Exposure/noise relationship

LWSC

LWSC

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Lucas
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I'm curious: why is it exactly that I can shoot at, say, ISO 400, underexposing the scene, and get a large amount of noise in the photograph, as much or more noise than shooting at 800-1000 ISO and exposing correctly?
 
General rule of thumb; underexposing is going to exaggerate noise. Much more so if you underexpose and then push it in post production.

If you want the least noise for any particular shot (and best colour, dynamic range, etc.) just expose it as close to the target final result as you can.

You can get slightly different 'looks' either by over or under exposing and pulling or pushing the final image. But for most intents and purposes, just expose 'properly', stick to full stops on the ISO setting, and concentrate or more important things.
 
It depends on the camera, some modern cameras (Nikon and Pentax particularly, less so Canon) allow you to push the shadows with little ill effect, have a Google for "iso invariant cameras".

But otherwise it is basically to do with the initial brightness of the shot - if the shot is bright then the signal to noise ratio is inherently low, if the shot is dark then the signal to noise ratio is high.
 
Nawty said:
It depends on the camera, some modern cameras (Nikon and Pentax particularly, less so Canon) allow you to push the shadows with little ill effect, have a Google for "iso invariant cameras".

But otherwise it is basically to do with the initial brightness of the shot - if the shot is bright then the signal to noise ratio is inherently low, if the shot is dark then the signal to noise ratio is high.
Click to expand...
Other way round.

20log10(signal/noisefloor)
 
LWSC said:
I'm curious: why is it exactly that I can shoot at, say, ISO 400, underexposing the scene, and get a large amount of noise in the photograph, as much or more noise than shooting at 800-1000 ISO and exposing correctly?
Click to expand...

You get more noise when under-exposing and brightening the image in post processing, than you do by shooting at a higher ISO in the first place. This is because the gain applied before the analogue-to-digital conversion stage, ie by raising ISO, is much more efficient than brightening in PP.

The new breed of so-called 'ISO-less' or 'ISO-invariant' sensors are much better in this respect and shadow areas can be brightened in post with with little or no noise penalty. You still get noise of course, but no more than if you'd raised ISO first. There is only a few cameras around like this, notably those with the latest Sony full-frame sensors (Nikon D810, Nikon D750, Sony A7Rii although the Canon 5Div is also very good) and this has major implications for exposure control.
 
HoppyUK said:
You get more noise when under-exposing and brightening the image in post processing, than you do by shooting at a higher ISO in the first place. This is because the gain applied before the analogue-to-digital conversion stage, ie by raising ISO, is much more efficient than brightening in PP.

The new breed of so-called 'ISO-less' or 'ISO-invariant' sensors are much better in this respect and shadow areas can be brightened in post with with little or no noise penalty. You still get noise of course, but no more than if you'd raised ISO first. There is only a few cameras around like this, notably those with the latest Sony full-frame sensors (Nikon D810, Nikon D750, Sony A7Rii although the Canon 5Div is also very good) and this has major implications for exposure control.
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The slightly earlier Sony sensors, such as on the NEX7 and A77, are pretty close to ISO invariant. It was that generation of sensors which started all the discussion of ISO invariance. Rather like focal length equivalences there are endless debates about how many factors have to be invariant, over what ranges, and how close to exact invariance they have to be, to count a sensor as near enough ISO invariant. When I upgraded to an A77 from an earlier generation sensor I found the difference in dynamic range and shadow liftability quite startling. I only heard about ISO invariance years later.
 
Musicman said:
I think you may have that the wrong way round :)
Click to expand...
Yup. When the amount of signal is low relative to the amount of noise, then the SNR is low... although the difference between levels may be greater. (SNR is a measure of how much *more* signal there is than noise. when noise is greater than signal the SNR is negative)
 
LWSC said:
I'm curious: why is it exactly that I can shoot at, say, ISO 400, underexposing the scene, and get a large amount of noise in the photograph, as much or more noise than shooting at 800-1000 ISO and exposing correctly?
Click to expand...
It's all about how much light the sensor actually receives, and ISO has no affect on that. There is a common misconception online that ISO is the sensor's "sensitivity" to light. It is not, it's amplification after the fact...like a volume/brightness adjustment on your computer.
It is also true that in very good conditions you can shoot at much higher ISO's without the same negative impact... that's because there is still enough light getting to the sensor to mitigate the issues. It's just that there's seldom any need to do so.

BTW, this is what makes many of the online high ISO tests misleading... they start with good light and adjust settings rather than reduce the actual light levels.
 
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Noise is to a large extent about the the amount of light captured (photons) or lack of it.
And the extent to which the signals have to be amplified.
Plenty of light, the better is the ratio between signal and noise.
The more the amplification the more the noise.

Cameras that can shoot at the Native ISO and not underexpose produce the least noise, if you move away from this ideal, increased noise is inevitable.
 
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HoppyUK said:
You get more noise when under-exposing and brightening the image in post processing, than you do by shooting at a higher ISO in the first place. This is because the gain applied before the analogue-to-digital conversion stage, ie by raising ISO, is much more efficient than brightening in PP.
Click to expand...

sk66 said:
It's all about how much light the sensor actually receives, and ISO has no affect on that. There is a common misconception online that ISO is the sensor's "sensitivity" to light. It is not, it's amplification after the fact...like a volume/brightness adjustment on your computer.
Click to expand...

This is all very interesting, thanks - I think that answers my question!
 
"ISO invarient" sensors are all well and good, but Google images seem to suggest this is encouraging some more 'excitable' photographers to produce images more akin to highly processed HDR images... Please, use it responsibly ;)
 
I have a Fuji XT2 which generally underexposes. There doesnt seem to be any noise when I brighten the shadows in PS
 
Or is it better to overexpose slightly and reduce it slightly later in PS
 
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