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Thread started 03 Oct 2011 (Monday) 23:57
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Noise - Underexposure or Skill or my 7D or something else

 
Rrudo74901
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Oct 11, 2011 06:43 |  #61

This thread was very informative, thanks to the OP and all who posted.


1D Mark IV / 7D / 50mm 1.4 / 24-70mm 2.8 L / 24-105mm L / 100-400mm L IS / 300mm 2.8 L IS / 1.4 III TC

  
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kfreels
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Oct 11, 2011 07:22 |  #62

nikmar08 wrote in post #13232405 (external link)
If it is underexposure that is the primary reason for noise, and having room for more exposure does not necessarily mean underexposure, then why the noise? Annooooying facepalm moment for you folks, eh? :p

Perhaps I wasn't clear. The exposure level being low is not the reason for noise. The noise comes when you boost the exposure in post to bring that exposure level back up to where you want it if you didn't shoot at the exposure you wanted in the first place. If you slightly overexpose (without blowing the highlights) you can bring it back down to the level you want it in post without noise issues. In other words, err to the right and not the left. If you get it wrong, going down doesn't create noise where going up does.

Again, I can't emphasize this enough....to get a good feel for what you are doing, go out and do some shots with bracketed exposures and review your results. That's what I love about digital. The learning curve is so incredibly fast compared to decades ago. I recall having to carry a notepad and shoot test shots bracketed while noting the exposure for each one and then waiting for the film and prints to be developed. Assuming the lab didn't make any corrections and they came back in proper order I could then compare the shots with my notes and learn from them. But if I forgot to take one note it was all screwed up and I would learn wrong. Now you can shoot, view, shoot again, and go through a process that would be a year's worth of practice in about two hours! So take advantage of it. :-)


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Canon 7D and a bunch of other stuff

  
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tdodd
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Oct 11, 2011 07:36 |  #63

It's probably worth a mention that when shooting at low ISO a bit of underexposure is not the end of the world, even with the 7D, but when shooting at high ISO, when you are already trying to make up for a shortage of photons, then underexposing the shot is not going to help. For example, as a rough approximation, underexposing a shot at 100 ISO by 2 stops is going to be a bit like shooting with a correct exposure at 400 ISO. That's equivalent to only giving the sensor 25% of the light it could have recorded. Not ideal, but not the end of the world.

But shooting at 1600 ISO and underexposing by 2 stops is going to be more like shooting with a correct exposure at 6400 ISO. Now the sensor is receiving only 1/64 or 1.5% of the light it could (should) have received. It would be no shock for such a shot to be noisy, especially at the pixel level on a 7D.

Here is an example of saving an underexposed image shot from a 7D at 100 ISO. I had exposure set manually for sunshine and as I was about to hit the shutter release a cloud robbed me of the light I was expecting. Even with a 2 stop push in Lightroom the resulting image looks quite tidy to me at 100%. Had I tried that same trick at 1600 ISO it would have been a very different story.

So, let's just not consider exposure accuracy and histograms, but also the logical thought process behind selecting camera settings. Why shoot a landscape (post #39) at 1/1000 and 1600 ISO? You could have taken the same shot at 1/60 and 100 ISO and had far more flexibility with the resulting file, as well as a superior IQ to begin with.


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kfreels
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Oct 11, 2011 08:34 as a reply to  @ tdodd's post |  #64

I know this is off the beaten path, but I was just reviewing my fantastic book, David Busch's Canon EOS 7D Guide to Digital SLR Photography, and ran into something I had forgotten about.

"Leaving Live View on for extended periods increases the temperature of the sensor, potentially causing noise or odd colors in your image. If you want to take a long exposure, turn off live view for several minutes before shooting to allow your sensor to cool. "

After seeing that and looking at your image, I'm really curious....are you using Live View or the viewfinder? If LiveView, is it on for quite some time when you shoot? Environmental variables could also play a part here. I rarely if ever use live view so I haven't run into this but I'm wondering if it might be contributing.


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tdodd
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Oct 11, 2011 11:51 |  #65

Understanding middle grey - http://www.sekonic.com …standing-Middle-Gray.aspx (external link) - might be of some use.




  
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Edwin ­ Herdman
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Oct 11, 2011 12:11 as a reply to  @ tdodd's post |  #66

I shoot with a T1i which is even more endangered by PNHS (poor noise handling syndrome). There have been some excellent tips in here (turning off Live View whenever you aren't using it is a great one - though we're getting into that time of year, for the northern hemisphere at least, where you'll not have to worry about this one quite so much - though it still does take a bit of time for heat to migrate out of the camear body), and I had a couple of my own:

1.) Instead of wasting your memory card space on bracketed exposures, focus on predicting your 7D's metering. This is not too hard to do: Shoot off a test shot, with no exposure compensation, and see if it is metered to your liking. If it is too bright or too dark, you'll know that if the lighting stays similar you'll have to dial in roughly that amount of EC. This is less about cheating noise, however, and more about creating adequate contrast or enhancing contrast in images.

