ISO Ratings on DSLRs

TwentySixDays

TwentySixDays

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Stuart
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I have read some information on CMOS, sensor arrays and things but Iam confused and there is probably a simple explanation...

Film Camera:
My understanding is that FILM has an ISO rating that reflects the time it takes for the light to be absorbed/capture by the chemicals in the film.
The more light the quicker the gains in the emulsions are saturated.
Also the bigger the grains in the emulsions the faster they absorb light.
Hence the gains in ISO100 film are smaller than in ISO400.
Which makes ISO400 faster and more "grainy" than ISO100.

Digital (CMOS) Camera
With a digital camera the sensor is fixed, the size of the sensors (pixels) is fixed.
I can understand that you may what to adjust the sensitivity to enable changes in aperture, DoF, etc. but...

Could someone please explain - in simple terms - why digital sensors have the same draw back of "noisey pictures"?
 
To achieve higher ISO setting the signals to/from CMOS sensor are amplified, so the higher the ISO the more the signals are amplified and a side effect of this is noise similar to grain.
 
Thanks Andy,
I knew there would be a bright spark who could explain it for me.
So this is simular to sound distortion when turn up the volume and the peaks of the waves are clipped.
 
TwentySixDays said:
So this is simular to sound distortion when turn up the volume and the peaks of the waves are clipped.
Click to expand...

Not quite.

It's a bit like turning up the volume when you play a quiet track on a cassette player (remember those?). The volume of the tape hiss (noise) goes up with the music.

Your example of distorted peaks is similar to overexposing with a digital sensor. Once you reach the maximum light level, it clips.



Steve.
 
Steve Smith said:
Not quite.

It's a bit like turning up the volume when you play a quiet track on a cassette player (remember those?). The volume of the tape hiss (noise) goes up with the music.

Your example of distorted peaks is similar to overexposing with a digital sensor. Once you reach the maximum light level, it clips.



Steve.
Click to expand...


Perhaps someone needs to invent the equivalent of "Dolby" noise reduction.
 
Terrywoodenpic said:
Perhaps someone needs to invent the equivalent of "Dolby" noise reduction.
Click to expand...
Most camera manufcturers have a form of noise reduction system built into their model firmware. You can also get applications that will reduce the amount of noise on subsequent images.
 
It's to do with the signal to noise ratio. There will always be some background noise on your images. Slight fluctuations in brightness/colour that aren't representative of what's there. When you amplify the signal from the sensor (increase the iso), you also amplify the background noise.
 
Thanks for all the info:
I don't think I'll be doing all calcs each time I take a picture through.:lol:
I've seen the Noise reduction (and increase) options in PSE I'll have a read up on those now I understand a little more.

Thanks again:thumbs:
 
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