This seems to be a well known issue amongst some sectors of the digital imaging world.
For stills work at hand held shutter speeds there was no significant noise issues to worry about until working with iso speeds in excess of 1000iso, bit it wasn't worth worrying about any particular intermediate settings.
Once you got to half second exposures 50,125,200,250 &320 all gave identical amounts of noise, it increased with 100 & 400, a little more with 160 & 800, more again with 640 & 1000 and above 1000 it gave incrementally more for each increase in iso setting.
It wasn't so much worrying as wanting to better understand how at camera, processing algorithms actually played into any kind of predictable progression...Having determined that, for my camera ,the apparent, by eye and RAW file MB data, is ISO 160, I now have the reason that the noise is less @ISO320 than 250 or 200, and that the noise is less @640 (significantly) than at ISO 500 and 400.
I had not understood that from the native ISO (160 for mine) reaching ISO 200 or 250 the manipulation went to ISO 320 first and then again more manipulation down to those ISOs. For me , I just like to know a little of the reasons why and have a starting point with bits such as these...
Sorry , I won't inflict my interest on such minutiae again. Just my nature to get excited and want to share what I found thought provoking...
The intermediate ISOs as well as ISO50 & ISOs3200+ are only useful for JPG shooters, in RAW you only lose dynamic range and get nothing in return.
Luis, Erik... Thank you for the input...
Erik, the ClarkVision article is amazing. As he says, generally better served to be out there shooting and refining...but the wealth of information from (his astrophotography) the various control positions in this paper are fantastic... that anyone would spend the time...I should stomp on my curiosity, but just can't help myself. I am indebted.
Photon noise (shot noise) is said to be much more naturally looking than readout noise.
Hi,
Can you kindly explain how?
Sincerely,
Joofa
Hi,
I wrote "is said", so it is not first hand observation by me, sorry!
Best regards
Erik
Dear Erik,
I guess then you should not believe everything that is on the net ;D. (Not picking on you. Was just trying to figure out what is the source of this claim.)
Thanks,
Joofa
Just an explanation. There has been quite a lot of discussion about Nikon D7000 and Pentax K5 having better DR than older cameras. To my knowledge the only way to increase DR are to increase FCW (Full Well Capacity) or reduce readout noise. Reducing readout noise would not affect shot noise. It also seemed to be assumed that FCW is in line with pixel pitch on the new sensors.
Best regards
Erik
Hi,
On the other hand, do you have any proof of the contrary?
It seems from photographers experience that Pentax K5 and D7000 has impressive shadow detail and it has been suggested that this may be because of more natural looks of shot noise compared readout noise, the latter haven more salt and pepper character.
(http://djjoofa.com/data/images/ns_shot_noise_hist.gif)
Seems like the wording from a certain guy at a certain website ;D.
Sincerely,
Joofa
Honestly, while I admire you guys with the talent for scientific analysis, I do not have neither the patience nor sufficient knowledge. So I did this extremely crude experiment, and I admit I did not give it a lot of thought, so please tell me what I did wrong.
I put a cap on the lens, used a manual 1/125s and f/5.6 and shot like that at every ISO. Then, without even loading it into the computer, I checked the file size on the camera's (Canon 40D) LCD and plotted them on a graph. My assumption is that the variations in file sizes have to do with noise only. And, not surprisingly, I got the same results as everybody else: !60 has the least noise, and the best ISOs are 160, 320, 640 and 1250.
Is it safe to conclude that for the best performance (noise and everything else) under sufficient light I should use ISO 160 (and not 100), and then if I need more speed to jump to 320-640-1250?
Hi,
Can you kindly explain how?
Sincerely,
Joofa
Honestly, while I admire you guys with the talent for scientific analysis, I do not have neither the patience nor sufficient knowledge. So I did this extremely crude experiment, and I admit I did not give it a lot of thought, so please tell me what I did wrong.
I put a cap on the lens, used a manual 1/125s and f/5.6 and shot like that at every ISO. Then, without even loading it into the computer, I checked the file size on the camera's (Canon 40D) LCD and plotted them on a graph. My assumption is that the variations in file sizes have to do with noise only. And, not surprisingly, I got the same results as everybody else: !60 has the least noise, and the best ISOs are 160, 320, 640 and 1250.
Is it safe to conclude that for the best performance (noise and everything else) under sufficient light I should use ISO 160 (and not 100), and then if I need more speed to jump to 320-640-1250?
I know which one I would prefer having to deal with in an image.
Hi Emil,
We don't do flat patches, and would welcome your graduation out of "baby signal processing", into the world of real images ;D.
But, more seriously, I have interpretted Erik's remarks (ref to "salt and pepper") to mean John Sheehy's usual statements on Dpreview regarding the "texture" of shot noise having anything to do with the signal. So, the basic premise was in showing that Shot noise has no correlation to signal. It has nothing to do with your personal ease to deal with it.
Ref.: http://forums.dpreview.com/forums/read.asp?forum=1032&message=37062241
Sincerely,
Joofa
I can do without the gratuitous insults, BTW.