2.) Stay away from non-even-value ISO settings (Daniel Browning has made some excellent observations in this area). Instead, go for numbers in the doubling sequence: 100-200-400-800-1600. ISO 50 is likewise, by all accounts, in modern cameras as a modification to the ISO 100 setting - the camera and its tiny onboard processor isn't doing as adequate a RAW processing job as would be possible with a desktop computer.

3.) Wonder why I left out ISO 3200? It may be better to simply underexpose and pull up ISOs later. The reason? There is a point in which applying gain fails to reduce read noise enough. To find the point where increasing the ISO will start to introduce obvious noise (to be dealt with later) you can consult a chart like Sensorgen.info's (external link), which is recompiled DxOMark data. Interestingly enough, for the 7D there appears to be a bump up in read noise reduction for ISO 1600 compared with ISO 800, which shows that Canon had to do something pretty drastic elsewhere to achieve that clean a result. That probably is reflected in the also-notable drop in DR in the third chart.




  
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Daniel ­ Browning
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Oct 13, 2011 17:10 |  #67

I just wanted to add one thing to this thread:

nikmar08 wrote in post #13202154 (external link)
the noise levels are not predictable.

There are many factors that go into visibility of noise, including (but not limited to):

  • Exposure
  • ISO setting (which is separate from exposure)
  • Actual ISO (which is different from both the exposure and the setting)
  • Color balance of the light
  • Image content
  • Raw conversion
  • Post processing
  • Display size, resolution, distance, etc.
  • "And more!"


When you find that the noise level between any two shots is different, it's probably due to one of those variables. Many posts in this thread have demonstrated what a big difference conversion and post processing can make.

nikmar08 wrote in post #13202154 (external link)
Sometimes, it stuns me producing fairly controlled levels of noise even at ISO 3200 and yet most other times, there is considerable amounts of noise.

One of the most common ones is color balance. Under some lighting conditions and white balance settings, the blue channel is effectively ISO 25600 while the green is 3200. So it's no wonder that it looks noisier than a shot where all three channels are only 3200 (such as in more balanced light).


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tonylong
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Oct 13, 2011 19:15 |  #68

Daniel Browning wrote in post #13247207 (external link)
When you find that the noise level between any two shots is different, it's probably due to one of those variables. Many posts in this thread have demonstrated what a big difference conversion and post processing can make.

One of the most common ones is color balance. Under some lighting conditions and white balance settings, the blue channel is effectively ISO 25600 while the green is 3200. So it's no wonder that it looks noisier than a shot where all three channels are only 3200 (such as in more balanced light).

Daniel, interesting stuff! I've seen statements about the White Balance having a possibly detrimental affect -- I wonder, what scenarios do you think that this could cause a real problem? Overly "warm" lighting where you are shooting at a high ISO and using the White Balance to "cool" things, like say a sundown setting or an indoor scene with low "warm" light?


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Daniel ­ Browning
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Oct 13, 2011 19:42 |  #69

tonylong wrote in post #13247682 (external link)
-- I wonder, what scenarios do you think that this could cause a real problem?

Well, everyone has their own personal standard of how much difference is required before something becomes a real problem, so I'll just put things in terms of stops.

In my experience, almost every photograph has at least one stop difference between the color channels (usually red or blue is less than green). Which means that channel will have the same noise level as if you shot at one stop less exposure and then pushed it one stop in post. But since it is so common, people already expect such noise levels. When all the color channels are in perfect unison, you get about one stop *less* noise than people generally expect, such as when you post an ISO 6400 image that looks like the noise of ISO 3200. That's also why magenta filters can be used to reduce noise: in typical lighting (e.g. 5600K daylight) they knock green down to the same level as blue and red, increasing dynamic range and reducing noise.

In other circumstances, the difference could be 2 or 3 stops, such as under Tungsten illumination. If you white balance back to neutral, the blue channel might be boosted 3 stops. So if you're setting is ISO 640, blue will actually be at ISO 5000. If you're in a studio and the light is ample, one solution is an 80A filter, which knocks down red and green to the level of blue.

tonylong wrote in post #13247682 (external link)
Overly "warm" lighting where you are shooting at a high ISO and using the White Balance to "cool" things, like say a sundown setting or an indoor scene with low "warm" light?

