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01-27-2008, 04:50 AM   #16
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Hmm ...

The photosensor is analog.
The light induces a certain amount of current.
This current can then be amplified or attenuated.
Only later the signal passes the A/D converter and gets digital.

But I think it is not that easy, because for some reason the attenuation does not work very good.

01-27-2008, 05:38 AM   #17
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I'll see I can help shed some light.

All light sensors are analog because light is not digital! Light has various wave lengths and the sensor must interpret it as such. Which is why they all have A/D converters to make it digital.

However, this sensor is still a circuit which requires voltage and current to operate. The circuitry was designed to operate at a specific voltage and current, any lower, and the circuity will no longer work (i.e. putting 2volts in a camera requiring 5volts will result in a non-operative camera).

With some tweaking, they can make the circuit have light capturing abilities very close to ISO standards.

So to summarize, the native ISO is the sensitivity of the sensor at the originally designed voltage and current.

To obtain higher sensitivities from the sensors, more power is sent to it (amplifying the signal). When you amplify a signal, interference is introduced and the A/D converters interprets this as slightly different colors. These random color inaccuracies are what we call noise

And to answer another question: Native ISO for the K20D is 100.
01-27-2008, 05:59 AM   #18
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QuoteOriginally posted by blende8 Quote
Where can I read more about Nikon's D300 base ISO?

QuoteOriginally posted by Nikon USA: D300 Website:
Sensitivity: ISO 200 to 3200 in steps of 1/3, 1/2 or 1 EV with additional settings of approx. 0.3, 0.5, 0.7 and 1 EV (ISO 100 equivalent) under ISO 200 and approx. 0.3, 0.5, 0.7 and 1 EV (ISO 6400 equivalent) over ISO 3200
Interpreting this:
The D300's native ISO is 200. It can amplify the signal (more power) to create up to ISO 3200 equivalent.

For the expanded ISO equivalents:

It can overexpose an image at ISO 200 to simulate ISO 100 (where it will take the digital signal after the A/D converter and decrease brightness/contrast/etc back to properly exposed levels).

It can underexpose an image at ISO 3200 to simulate ISO 6400 (where it will take the digital signal after the A/D converter and increase brightness/contrast/etc back to properly exposed levels).

Last edited by AVANT; 01-27-2008 at 06:03 AM. Reason: Updated wording
01-27-2008, 06:11 AM   #19
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Thank you Avant - and I stand corrected in my theory. Of course the sensor is analog - DUH!

01-27-2008, 06:29 AM   #20
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QuoteOriginally posted by d.bradley Quote
ISO 100 is needed for bright conditions when you don't want to stop down. Given that sensors only have a limited dynamic range and available 'gain', setting the 'base' ISO at 200 means you should be able to get better high ISO performance, but requires that you use a negative gain (like -6db in my Pro JVC video camera) to get back down to ISO 100.
you can also use nd filters. stop down the light but keep the lens open.
01-27-2008, 02:40 PM   #21
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The italian guy at PDML who went to the Dubai conference said the base ISO is 200, and his pictures confirm this:

01-27-2008, 04:10 PM   #22
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QuoteOriginally posted by d.bradley Quote
ISO 100 is needed for bright conditions when you don't want to stop down. Given that sensors only have a limited dynamic range and available 'gain', setting the 'base' ISO at 200 means you should be able to get better high ISO performance, but requires that you use a negative gain (like -6db in my Pro JVC video camera) to get back down to ISO 100.
Thanks, that makes sense. I am fairly new to photography and brand new to the world of DSLRs. I have lots of questions.
01-27-2008, 06:15 PM   #23
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Here is a good article on 'Digital ISO Speeds':

Digital ISO Speeds | Digital Photo Pro Magazine

In the article, Mike Stensvold notes that ISOs below the native sensitivity of the sensor are achieved in the image processing software AFTER the A/D converter, resulting in a reduction of the dynamic range of the image. ISOs above the native sensitivity are achieved by additional gain BEFORE the A/D converter, increasing noise. Hence, the native sensitivity is optimal (what ever it actually is). Mike does offer an interesting note, however, regarding using higher ISOs versus longer shutter times, both of which tend to increase noise... so there is a trade-off, depending on what the scene you are caturing consists of - i.e. there is no simple answer!

Curt

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