# Could someone explain ISO?



## brookie418 (Oct 13, 2008)

Could someone please explain ISO to me?  Thanks!


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## GeneralBenson (Oct 13, 2008)

Iso is basically your film/sensor's ability to gather light information.  It effect the outcome of an exposure, just like aperture and shutter speed; although they all have their  differences in outcome.  Where shutter speed and aperture change how much light will hit the film/sensor, ISO changes the sped at which the medium can record that amount of light.  So let's say you take a shot at f/5.6, 1/125th, ISO 200; then you take one at f/5.6, 1/125th, ISO 400.  The second will be 1 stop brighter becaude ISO 400 is gathers twice as much light as ISO 200.


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## table1349 (Oct 13, 2008)

http://digital-photography-school.c...settings-understanding-iso-a-beginners-guide/

The whole series
http://digital-photography-school.com/blog/digital-photography-tips-for-beginners/


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## ksmattfish (Oct 13, 2008)

ISO is a scale used to indicate how light sensitive the film or sensor is.  Lower ISO numbers are slower, and require more light/exposure than higher ISOs.  Halving or doubling the ISO number is a stop.  A stop is half or double the amount of light/exposure.


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## Sim (Oct 13, 2008)

Just to add to ksmattfish's excellent definition, the higher the ISO, the more noise is introduced into your pictures so generally using a lower ISO is preferable to a higher one.


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## astrostu (Oct 13, 2008)

The first poster is not technically correct.  It does not change how sensitive the CCD is nor the speed at which it records light.  The CCD records light as fast as it can given the optics you have (aperture) and length of time exposed (shutter).  The CCD then has the option to multiply that light by a certain amount dependent upon the ISO you set.  This is called "gain."

The problem is that this does not increase the amount of light you get, it just increases how bright the final saved image will be (if you increase the ISO; reverse that for decreasing the ISO).  This means that any noise in that signal ALSO gets magnified when the ISO is increased, which is why high-ISO images look noisier.

In my opinion, ISO should always be at the lowest setting you can get it at while still getting a proper exposure.  It should not be thought of as a substitute for a longer shutter speed in terms of getting an equivalent exposure brightness.


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## Extreme Detail (Feb 11, 2009)

brookie418 said:


> Could someone please explain ISO to me? Thanks!


 The lower the iso the faster the shutter speed.


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## Big Mike (Feb 11, 2009)

> The lower the iso the faster the shutter speed.


Not necessarily.  It's one part of the exposure equations which involves ISO, shutter speed and aperture.


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## Prophet (Feb 11, 2009)

All praise to the holy trinity.... Can I get an Aye MEN?

-JD-


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## Ron Evers (Feb 11, 2009)

Extreme Detail said:


> The lower the iso the faster the shutter speed.



In ancient history (the 50s) when I was first learning photography with an SLR, film speed was rated as ASA, now ISO.  Then, ASA 100 colour was a super fast film & one needed a shutter speed one stop less than ASA 50 film for a given aperture.  Methinks this post is ass-backwards.  

Ron, who used ASA 12 Kodachrome for slides.


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## rodman (Feb 11, 2009)

Prophet said:


> All praise to the holy trinity.... Can I get an Aye MEN?
> 
> -JD-



Amen brothah! AMEN! PRAISE THE LORD!:thumbup::thumbup:


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## tsaraleksi (Feb 11, 2009)

Ron Evers said:


> In ancient history (the 50s) when I was first learning photography with an SLR, film speed was rated as ASA, now ISO.  Then, ASA 100 colour was a super fast film & one needed a shutter speed one stop less than ASA 50 film for a given aperture.  Methinks this post is ass-backwards.
> 
> Ron, who used ASA 12 Kodachrome for slides.



Methinks you're right. 

I think it's a shame we've stopped using ASA in favor of ISO.


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## jlykins (Feb 11, 2009)

I'm going to paraphrase here. I got this definition from Understanding exposure: Think of iso as bee's. If you have a set amount of light to collect, and 200 bee's can do it in say 4 seconds, then if you double that and have 400 bee's you will be able to collect the same amount of light in 2 seconds or half the time. Does that make sense?


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## Ron Evers (Feb 11, 2009)

Extreme Detail said:


> The lower the iso the faster the shutter speed.





jlykins said:


> I'm going to paraphrase here. I got this definition from Understanding exposure: Think of iso as bee's. If you have a set amount of light to collect, and 200 bee's can do it in say 4 seconds, then if you double that and have 400 bee's you will be able to collect the same amount of light in 2 seconds or half the time. Does that make sense?




YES, but more simply stated, the higher the ISO the faster it will collect light.  DUH!


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