What does changing ISO actually do ?

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rogertb

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Roger
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Hi chaps ... now I'm getting a little more into the photography thing, and starting to use manual more I wondered what does changing ISO actually do ... I know altering the speed, well, changes the time the shutter's open ... and the aperture changes the size of the hole in the lens ... but ISO ? - in film days a high ASA film was more 'sensitive' but I'm pretty sure a higher ISO isn't altering the sensitivity of the 'sensor' so anyone know what's happening ?

Regards Roger
 
rogertb said:
I'm pretty sure a higher ISO isn't altering the sensitivity of the 'sensor'
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The ISO sensitivity is increased by turning the gain up on the output from the sensor. There's people on here who can give you a far better in depth description of this than me, but that's the basic principal :)
 
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For a simple-ish explanation. Think of your cameras sensor as an array of light gathering buckets. Light fills the buckets and once full the picture is assembled.
If you up the ISO it's like using buckets with slightly less volume, they still need to be filled to make the picture but filling them takes less time. You still get the picture but it contains less information.
 
MikeDB said:
For a simple-ish explanation. Think of your cameras sensor as an array of light gathering buckets. Light fills the buckets and once full the picture is assembled.
If you up the ISO it's like using buckets with slightly less volume, they still need to be filled to make the picture but filling them takes less time. You still get the picture but it contains less information..........
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.......but more noise. Good way of putting it! :thumbs:
 
rogertb said:
Hi chaps ... now I'm getting a little more into the photography thing, and starting to use manual more I wondered what does changing ISO actually do ... I know altering the speed, well, changes the time the shutter's open ... and the aperture changes the size of the hole in the lens ... but ISO ? - in film days a high ASA film was more 'sensitive' but I'm pretty sure a higher ISO isn't altering the sensitivity of the 'sensor' so anyone know what's happening ?

Regards Roger
Click to expand...

Technically, it doesn't alter the sensitivity of the sensor, but it increases the amplification (gain) of the signal which is effectively the same thing.

The downside is you get more noise (coloured speckles on the image) that comes from the amplifier (the 'processing engine'). It's called noise because it's the same thing as you get from an audio amp, the buzzing/humming sound, when you turn up the volume during quite bits.
 
There is a very small pixie inside the camera.

That is why digital camera can't easily be opened - you don't want it to escape! When you increase the ISO, it gets an extra jolt of electricity from the camera battery (pixies live on electricity), so it paints faster. The 'noise' is actually the paint splattering a bit from the speed that the brush moves. More speed, more splatter, obviously.

Advanced cameras have more than one pixie, so need bigger sensors to allow them space to work (up to four in 'full frame' models). Hence you can use a higher ISO in these because with each pixie (or 'pixel' to use the correct name) doing a section of the image, they don't have to work so fast, hence less splatter.

The pixies came from another planet (rumoured to be flat, rather than spherical), and the authorities tried to hush it up, the way they do. But some escaped and have become fabulously rich working for camera corporations.

ISO stands for Iconographic Synthesizing Operative.

Hope this makes things clear. :thumbs:
 
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jon ryan said:
There is a very small pixie inside the camera.

That is why digital camera can't easily be opened - you don't want it to escape! When you increase the ISO, it gets an extra jolt of electricity from the camera battery (pixies live on electricity), so it paints faster. The 'noise' is actually the paint splattering a bit from the speed that the brush moves. More speed, more splatter, obviously.

Advanced cameras have more than one pixie, so need bigger sensors to allow them space to work (up to four in 'full frame' models). Hence you can use a higher ISO in these because with each pixie (or 'pixel' to use the correct name) doing a section of the image, they don't have to work so fast, hence less splatter.

The pixies came from another planet (rumoured to be flat, rather than spherical), and the authorities tried to hush it up, the way they do. But some escaped and have become fabulously rich working for camera corporations.

ISO stands for Iconographic Synthesizing Operative.

Hope this makes things clear. :thumbs:
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OR, in some cases - Idiot Speaks Out. :D
 
jon ryan said:
There is a very small pixie inside the camera.

