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05-10-2010, 12:13 PM   #1
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shadow noise?

Hello all,

Curious about shadow noise. I think I have been naively operating under the assumption that-- as long as exposure is decent and ISO low-- noise (of all stripes) will be minimal.

Was shooting indoors yesterday-- sunny day in SF, all shots via available light from windows, late morning/early afternoon. Was testing my manual focus skills with the SMC 50 1.7, candid shots of family members. K2000 (have been shooting it about a year now).

Took most shots at ISO 100 or 200. Shutter speeds from 1/20 to 1/200 or so. Aperture set to f2.8 for most. Manual mode (stop down metering from K2000 with some adjustment from me). Mostly concentrating on getting the shots in focus (it can be tricky!).

Under the conditions, I got what I mostly expected; a few stinkers from misfocus, a few shots under or over-exposed, a few shots blurred from subject movement (serves me right for dipping below 1/50, okay). But a fair number of shots I had hopes for according to the viewfinder preview.

Once uploaded to Lightroom, I noticed that I got a number of shots with decent looking histograms that are nonetheless "noisy." Assuming this is shadow noise; in many cases, looks like blotchiness in skin tones where shadows fall. I've seen it before but don't recognize the conditions that produce it. In out of focus areas, looks even a bit like pixellation or a "watercolor" effect (posterizing?).

What can I learn from this experience?

1. This is a dynamic range issue? Some of the shots were backlit to one degree or another, so necessary bandwidth exceeded the camera DR, resulting in loss of shadow detail?
2. This is a function of how I shot? I tend to treat ISO/aperture/shutter speed as interchangeable, but in fact they are not (I could have chosen another combination that would result in the same level of light falling on the sensor-- and the same histogram-- but it would lead to a better result)?
3. This is a function of the lens? I shot without a hood (would think this would be mostly a contrast issue)?
4. This is a sign from the photography gods that I should stick to auto modes (rejected out of hand, sort of like singing the loudest even if you can't carry a tune)!

Any thoughts on how to handle this type of lighting situation in the future?

thanks!

-s

05-10-2010, 12:27 PM   #2
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you should post a shot for us
05-10-2010, 05:33 PM   #3
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I would not expect to see noise at ISO 100 or 200 except in really dramatic situations. My wild guess is your software decided to boost your exposures and "turned up the volume" on your noise. That would make it much more visible.
05-10-2010, 09:18 PM   #4
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QuoteOriginally posted by skoobie Quote
Hello all,
1. This is a dynamic range issue? Some of the shots were backlit to one degree or another, so necessary bandwidth exceeded the camera DR, resulting in loss of shadow detail?
2. This is a function of how I shot? I tend to treat ISO/aperture/shutter speed as interchangeable, but in fact they are not (I could have chosen another combination that would result in the same level of light falling on the sensor-- and the same histogram-- but it would lead to a better result)?
3. This is a function of the lens? I shot without a hood (would think this would be mostly a contrast issue)?
4. This is a sign from the photography gods that I should stick to auto modes (rejected out of hand, sort of like singing the loudest even if you can't carry a tune)!

Any thoughts on how to handle this type of lighting situation in the future?

-s
Noise resides in the dark, especially if you pull up the exposure in lightroom or Photoshop. I've seen some noise even in ISO 100-200 when I pull up the exposure in post production.

1) It's not a dynamic range issue but I do like your bandwidth/DR/shadow-detail theory that you've got going there.

2) Yes, this is definitely a function of how you shot. ISO/Aperture/Shutter is indeed interchangeable. However, depending on the situation one will often take priority over the other two.

3) Not a function of the lens. Stop blaming your equipment. 90% of the time it's user error. Hoods will only prevent flares and loss of contrast. It will not prevent clipping/noise/tsunamis/high crime rates and abortions.

4) No, this is a sign for you to stop blaming your equipment, suck it up and learn how to use manual mode.

The best way to prevent noise it is to over expose your shot by a tad bit, 1/3 to 2/3 of a stop, then pull down the exposure in post production. This is tricky as you will frequently clip highlights, but this is a legitimate method to prevent noise. Be sure to turn on CLIPPING preview and histogram so you can judge the range correctly. It's difficult to check for clipping on your LCD under bright sunlight, so having the flashing red is crucial.

My guess as to why you're seeing noise is because you're under exposing and pulling up the exposure in Lightroom, thus seeing the noise. I'd use ISO 400-800 indoors. If you're using the K-x, ISO1600 is totally usable. If you're most concerned about getting shots in focus, switch all the way up to f5.6 or f8 for the wide depth of field and consider even ISO 3200 to get it done.

Also, try using the spot meter. If the subject is wearing white, meter that, try to get it to be 2-3 stops over exposed, take a few test shots, check your histogram and clipping and adjust as needed. You can also meter off the skin tone, depending on their skin color (+1.66 for caucasians, -2 for dark blacks, etc) Since you're indoor, you're not under the mercy of clouds as much, so once you set up the aperture, ISO and shutter in manual mode, you can keep on taking photos. If you notice shifts in light caused by clouds, you can always adjust to compensate. I'd also suggest using flash so you can tone down the ISO and keep the aperture small.

Once you've done your due diligence during the shoot, you can also use the noise reduction feature of Lightroom 3 BETA, by far the best denoising software I've seen so far. Even beats Noise Ninja.

05-10-2010, 10:23 PM   #5
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Take hold of your histogram and make it work for you...

I also agree with hangu.... get to know your equipment. Good photo's don't come easy, but it is worth learning how to take them imo.

Good luck.

PS. there's always noise in image(even the clean ones). We just don't see it most of the time as it falls . bellow the visible threshold. However... if either expose or shoot outside that threshold, it will become visible in PP etc. So it's important to take control of this early on.

