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Old Feb 4, 2003, 4:58 AM   #1
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Default What's the reason for ISO in a digital camera

Hi. I've been an amature photographer for many years, and partially understand the reasoning for iso. Different iso grades of film emulsion are used to allow for better photography in diff shooting situations. A higher iso allows for better shooting in low light or faster action shots but also increases grain.

What is the purpose of implementing iso levels in a digital camera? Both are basically noise (I know grain can be used artistically). But in a digital medium, aren't we fighting to eliminate noise from our images? It's easy enough to introduce noise into digital images post snapping with software, so why would we want to play around with iso (noise levels) in a digital camera?

I know its used to get faster speeds in low light etc, just like a film camera, but film is an analog chemical emulsion. It seems like iso simply simulates film grain by using noise. Ccd's I guess induce noise in the absence of adequate light, but why switch to a higher iso which seems to introduce more noise just to get off a picture with more noise in it? For example, why don't all digital cameras simply offer or default to shoot at a standard iso of 50 giving you the cleanest noise free image possible? I know I wouldn't want to shoot anything in iso 400 judging from the amount of noise I've seen in cameras offering this iso level. I'd always want to shoot at 50 or 100 max.

Is this a ccd engineering or design limitation?

Thanks.
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Old Feb 4, 2003, 7:33 AM   #2
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I think you pretty much answered your own question. But higher end digital SLR's have more usable ISO levels, keeping noise below that of consumer digicams. One pays dearly for such prowness, but if one also makes money with the camera equipment, then he or she can not be limited by 50 or 100 ISO. Sometimes 800, 1600, or even 6400 ISO is required to capture a fleeting moment, and depending on usage these very noisy 6400 ISO images are still usable indeed (reducing for web display, or for infrared prints that are already expected to be noisy as film is).
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Old Feb 4, 2003, 7:48 AM   #3
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it is a reference point from which those coming from the film world can relate to.

just as f stops in the still world are the same as t stops in the cinema world only different.

its all a matter of reference.
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Old Feb 4, 2003, 8:10 AM   #4
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FYI Interesting discussion overhere:http://www.fillfactory.com/htm/techn...00concepts.pdf

Page C11: Pixel voltage vs time
Page C26: Sensitivity - See a faint signal in a fixed shutter time
Page C17: Silicon vs Film
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Old Feb 4, 2003, 8:28 AM   #5
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The idea is to have more creative control over your pictures. Why have controls you can override (shutter speed, aperture) anyway? There have been times that I've needed a higher shutter speed since I was shooting a long zoom without a tripod inside...being able to override the ISO gives me the flexibility to shoot at that higher speed, just like in a film camera (the digital has the extra advantage of changing the ISO on the fly, and not have to remove a roll of film that's half exposed to put in a higher speed film).

As for noise in higher ISOs, they're not trying to emulate film...just like your film, and your eyes, in lower light there's more noise due to the fact you're pushing the light sensitivity of the CCD beyond the normal range. Newer digitals have noise reduction to reduce the noise, but there's also software that's designed to get rid of this type of noise.

You can buy cameras that have one ISO, but they're cheap, low quality, and give medium results. When I bought my digital, I looked for a camera where I can override as much as possible, including ISO, just like my 35mm SLR.
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Old Feb 4, 2003, 1:56 PM   #6
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Well I'd be just as happy with decibels voltage gain, coming from the electronics world (it's there on TV cams). So I think it's just a convenient way of linking old and new technology.

What I'd like to see is some distinction between electronic gain and equivalent lens gain, so I know if I'm buying a nice big sensitive piece of glass (good signal to noise) or a smaller cheaper insensitive lens, with noisy voltage gain.

ASA is no good as a yardstick on its own, unless you also know the sig/noise performance corresponding to each setting (and that may be distorted by noise reduction processing). One question which has puzzled me is what the maximum lens f stop means in the above context for digicams, when there is electronic gain. Can we be fooled?
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Old Feb 4, 2003, 8:31 PM   #7
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Quote:
ASA is no good as a yardstick on its own, unless you also know the sig/noise performance corresponding to each setting (and that may be distorted by noise reduction processing). One question which has puzzled me is what the maximum lens f stop means in the above context for digicams, when there is electronic gain. Can we be fooled?
I tend to agree with Vox here, sensitivity in film can be dictated by the emulsion chemistry, but isn't this fixed by the CCD and constant in every camera? When an ASA/ISO is changed isn't it just like a program curve that a particular camera is mapped to since the shutter speed is also controlled by the CCD... ie the only light control variable is the aperture. Aren't we all fooled by the electronics/firmware algorithm?

BTW in checking back a few CCD datasheets it seems to me that their outputs are all analog voltages from every single pixels... The A/D conversion happen way downstream from the sensor!
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Old Feb 5, 2003, 7:25 AM   #8
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NHL...that's precisely my thinking, that film can be standardised allowing the cams to be compared. I have had some experience of ccd TV cam testing, the analogue sig/noise must be considered just as important as the sensitivity, the gains come in the first stage from lens and ccd design e.g using multi-chip and hyper HAD sensors. Noise correction afterwards, as in TV cams, is always a compromise and can actually add more artefacts than it removes.

It always used to be the case that colour sensitivity was not the same for RG and B. So noise isn't just an issue for luminance(Black and white brightness), It will vary in level depending on the actual colour and its saturation. So measuring (in dB's!) the noise output on a red/green/blue step or ramps at each ASA should tell a story.
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Old Feb 5, 2003, 8:01 AM   #9
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Vox

Right on! look @ what I just came across:
http://www.dalibor.cz/minolta/iso.htm

In looking further into this website I also came across the companion frame readout for the CCD (the D7's use an custom LSI though) that if you follow through explain how the sensor data is actually organized, or how the shutter speed works (p.24-25) 8)
http://www.sony.co.jp/~semicon/engli...1/a6803152.pdf
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Old Feb 5, 2003, 8:39 AM   #10
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It's to give the user a familiar frame of reference. Look at what computers were originally, and what they are today...computers used to be just lights and switches, osciliscopes, punched cards; today they're TV screens with text and pictures, keyboards, things that the average person can relate to (although a mouse isn't familiar, many kids grew up pushing toy cars around so in that sense it is).

Advanced users will know what shutter speed is, what happens when you drop below 1/30th of a second, what the lens opening does for depth of field, and what ISO does in relation to those other things.

I know in my case I've never used a camera with EV (Exposure Value compensation), so I'm having a tough time learning how to use it...just imagine those who have never taken electronics who would understand decibels (they'd think of sound), voltage gain (is that a surge), etc.

Think of modern cars, (hypothetical, these are made up numbers) do you think of pushing the gas pedal to increase the fuel injector's rate from 300 pulses per minute to 500, or do you think of increasing the car's speed from 25mph to 35mph (40km/h to 60km/h in metric)?
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