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Old Sep 27, 2010, 10:40 AM   #1
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Default Noise vs. ISO

I have just done a comparison of noise effects due to high ISO's on two identical images. The first is done at ISO 100 and the second at ISO 1600. The most pronounced difference is that of brightness which would indicate that higher ISO's would call for a half or full stop down. These images are both 100% crops. Camera used was a Sony A200 with flash at f13. Lens employed was a Minolta 28-105mm si zoom. NO POST PROCESSING WAS DONE OTHER THAN CROPPING.
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Old Sep 27, 2010, 11:30 AM   #2
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I think these photos miss the point. Since you used flash, you had adequate light. The "gotcha" for high ISO is that, while it is perfectly acceptable in adequate light, it is obviously noisy in low light -- precisely where people want to depend on high ISO. It is the performance in the shadows that make it less than ideal, and that is also why people take night shots at low ISOs. Personally, I tend to take photos at higher ISOs than most folks think is appropriate. You photos may help dispell the notion that you should never use a setting above ISO 200. But you need to be careful not to mislead people on what they can achieve with high ISOs, too.
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Old Sep 27, 2010, 11:39 AM   #3
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I think even with the flash the IQ is considerably worse in the second shot. What I find more interesting is that the camera appears to be having trouble compensating for the flash with the shorter exposure time of the high iso shot.
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Old Sep 27, 2010, 12:12 PM   #4
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Martin, your point is well-taken, but that isn't a fault of the ISO. The camera just doesn't properly expose images at the higher ISO. I've noticed this (and a limitation on shutter speeds) as one of the changes in more modern cameras that is a step back from the norm on older cameras. It seems that camera manufacturers have decided that anyone who isn't a pro doesn't know how to use flash anyway, so only work properly if they chose ISO 100 and 1/100 second for the shutter speed. It's shocking how crappy the D5000 is wrt shutter synchronization, for example. You're out-of-spec if you set the shutter to faster than 1/250 sec! And it is far from the worst offender in this department.
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Old Sep 27, 2010, 12:55 PM   #5
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Noise exists in all digital photographs, whatever the ISO setting might have been when the photo was taken. Throughout a digital photograph there are errant pixels that record light intensities that are just a little bit too high. The difference between the value of any errant pixel and all the adjacent pixels, at the base ISO, isn't enough that you could notice it except under the most intense scrutiny.

When you double the ISO, you double the measured intensities of each pixel, and as you continue to double the ISO, doubling the measured intensities, the errant pixels become more and more obvious. Notice that this has nothing to do with the other exposure settings or the brightness of the scene.

So Noise isn't just a result of shooting in low-light conditions. It's a result of higher ISO settings, and is independant of other settings or circumstances. Noise is just easier to see in lower light situations, but it will occur anytime the ISO setting is higher than its base setting. (In fact, even then, though it is barely visible.)

Also, the slight difference in the brightness of the images is a natural variation. Any time you take multiple photos of the same scene, you are likely to have slight variations in the exposure settings (in this case, the duration of the flash.) Remember that all exposure settings are adjustable in 1/3 EV increments. One may have been just barely dim enough to use one set of exposure values, but the second shot may be just slightly brighter (a light in another room may have been turned on during the intervening seconds, Streets may be standing in a very slightly different position such that he's blocking slightly less light from the lamp behind him, etc.), so the exposure is just 1/3 EV higher. The EXIF data shows a Brightness Value of -0.63 for the first, and -0.50 for the second. That's enough for the duration of the flash to be slightly different, in 1/3 EV increments, and thus produce a slightly brighter image. It has nothing to do with the change in the ISO setting.

What I noticed, when looking at the images at 400%, was that the second image had a lot more noise, which is what you'd expect, given that the difference between the errant pixels and the normal pixels had been doubled 4 times.
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Old Sep 27, 2010, 1:03 PM   #6
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Of course, at that level of magnification, chromatic aberration and oversharpening artifacts also become visible.
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Old Sep 27, 2010, 5:29 PM   #7
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The ISO numbers may not reflect the actual sensitivity of the camera at that setting. I have seen reviews which test this, and most cameras seem to be more or less sensitive than the displayed ISO setting. The exposure system doesn't know this, though, and exposes based on the numbers. Yup, even with the pre-flash. So, it pays to know your camera.


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Old Sep 27, 2010, 5:49 PM   #8
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Saying that noise is present at all levels, but is more visible in the dark regions may in some sense be true, but is wildly misleading. Noise is independent of signal. What this means is that, if you increase the variation due to noise to, say, +/- 3 counts, it will be huge if the signal is on the order of magnitude of 3 counts, and will be minimal if the signal is on the order of magnitude of 1000 counts. If you choose to report the noise level in counts, in a misleading sense you can say that it is constant. But, if you state the more informative equivalent statement that it is +/- 100% of signal at the first level and +/- 0.3% of the second signal, you get an accurate representation of how it affects a photo.
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Old Sep 27, 2010, 8:26 PM   #9
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I appreciate all of your inputs but I am getting a serious brain lock. You guys make me miss my Brownie Hawkeye.
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Old Sep 27, 2010, 8:36 PM   #10
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If you can get away with ISO 100, then let's say that 18% gray is 1000 eV (electron Volts), an errant pixel might have an extra 10 eV, or 1010 eV. That's a difference of 1%, and hardly noticeable to the unaided eye.

If you increase the ISO and increase the shutter speed by a corresponding amount, then let's say that 18% gray is now 100 eV and the errant pixel has the same extra 10 eV, then (because of the ISO setting) you amplify it to the same level as in the first instance, then you've got 1000 eV for the 18% gray and 1100 eV for the errant pixel. That's an increase of 10% and clearly discernable by the casual observer.

Noise is present all the time, and always at about the same magnitude. What changes is not it's levels before you amplify it, but the difference between the signal and the noise+signal after you've amplified it.
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