Yes, those are two good examples. The more balanced the light is, or the less you change it (with white balance), the less the noise will be.


Daniel

  
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kfreels
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Oct 13, 2011 20:07 as a reply to  @ Daniel Browning's post |  #70

Hey. We just had a really nice rainbow and I went out and shot it and ran into similar noise. I'm messing around with it right now and it seems to be as you say in the color channels. A simple adjustment of the white balance to about 4500k seems to have rid a good deal of the noise and that's just in the few minutes I messed with it. I'm going to try to get some time tonight and really dig in and identify where this is coming from. I'll post what I come up with if anything. Maybe it will be worthwhile. Maybe it will just be the wold rantings of a lunatic. We shall see!


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Snydremark
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Oct 13, 2011 20:27 |  #71

Maybe it will be both...:D


- Eric S.: My Birds/Wildlife (external link) (7D MkII/5D IV, Canon 10-22 f/3.5-4.5, Canon 24-105L f/4 IS, Canon 70-200L f/2.8 IS MkII, Canon 100-400L f/4.5-5.6 IS I/II)
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kfreels
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Oct 13, 2011 20:51 as a reply to  @ kfreels's post |  #72

OK. I messed with this rainbow shot some but I have to go get my daughter in a few minutes so I won't have more time to mess with this tonight. From what I can tell initially is that it appears to be a combination of white balance being off - maybe from the rainbow and various other things going on in the atmosphere, and luminance noise.

First I had changed the white balance to 4200 from 5200 which is what the auto setting had it at. When I made that change, a good chunk of noise fell away. I didn't think to save a pic of that one as well and I don't have time to go back now and do it.

There was still quite a bit of noise though. Then I tried the noise reduction in DPP. Chrominance NR didn't do a thing, but luminance got rid of the remaining noise almost completely.

I did some side by side screen caps that looked rather nice and you could see the difference but in my haste to get out the door I saved them in jpeg and the compression artifacts made the comparisons worthless. I never noticed until after I had uploaded them and posted them here then previewed the post! They were really ugly so I deleted them.

And now I'm late getting out the door but I'll leave you all with the RAW file if you want to experiment with it. Can't wait till marching band season is over! Here's a link.

http://dl.dropbox.com/​u/44688334/_MG_2705.CR​2 (external link)

Gotta run!


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nikmar08
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Oct 13, 2011 22:17 |  #73

Thanks for more people chiming in and adding to my confusion :p I'm still ruminating on the thoughts expressed earlier, and boy, now I have to deal with fitting in more aspects of noise into the equation!! Never hurts to know more, eh?

kfreels - while I do see some noise in your shot but not too much. I have some observations / questions though:
1. I see you shot it at 17mm f/3.2 with a SS of 1/640 and at ISO 200 and there's almost 2 stops of headroom on the right of the histogram. Was that deliberate because you wanted to specifically test noise in the context of color channel and white balance aspects that Daniel introduced in this thread? Or was it because you took it in the same hurried way as I took my shot - it's just that my settings were worse, 1/1000s, f/9 and such and it is natural to goof up? :p

2. Would you agree that even your shot looks a bit soft like mine? Even if not, what did you lock focus on and what portion of the frame did you meter off of?


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nikmar08
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Oct 13, 2011 22:21 |  #74

Daniel Browning wrote in post #13247207 (external link)
There are many factors that go into visibility of noise, including (but not limited to):
  • ISO setting (which is separate from exposure)
  • Actual ISO (which is different from both the exposure and the setting)

Can Daniel and/or somebody else elaborate what is the distinction between those two?


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Daniel ­ Browning
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Oct 13, 2011 22:31 |  #75

nikmar08 wrote in post #13248532 (external link)
Can Daniel and/or somebody else elaborate what is the distinction between those two?

The ISO setting on the camera should really be labelled "gain". Changing that setting modifies how much of the various types of gain are applied to the raw file, including analog gain, digital gain, and metadata gain. For a longer explanation, see the thread I started here:

https://photography-on-the.net …/showthread.php​?t=1081982

The other type of ISO is more like an "Exposure Index". You take the amount of light coming out of the display (such as middle gray in a JPEG) and compare that to the amount of light that went into the camera. The exact conversion depends on what particular option and interpretation you use from the ISO standard.

For example, the ISO setting could be "200", but using one interpretation of the ISO standard (e.g. the one common to digicams), the resulting JPEG may be called ISO 100. Using another interpretation (e.g. the one common in cinema), it may be called ISO 400. Furthermore, if you apply a one stop push to the raw file, the ISO of the resulting JPEG is now double.


Daniel

  
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Noise - Underexposure or Skill or my 7D or something else
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