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I have a question about image quality. Does noise in an image is depends upon "Megapixels" or on "ISO"?

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4 Answers 4

up vote 12 down vote accepted

Noise originates due to a number of factors, see:

What types of noise can be present in digital photographs?

Increasing the number of megapixels keeping everything else constant (sensor size, technology etc.) will increase noise per pixel, but also has the effect of making the noise finer grained which is less objectionable.

ISO does not by itself increase noise, only if you combine increasing ISO with decreasing shutter time / closing aperture.

It's [probably] worth repeating this again here...

Increasing ISO whilst keeping shutter-speed/aperture constant does not increase noise:

Here is an example, as the ISO 100 shot was underexposed, raising the ISO to 1600 yielded a much less noisy result!

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I'm not sure why this ISO topic always yields such a heated debate. –  rfusca May 16 '11 at 15:57
    
@fusca: because people confuse the noise level with the signal to noise ratio, and because you don't need a degree in optical engineering to be a good photographer :) –  Colin K May 16 '11 at 16:07
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Because it's counter-intuitive, and because noisy images tend to have been shot with high ISO, by association they attribute this to high ISO. So when you come along and say "increasing ISO actually improves the signal to noise ratio (all else being equal)" people refuse to accept it, even when you provide the results of a simple experiment that proves this. –  Matt Grum May 16 '11 at 16:33
    
Thank you for the explanation. –  subeer haldar May 16 '11 at 18:10
    
@MattGrum I'm missing something here, how did you take the first photo with the same aperture and shutter speed as in the second one but with higher ISO and the first photo is not brighter than the second one? –  akram May 16 '12 at 19:10

When it comes to ISO, it's a misnomer that increasing the ISO increases noise. It's doing nothing of the sort. The reason high ISO produces noisy images is entirely due to the fact that for a higher ISO, you have to cut the amount of light (signal) hitting the sensor, ultimately increasing noise ratio before ISO amplification even takes place.

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Both of those factors can have an impact on noise. Of the two, the effect of ISO is the most immediately visible. All digital cameras exhibit image noise that rises as ISO increases. Depending on the camera, you may see little or no noise at ISO 100, for instance, and proportionally more as you approach the maximum ISO setting for your camera (typically 1600 to 6400 or more). There are plenty of examples of this for just about every camera on the market (reviews for each specific camera will often show examples).

Image size (megapixels) doesn't directly impact noise, but it indirectly impacts noise when you consider two sensors of the same size. In this case, all things being equal, the larger-megapixel sensor will show more noise as a side-effect of trying to "squeeze" more pixels onto a sensor of similar size. Although this can be shown in certain cases within a camera line, this factor can be difficult to pin down because cameras also often have upgraded sensor or processing technology that manages noise more effectively.

There are some great answers on this site already regarding specific aspects of noise. If you search for "noise", you'll find all sorts of great reading.

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All digital cameras exhibit image noise that rises as ISO increases not true. In fact the only way noise increases is if you decrease the amount of light coming in. Raise the ISO whilst keeping the same shutter speed and aperture will cause noise to decrease (or the highlights to clip). –  Matt Grum May 16 '11 at 14:52
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@Colin: Raising ISO raises the noise, if the aperture and/or shutter speed are adjusted to reduce the amount of light getting to the sensor, as is typically the case in autoexposure modes. To some people, this assumption is so natural that it is too obvious to state. To others, the assumption is so abhorrent that they break out in hives. –  coneslayer May 16 '11 at 16:02
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@Matt Grum: But what did you have to do to take those photos? With the same shutter and aperture settings, you must have either used a lot of ND on the high ISO shot, or boosted the image brightness a lot in post-processing for the low ISO image (or some other manipulation of the light level). That means that your SNR on the sensor was actually much worse in the low ISO shot. That changed the actual SNR at the sensor, so anything you do after that to make the images comparable is going to amplify a lot of noise in the dimmer image. –  Colin K May 16 '11 at 16:16
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@Colin sorry I didn't read the bit about SNR, yes you get a higher absolute noise level when you increase ISO but you get a better signal to noise ratio. When I talk about noise I mean signal to noise ratio, as this is what determines the amount of noise you see in an image. To answer your question the ISO100 shot was underexposed. By boosting exposure digitally I amplified both the image noise and read noise giving a worse SNR compared to the analogue amplified high ISO image. –  Matt Grum May 16 '11 at 20:20
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@Matt: Then it was the proper exposure which gave you a better SNR. Applying gain to a signal (analog or digital) has, at best, no impact on SNR, and in practice actually reduces SNR somewhat. A higher ISO only gets you a better SNR when it allows you to get a better exposure. You demo was flawed in the sense that the 100 ISO shot was not properly exposed. If you repeat the test, but use ND filters or a change in lighting to make the low ISO shot exposed correctly, it will have equal or better SNR than the high ISO shot. –  Colin K May 16 '11 at 20:28

Response: ISO

Examples:

  • ISO 80 (low iso) -> image without noise

  • ISO 6400 (high iso) -> image with a lot of noise

If you raise the ISO level, you will five the photo more noise.

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Counter example: ISO 100 (low iso) -> image with a lot of noise, ISO 6400 (high iso) -> image with little noise. Proof: mattgrum.com/ISOcomparison/ISO_100_vs_ISO_1600.jpg –  Matt Grum May 16 '11 at 14:48
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@Matt: You are certainly correct, but I think the way most photographers think about things is that when you increase ISO, you offset that increase with a faster shutter speed or smaller aperture, to produce (roughly) the same photograph. (In most autoexposure modes, this way of thinking corresponds to the mechanics of shooting.) I think we'd be better off encouraging people to state that assumption in their writing, rather than just saying they're wrong. –  coneslayer May 16 '11 at 15:16
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Ok so every noisy high ISO image would be even noisier if shot with a lower ISO. Likewise every noisy high ISO would be less noisy if shot with a longer exposure. So I don't understand why people call it "high ISO noise" and not "insufficient light noise". ISO doesn't create noise, which is exactly what statements like "all digital cameras exhibit image noise which rises as ISO increases" implies. If you instead say "all digital cameras exhibit noise which increases as incoming light decreases" you would be correct without the need to state any assumptions! –  Matt Grum May 16 '11 at 15:39
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"Ok so every noisy high ISO image would be even noisier if shot with a lower ISO." Only under your unstated assumption that shutter speed and aperture remain fixed. Which is not necessarily how most photographers actually think about things. –  coneslayer May 16 '11 at 15:49

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