But, more seriously, I have interpretted Erik's remarks (ref to "salt and pepper") to mean John Sheehy's usual statements on Dpreview regarding the "texture" of shot noise having anything to do with the signal. So, the basic premise was in showing that Shot noise has no correlation to signal.
Ref.: http://forums.dpreview.com/forums/read.asp?forum=1032&message=37062241
I took the time today to test the noise from L thru H2 with the 5DM2 and was stunned to see the surprises that were the result. ie less noise @ iso 640 than iso 125. ; less @iso160 than 100. Does anyonyone know if this is across the board , or camera sensor to camera sensor.
I used the manual, cap on method for test shots, opened raw files @ 100% and did auto process to compare blind...the results matched the file size in metadata... I have not figured out how to do the uncapped test, but so far based on this it appears I need to rethink my iso settings...
I know which one I would prefer having to deal with in an image.
Hi Emil,
Indeed. However, the "read noise" example consists of more than just read noise, specifically pattern noise and PRNU. The result of subtracting two black frames (divided by Sqrt(2), and adding an offset) will show a better approximation of real read noise. AFAIK, its distribution looks a lot like Poisson/Gaussian noise. but you are correct that in every day use, lots of photons will produce a nicer noise pattern than some sensor arrays do with lower signal levels (unless we do postprocessing of multiple frames at the Raw level before demosaicing).
I agree with your suggestion that ISO 100 can produce a smoother image than ISO 160, even if "read noise" seems lower at ISO 160, purely due to the more random nature and higher S/N of shot noise. You also are right in mentioning that the Photon statistics are altered a bit by the quantization differences at different ISOs (up to unity gain).
Cheers,
Bart
Hi Bart,
What I meant by 'read noise' is all the non-signal added by the camera electronics. In many cameras it has a substantial patterned component which is hard to deal with using standard NR filters since it looks like a bunch of edges, which the filters are designed to try to preserve. And the totality of all this non-signal variation is also rather impulsive too, and tends to be preserved as well by the filters.
I have never found 'unity gain' to be a useful concept; current sensors have too much analog noise relative to the quantization step.
I suspect what John is talking about is that shot noise variance is correlated to signal, as your example shows. You don't need multiple samples to see that; for instance,
I hope that you realize that this is not the same as saying that noise is correlated with the signal. Multiple images were only for people to see what is going on here.
The only real ISOs in the 5d2 are 100,200,400,800 & 1600. The others are just software.
Hi,
Please see the graphic below where the image on the top left is treated as representing the "true" signal. On the top right is a sample shot noise simulation derived from the statistics of this image. Seems pretty random to me and devoid of any "natural texture" as claimed.
Hi Joofa,
I'm not entirely sure what you did, but if you can reconstruct the original image from the noise, then the noise isn't random.
If your parents gave you a name, what's wrong with sharing it with us?
The bottom two images are precisely the visualization of the well-known relation that shot noise standard deviation is sqrt (mean signal). So, noise and signal are related, in some sense, but that does not make shot noise non-random. Because, the noise values are themselves random, but the standard deviationof these values is not random.
If you are in doubt then Google is your buddy ;D
Hi DJ (?),
Your example in post no. 15 (top right quadrant, labeled "sample shot noise") is a misleading presentation of shot-noise. What you are actually displaying is a large number of pixels/samples, each and every one from Poisson distributed random noise. Each pixel is the product of a different level of signal, hence it has different mean levels with an accompanying probability distribution at each spatial sampling position. You are not showing "sample shot noise", but you are showing multiple samples (one sample at each pixel position). A better label would have been "many shot noise samples", but it would still not show the nature of Poisson (shot) noise, which was the topic.
You also do not mention the fact that the human visual system will notice noise more when the signal levels are spatially more uniform. In parts of the image where the signal spatially fluctuates rapidly, IOW lots of detail, the character of the noise is much harder to appreciate and often less of an issue.
Hi Bart,
I respectfully beg to disagree. The issue is that a real image is an example of non-stationary Poisson noise process - i.e., in theory, the noise statistics are different at each pixel location, which you yourself noted, hence, the top right image does indeed show a single sample sequence of such non-stationary noise, and as I mentioned in the post you referenced, derived using the top left image as the "true" signal.
The way I see is that how to develop a simple model of noise vis-a-vis signal even in such signal fluctuation cases, which is btw what happens in a real image, so that we can move forward to questions such as snr at image level, the effect of resampling on that snr, best possible window sizes in spatial averaging, frequency analysis of non-stationary noise, and all sort of other interesting questions.
Sincerely,
Joofa
... With the lens cap on, you are looking at the std dev of read noise in raw levels...
To continue Erik's point (about real-world relevance), here is my next question ... so it is a read noise, but even in that case, why is there less noise at 160 than at 100 (and so on) and what (if any) real-world relevance does it have?
... (unless your interest is photographing the inside of your lens cap as an end in itself).
... why can't photographers exhibit pure black canvas? ;D