That is why digital camera can't easily be opened - you don't want it to escape! When you increase the ISO, it gets an extra jolt of electricity from the camera battery (pixies live on electricity), so it paints faster. The 'noise' is actually the paint splattering a bit from the speed that the brush moves. More speed, more splatter, obviously.

Advanced cameras have more than one pixie, so need bigger sensors to allow them space to work (up to four in 'full frame' models). Hence you can use a higher ISO in these because with each pixie (or 'pixel' to use the correct name) doing a section of the image, they don't have to work so fast, hence less splatter.

The pixies came from another planet (rumoured to be flat, rather than spherical), and the authorities tried to hush it up, the way they do. But some escaped and have become fabulously rich working for camera corporations.

ISO stands for Iconographic Synthesizing Operative.

Hope this makes things clear. :thumbs:
Click to expand...

They're not just any old pixies, they're megapixies :naughty:
 
jon ryan said:
There is a very small pixie inside the camera.

That is why digital camera can't easily be opened - you don't want it to escape! When you increase the ISO, it gets an extra jolt of electricity from the camera battery (pixies live on electricity), so it paints faster. The 'noise' is actually the paint splattering a bit from the speed that the brush moves. More speed, more splatter, obviously.

Advanced cameras have more than one pixie, so need bigger sensors to allow them space to work (up to four in 'full frame' models). Hence you can use a higher ISO in these because with each pixie (or 'pixel' to use the correct name) doing a section of the image, they don't have to work so fast, hence less splatter.

The pixies came from another planet (rumoured to be flat, rather than spherical), and the authorities tried to hush it up, the way they do. But some escaped and have become fabulously rich working for camera corporations.

ISO stands for Iconographic Synthesizing Operative.

Hope this makes things clear. :thumbs:
Click to expand...

Post of the week.
 
DuncanDisorderly said:
I'm not sure...
I can see how the relationship works between pixies and digital (thanks for clearing that up Jon), but I associate ASA with film.
Is it still pixes?
Click to expand...

:thinking:
 
ASA (American Standards Association, now called ANSI) refers to the American way of expressing film speed (the German DIN system is also used and shown on film boxes and older cameras) and is exactly the same as the ISO as the International Standards Agency simply adopted that as the international standard.
 
ISO and ASA are film speed settings one of which (ISO) has carried over from film to digital where its used as a reference to the gain from the sensor.

In order to fully understand it you need to look at its origins in film.

With film different films would have different sensitivities to light, so whilst a low sensitivity film (ISO 50-100) would need a long shutter speed in low light conditions to achieve a well exposed image a film with a higher light sensitivity (IS0 400 - 1600) would allow you to shoot the same image in the same low light conditions but with a faster shutter speed and/or a smaller aperture.
Choosing the right film speed for the circumstances was just as important as choosing the right shutter speed/aperture.
Of course back then we had to rewind and remove the ISO 100 film, then load a fresh roll of ISO 800 for instance. In the digital version all we have to do is flick a switch or rotate a dial and we can keep shooting.

As said what happens now is that the sensor gain is increased when you increase the ISO setting, this has the apparent effect of raising the sensors sensitivity to light allowing you to use a faster shutter speed.

It's ironic in a way that whilst with film you always got increased film grain shooting on faster (high ISO/ASA film) similar has happened with digital mediums due to "noise", there are various types and its a bit technical to go into in depth.

One of the factors that influences "noise" at higher ISO's is photosite size, the larger the photosites and hence the larger the sensor the more light the photosites can grab hence better low light performance.

Sometimes this can go against the pixel peeper who will only look at sensors with x amount of megapixels, in short if you took two sensors of the same physical size and crammed 24mp onto one and 12 larger MP onto the other, the 12mp would outperform in low light conditions, the resulting images from it would show less noise.

Technical, but with a grounding in film its very easy to understand... so just think of it as ISO = light sensitivity, low ISO for bright conditions, high ISO for low light conditions.
 
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Thank you all so much, informative and entertaining, what more could I ask for on a rainy day ... I favour the pixies paint flicking explanation, it seems to make more sense than the other suggestions and I now understand why Canon don't like their cameras getting too hot or too cold !

Roger
 
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