ISO: Film vs. Digital Sensor

I grew up using a Rebel SII and about 2 years ago I went to an XTI and since

then now have a 5D. My question is how does one digital sensor reproduce the

effect of numerous film ISO ratings. Is it in the sensor or more done by the

programing. I'm trying to explain what I think is going on to my father (who

is not in any way computer literate) and failing miserably. Any help would be

greatly appreciated!

 

Jake

it is done in the hardware. the image processor and all that good stuff. I think the image signals get different amplifications at different ISO settings.. the closest analogy that I can imagine is similar is an amplifier for music. you can take a weak signal and boost it but you will get lots of noise or take a strong signal and not boost it as much. and get better quality.

Yes amplified signal. Then you get noise.

Increasing the ISO is like turning up the volume on the stereo.

Thanks a lot! My dad was an old stereo head and that will really make sense to him. You guys rock!

Could someone explain to me what the term Noise means in reference to digital Photography? Althought iv been into photography for well over 10 years, I use Film. Thanks, Tim.

Tim,

 

http://www.dpreview.com/learn/?/Glossary/Digital_Imaging/Noise_01.htm

Noise is mostly what's there when there is no signal. It's the result of random movement of electrons in the electronics due to thermal processes and the discrete nature of electron movement (shot noise). It's also due to the quantum nature of the statistics of photon arrival (poisson noise) at low light levels.

 

It's the digital equivalent of "grain" in film images (though they don't look exactly the same).

Indeed, increasing iso is like turning up the amplifier on a stereo when there's a cassette tape playing. The hiss in the background is noise. Think of quiet parts like dark areas in a photo, and loud parts like highlights in a photo. When the music is loud you don't hear the tape hiss. When the music is quiet and you turn up the volume, you start hearing tape hiss. Same goes for digital photography, you turn up the iso and things get noisy.

Of course, on some Canon models (e.g.30D) the intermediate ISO settings are managed via computation, not amplifier feeds. The base ISO is that nearest the setting.

-Mark

As I understand it, noise is basically made-up pixels that are installed when the sensor doesn't get enough light to render anything.

 

It is often compared to graininess in film images, but looks much different, as it has nothing to do with resolution, and it is also noticably colored (purple, blue, something in that area of the spectrum), and really square looking.

 

This is why I still prefer film for extremely low light shooting. Digital gets better all the time, though.

 

Keith

Bob I might be all wet but I thought that noise was increased by increasing the ISO. Noise was caused by the proximity of the pixels to each other on the chip as they are electronically interfering with each other. As the sensitivity(increase in ISO) is increased They interfere with each other more making more noise? Bill

Hmm.. see I thought noise was what Bob said it was.. And that it was the deviation in the average result .. say red.. some get red. some get .. redish.. some pixels go crazy and get blue.. of course that is a result of the pixels (red ones green ones and blue ones) all getting different levels.. anyway.. the over all result is red .. on a red object.. but up close it looks like a red fuzzy flea race on TV when you have no signal.

 

you get more of it when you turn up the ISO because you amplify the differences. I think :)

With a low iso ; you have a good signal to noise ratio. With a super high iso setting; remember onw is boosting signal and the noise too; and one has a lower signal to noise ratio. Try recording whispers in a libary; and at chucky cheese during a kids birthday party.

This set of lecture notes covers a lot of the different sources of noise in CMOS sensors:

 

http://www.cse.yorku.ca/~visor/pdf/CMOS_3.pdf

 

Bear in mind it is about 10 years old, so technology has moved on in many ways, and sensor performance has improved dramatically since then. However, it does cover a lot of the relevant ground.

And depending on the white balance, different channels (R,G,B) get amplified different amounts, so the noise takes on the color of the most amplified channel (ignoring image processing). The green channel is also more noise resistant, since it has twice as many pixels.

Tell your dad ,

 

That the red ,green and blue color adjustment in a dslr is like the bass, mid, and treble in stereo, to further increase his understanding. hehehehe :D

 

Cheers!

Actually, I think all pixels are nominally amplified by the same amount (arranging anything else would be a nightmare for a chip designer). White balance adjustment is done in the digital domain using arithmetic. True, it can result in increased noise for a channel that is under-illuminated in the original image. It's slightly splitting hairs, but the effects of digital image processing are subtly different and complex in their own right.

Given a 30D is the image actually better, cleaner or whatever at ISO 100 or 200? I have heard both from different people and some say 200 and 100 are exactly the same as far as quality, just that one is slower than the other. Any truth to this?

Wait a second. 2 years ago you had an XTi? ?