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Strange noise artifacts (Canon 350D)

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Old Tuesday 30th January 2007, 22:59   #1
gatafrancesca
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Strange noise artifacts (Canon 350D)

Hi everyone,
in the last few days I noticed strange noise artifacts appearing on my photos and I am quite concerned as it had never happened before. Yes there was some noise on pictures taken at ISO 800, but I had no trouble fixing it with Noise Ninja. But on my latest photos the noise appears like strange "bands", I can't describe it better, I will try to attach a photo to show you the nature of the problem. This kind of noise appears even in photos taken at ISO 400, like the one I am posting and it's not possible to remove it completely, it gets even worse at ISO 800. Is something wrong with my camera?? Please help!
The photo I am posting has received no post processing apart from some cropping.
Cheers
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Old Wednesday 31st January 2007, 00:02   #2
Chris Oates
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Quote:
Originally Posted by gatafrancesca
Hi everyone,
in the last few days I noticed strange noise artifacts appearing on my photos and I am quite concerned as it had never happened before. Yes there was some noise on pictures taken at ISO 800, but I had no trouble fixing it with Noise Ninja. But on my latest photos the noise appears like strange "bands", I can't describe it better, I will try to attach a photo to show you the nature of the problem. This kind of noise appears even in photos taken at ISO 400, like the one I am posting and it's not possible to remove it completely, it gets even worse at ISO 800. Is something wrong with my camera?? Please help!
The photo I am posting has received no post processing apart from some cropping.
Cheers
Nasty isn't it - had a look around and it seems a fairly common complaint - and not only with the 350d.
I've seen one person receive a replacement camera only to have the same problem - Canon suggested it was the lens ?????????????
Been a while since I was close to people with 350d's but when I was it was their superior high ISO noise performance thst made people buy them.
Under exposure has been said to aggravate banding.
I use Nikon and always use a high power speed light to try to get as much light (even a little) on the subject a possible to avoid high ISO and avoid high contrast.
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Old Wednesday 31st January 2007, 00:22   #3
Keith Reeder
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Hi Fransesca,

as Chris mentions, if you had not already said you haven't post processed this, I'd have said this was definitely a case of an under exposed picture being pulled back up to proper brightness in post processing.

I notice that the camera was set to -1 EV, which can result in under exposure.

Try setting the EV to 0 and take a few more pictures: it might be that you've set the camera to -1 without realising it.

That said... it is worrying that the banding only appears in one part of the picture: is it in the same place in all pictures where it appears? If it is, it might be a camera fault...

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Old Wednesday 31st January 2007, 00:33   #4
Chris Oates
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Originally Posted by Keith Reeder
Hi Fransesca,

That said... it is worrying that the banding only appears in one part of the picture: is it in the same place in all pictures where it appears? If it is, it might be a camera fault...

Good call on the -1EV - didn't notice that.
See the repost of the pic - there's actually banding everywhere most noticeable at areas of contrast.
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Old Wednesday 31st January 2007, 01:16   #5
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Hi again and thanks for your replies.
Well, actually I underexposed that photo on purpose and I usually tend to underexpose a little bit for two reasons:
1) I can get a faster shutter speed.
2) Sunshine is very strong here in the tropics and if I don't do it many of my images look overexposed. I could use iso 200, but I seldom do that as I always try to get a faster shutter speed, especially when photographing hummers.
I must add that my experience is limited but this problem never happened before, it mainly appears with dark backgrounds.
Now, let me post a picture that will make you laugh , this one comes straight out of the camera, just converted from raw, no post processing and no cropping at all. It was one of a series took in burst mode while a squirrel was running up a tree. Not a very artistic one, but very good to show you another example of this awful banding effect. This one was taken at iso 800, I was taking some flying hummingbirds pictures in a darker part of my garden, so I decided to use a higher iso. When this squirrel suddenly arrived (I feed them) I just had the time to turn my camera on that direction and take a few quick pictures. I forgot about the high iso setting, iso 400 would have been ok in the sunlight, but it was just a matter of seconds before the squirrel was gone. Underexposed by -0.67EV. It looks to me that the banding appears on the whole background, not just in a part of it.
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Old Wednesday 31st January 2007, 14:27   #6
mjmw
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Quote:
Originally Posted by gatafrancesca
Hi again and thanks for your replies.
Well, actually I underexposed that photo on purpose and I usually tend to underexpose a little bit for two reasons:
1) I can get a faster shutter speed.
2) Sunshine is very strong here in the tropics and if I don't do it many of my images look overexposed. I could use iso 200, but I seldom do that as I always try to get a faster shutter speed, especially when photographing hummers.
Hi Francesca, this looks to me like the noise in the dark background is being posterized. This is caused by 'stretching' the tonal data in the shadows, which is easy to do because the least amount of tonal data is recorded there. Shot at ISO 800 and underexposure could lead to this kind of noise if exposure is corrected in post processing. If you are not adding exposure in RAW conversion then the camera may be doing it. Most, if not all, DSLRs will apply some level of shadow/highlight adjustment to the captured image data. This is mainly to increase the available tonal data in the darker areas of an image, but has the negative effect of increasing noise in dark, underexposed areas. I am not sure what control you have of this, if any, but will endeavour to find out.

Assuming that this is the cause, the quickest fix is to create a 16 bit file (for example TIFF) from the RAW, select the dark areas and apply aggresive noise reduction. Other techniques can also yeald very good results.

How to avoid it? Nail your exposure Ensure that you have the correct exposure for the subject and avoid increasing exposure in post processing (PP). Aim to expose to the right of the histogram without blowing highlights and then lower exposure in RAW conversion. Remember that dialing in negative exposure compensation is not always under-exposing - you may be correcting for a complex scene to expose the subject correctly (for example a white Swan against a dark background often requires negative exposure compensation).

Extreme tonal ranges like this squirrel shot will probably still have noise in the darks and you can only eliminate this with selective noise reduction in PP or take two exposures - one for the squirrel and one for the bg, blending them afterwards.

Hope this helps.
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