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Old Apr 15, 2003, 4:39 PM   #1
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Default ASA Settings

I am quite familiar and comfortable with the effects and uses of different ASA films and settings in film photography but frankly am a little mistified with the process behind multiple ASA settings on digital cameras.

Can anyone explain the background (I think I know the effects of using differing settings, i.e. 1600 vs 100) . Obviously (I think) we're not changing the actual capabilities of the photo chip so what's the process??

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Old Apr 15, 2003, 5:04 PM   #2
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To put it simply, it's adding more electrical gain, or amplification after the ccd sensor - which is why you get the gain with pain - which is increased electrical noise on your pics as you increase ASA. The camera internal processing attempts to reduce or mask the appearance of noise, but it never goes away. Just like film grain, so not much has changed!

To complicate matters, some cameras like the Fuji 602, can combine the output of pixels to enhance sensitivity, but resolution is traded off and in high ASA modes the camera functions in 1Mpix mode.
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Old Apr 15, 2003, 5:10 PM   #3
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From what I can gather, increasing the ISO setting on a digital camera just increases the voltage to the sensor chip. Goosing up the voltage tends to introduce noise to pictures. My camera (Canon Pro90) has an Automatic ISO setting which lets the camera choose between ISO 50 or 100. I can also pick from settings of 100, 200, 300 or 400. Things get pretty ugly above 200.
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Old Apr 15, 2003, 6:27 PM   #4
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Just to add My two Cents. You can deal with the noise after the fact. There are programs like NEAT http://www.neatimage.com/ that will let You work on the noise. There is a cost in image detail.
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Old Apr 16, 2003, 11:34 AM   #5
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Quote:
ASA
Now there's a name I haven't heard in a long time!
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Old Apr 16, 2003, 1:37 PM   #6
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Thanks to all.

Now it make's sense. Understood chemical process and potential problems with film processing & now have at least a glimmer of understanding electronic processes.
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Old Apr 16, 2003, 2:02 PM   #7
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Perhaps one of us should have said, that whilst ASA standards were set specifically for film, in digicams the ASA number is supposes to be representative of the equivalent in a film cam.

So if you've been a chemical shooter, the digicam world just puts you at ease with some friendly links to the past!
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Old Apr 16, 2003, 8:30 PM   #8
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The meaning of the f/stop and shutter speed are exactly the same in digital and chemical cameras. I suppose the sensitivity of the detector could have been stated in terms of quantum effeceincy, but why bother? Seems like the ASA/ISO rating for film specifies the amount of light needed to get an exposure - just the kind of number that works well to describe exposure. So just use the "same" system.

High "speed" film has grain: high "speed" digital has noise. Film has been around for more than 100 years so it doesn't change much over a few years (- though I do remember ASA10 Kodachrome). Digital is still young and is improving fast. And more money buys better detectors in digital.

Anyhoo - to change the ASA of the film in your camera, you change the film and/or how it is devleoped. To change the ISO in your digicam, you change the amplification within the camera and/or change how you handle it in the photo editor - the digital darkroom. Different processes, but the problems, other than electronic or chemical, are the same.
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Old Apr 20, 2003, 6:05 PM   #9
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This is the first time I have seen examples of a noise reduction program. Results appear to be very good! Thanks for the link to NEAT, rlpumphrey!
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