on
 
ianp5a said:
I thought a hot pixel was a single pixel. You can see there are more than one pixel involved in each location. Does that mean some processing has already taken place? Is there just a single affected pixel at each location in the raw file?
Click to expand...



It will be photoshop's anti-aliasing in the preview most likely, or JPEG artefacts.
 
Last edited:
Thanks.

On closer inspection I've discovered I have a similar problem when photographing the night sky.

hot pixels.JPG

Maybe I should switch on anti-aliening?
 
Last edited:
Whatever you do, dont use the cameras noise reduction function for shooting star trails. As MWHCVT has said, you'll end up with a dotted line instread of a solid line for your star trails. Any time longer than a couple of seconds is enough to give you gaps in your trail.

Best bet is to use LR/PS's noise reduction ability to reduce any overall noise, and clone out those hot pixels manually.
 
Carl Hall said:
Whatever you do, dont use the cameras noise reduction function for shooting star trails. As MWHCVT has said, you'll end up with a dotted line instread of a solid line for your star trails. Any time longer than a couple of seconds is enough to give you gaps in your trail.

Best bet is to use LR/PS's noise reduction ability to reduce any overall noise, and clone out those hot pixels manually.
Click to expand...
I know very well how many seconds can create gaps in star trails with my many failed attempts lol. I'm not that bothered about noise when doing trails because when you stack all the images together it reduces the noise. Anything helps though because the 550D isn't the greatest when it comes to high ISO.
 
Your camera should be able to map out hot/stuck pixels permanently. You use the 'sensor cleaning' functionality which lifts the mirror to gain access to the sensor. Remove lens. Attach camera body cap firmly. Cover viewfinder. (Or do this in a dark room: the idea is that NO light enters the camera.) Put camera into manual sensor cleaning mode for 30 seconds. Switch off. attach lens. Shoot.

I have seen suggestions that this procedure will only map out pixels which are really bad, ie which get stuck/hot on relatively short exposures. If you want to also map out pixels which are prone to getting stuck/hot on longer exposures, it is suggested that you should run the camera in Live View mode for a couple of minutes before performing the sensor cleaning operation. The idea is that this heats up the sensor and triggers more pixels to stick. I should stress that this is only something I've read and I can't vouch for its effectiveness..
 
I think the canon in camera solution only works when you use the RAWs with Canon DPP as it just writes corrective information into metedata instead of actually removing. I think they'll still be there in ACR or Lightroom. I'm prepared to be corrected here BTW as I don't shoot Canon, but I remember reading this in the Magic Lantern forums.
 
David, do you know if there's a similar trick/solution for Nikons? D70 and D700 in particular.
 
Nod, i havent done this on a nikon, but the term you need to google is 'pixel mapping'. if any software does it it will be capture nx.
 
Nod said:
David, do you know if there's a similar trick/solution for Nikons? D70 and D700 in particular.
Click to expand...


The same trick doesn't seem to work for Nikon, no. There's loads of stuff on the net that says it does, but I've never managed to get it to work.


Adobe Camera RAW will automatically remove hot pixels apparently... as does Lightroom... if you shoot RAW. can't say I've ever noticed it doing anything myself though.
 
Last edited:
Cheers for the answers. I shoot JPEG which makes it a bit harder but then again, I also tend to use the D700 for longer and high ISO stuff so the noise problem is less than it used to be on the D200!
 
You must log in or register to reply here.
Back
Top