Good WB and Exposure... it will make shooting much easier on your
Good-luck
05-10-2010, 10:25 PM   #6
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If you shoot jpg, use the highest quality. Low quality jpg (high compression) can create blotchiness.
05-11-2010, 07:29 AM   #7
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Perhaps a few more more words to explain things will help you understand better...

There is always noise in all photo's.
Signal noise is generated by all electronic equipment.
Noise that shows in bands is actually something called cross talk and comes from high frequency signals inside the camera influencing each other.
The main source of signal noise is heath. The warmer your sensor is, the more noise it will generate.
Electronic circuits can be designed to have low noise, but they cannot be designed to have no noise.
If you turn up the volume of an audio amplifier without anything attached to the inpuuts, or any other audio device, you can hear noise as well. It is the same thing.

The question with your pictures to answer is: what is the signal to noise ratio in there?
Like with the radio, if there is music, you cannot hear any noise. You can only hear the noise when there is no (or a low) signal.

If in a dark section of a picture, the signal generated by light on a pixel is let's say: 100, and the sensor pixel noise generated is: 1, you have a s/n ratio of 100:1 and you are likely not able to see it.
When in dark sections of the same picture, the light generated signal is: 5, your s/n ratio is 5:1 and you will see the noise in that section.

The general idea of overexposing is that you up all signals, for instance if you would double the light intensity.
I.e. if you would overexpose by 1 stop or +1Ev your light generated signals would double (100 -> 200 and 5 -> 10, but the noise signal remains 1), therefore the s/n ration would also double.
When the exposure is brought back to the right value (-1 Ev) in post processing, you will also half the noise signal, maintaining the better signal to noise ratio.

When you increase the ISO value, you will amplify the sensor signal. Turn up the volume!
Both the signal generated by light taken in by the sensor *as its noise* will be amplified.
So, overexposing using a higher ISO value in general does not help.
You will first amplify the noise signal, after which you will bring it down in PP. That makes no difference in the s/n ratio....

I hope this helps.

- Bert
05-12-2010, 01:59 PM   #8
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Original Poster
Thanks for all the thoughtful responses!

As context, I've been shooting the k2000 for about a year and have generally been an omnivore in reading about and improving my knowledge and skills. I consider myself a novice with respect to others with much deeper experience in photography, though, so this seemed a good place to pose the questions.

I went back and looked at the photos more closely in LR; in most cases, the problematic ones were "underexposed"-- to the left of the histogram. I say that with some hesitation, as there were still blown highlights in some these shots where light from the windows fell directly on individuals, or due to strong backlighting. A tricky environment for exposure.

Dave: My experience with the K2000 is that noise is visible (certainly if pixel peeping) in ISO 200. I shot in RAW, and could see noise even before adjusting exposure in LR. Perhaps what I'm seeing isn't "noise." That's possible. I'll try to post a few pictures (pre adjustment) to the thread tonight.

Hangu: Was joking a little about abandoning manual shooting. With my 50mm, manual shooting and I are stuck with each other! Thanks for the tip on spot metering; I think I had center-emphasized multi-metering, but spot would have worked better in that setting, I'm sure. Also good tips on easing focus by using a higher f stop. Good advice, all of it. Using LR 3 now, think I'm not going to be able to live without it when the beta ends!

One tip for beginner LR users; while I don't usually use the "auto" exposure feature, in this case, it led me to the right solution for some of my problematic photos: shift exposure down (often by half to 2/3 a stop), and shift brightness up (+25-50 on the slider), usually with a drop in contrast. This minimized bright highlights and evened out the exposure across the frame. My sidelit shots were so strongly lit by the sun coming in from the windows that one side of the subject's face would be bright, the other in shadow... I think this processing technique something I'd be hard pressed to reproduce with my shooting choices (i.e. my choice "in-camera" is either blown highlights or a photo that looks "underexposed")?

thanks again!

-s

05-13-2010, 12:04 PM   #9
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QuoteOriginally posted by skoobie Quote
Dave: My experience with the K2000 is that noise is visible (certainly if pixel peeping) in ISO 200. I shot in RAW, and could see noise even before adjusting exposure in LR. Perhaps what I'm seeing isn't "noise." That's possible. I'll try to post a few pictures (pre adjustment) to the thread tonight.
I normally kept my *ist DS welded to ISO 200 to avoid dealing with noise. I only saw noise issues at that level if I really altered exposures in processing. Increasing exposure by a stop in processing is more or less the same as increasing ISO by a stop. I notice a lot of comments about the K-7 having noise at ISO 100 and I don't see that either. Different ideas about what noise is bothersome mean no one is going to be wrong. I also have no experience with that sensor.

QuoteQuote:
One tip for beginner LR users; while I don't usually use the "auto" exposure feature, in this case, it led me to the right solution for some of my problematic photos: shift exposure down (often by half to 2/3 a stop), and shift brightness up (+25-50 on the slider), usually with a drop in contrast. This minimized bright highlights and evened out the exposure across the frame. My sidelit shots were so strongly lit by the sun coming in from the windows that one side of the subject's face would be bright, the other in shadow... I think this processing technique something I'd be hard pressed to reproduce with my shooting choices (i.e. my choice "in-camera" is either blown highlights or a photo that looks "underexposed")?
I have a black and white dog that I practice on. In the sun, the white parts are six stops brighter than the black parts. If the black parts are in the shade, that range is more like eight stops. I have to do something similar in processing, getting the very wide dynamic range of the photo into a more balanced overall image.

I think the only technique you can use when shooting is some fill-flash. You don't need a lot of power, just a bit of extra light in the shadows. Unfortunately, with your lens and camera, you won't have any control over the onboard flash.
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