Digital Camera - ISO Setting - Understanding

[Luminosity]

Hello

Film Camera
I can understand the reasoning behind ISO setting in a film camera.
Different film makeup reacts to light intensity differently for each film.
Each camera / film has an optimised preferred ISO setting for best results.

Digital Camera
Has a semiconductor CCD.... no film to change.
So why not just have one maximised preset ISO setting for the CCD?

What might I be ovelooking?

[NikoJorj]

So why not just have one maximised preset ISO setting for the CCD?

Because in some (if not many) cases, pushing the ISO in-camera means signal amplification before the main sources of read noise kick in, hence the noise remains constant while signal is higher, maximizing the signal-to-noise ration.
This is the case mostly until not-too-high ISOs (often 1600ISO), to have the shot noise well below the read noise (because shot noise is always amplified with signal, being due to the stochastic nature of light). The exceptions are cameras with a very low read noise, as are the recent Sony sensors (D7000, D5100, K5...).

See http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/ for more details about the different sources of noise.

[k bennett]

In general, the best image quality comes at the lowest ISO setting for the sensor. I suppose you could call that the maximized setting and never change it....

However, many photos can't be made at ISO 100, which is why we have the ISO dial. There is a reduction in absolute image quality, of course, but that is often acceptable when it means getting the image in the first place.

[NikoJorj]

Btw, more info here http://www.luminous-landscape.com/forum/index.php?topic=52178.0 and here http://www.luminous-landscape.com/forum/index.php?topic=56906.0 on the topic "should one push the ISO or not".

[BobFisher]

Oh geez!  Don't do that to the poor guy.  He's new.  Let him get his feet wet before tossing him into the measurebator/engineer-or-physicist-posing-as-photographer lion cage.   

[hjulenissen]

In general, the best image quality comes at the lowest ISO setting for the sensor. I suppose you could call that the maximized setting and never change it....

However, many photos can't be made at ISO 100, which is why we have the ISO dial. There is a reduction in absolute image quality, of course, but that is often acceptable when it means getting the image in the first place.

All images "could" be shot at ISO100 but it might not give satisfactory images or be the best choice in some cases.

Given a shot that tends to be under-exposed, and that other options are at their limit or fixed (exposure time/IS, aperture, flash,...). For most cameras, the image may be "less noisy" and perhaps "look better" if you increase in-camera ISO rather than just fixing the exposure in e.g. Lightroom, although there is a greater risk for blowing highlights. For certain cameras, the same can be achieved by using the sliders in Lightroom with the added benefit of not clipping the highlights in the raw file.

-h

[hjulenissen]

What might I be ovelooking?

As has been mentioned here and elsewhere:
signal->additive noise 1->analog amplifier->additive noise 2->ADC->digital amplification

If additive noise 2 is large, then doing analog signal amplification can be an advantage.

If you have a tv antenna, you might have an amplifier in the antenna/base. For some cases this is a great advantage, as a long cable from the antenna to the tuner may add noise that an amplifier later in the signal chain never can recover. The early amplifier has an inherent advantage.

-h

[digitaldog]

Oh geez!  Don't do that to the poor guy.  He's new.  Let him get his feet wet before tossing him into the measurebator/engineer-or-physicist-posing-as-photographer lion cage.  

Actually the 2nd URL provided is a real eye opener and by reading just a few of the posts (#49 for one), and running some tests, its fascinating to see (based on the camera) that an ISO 800 capture can indeed have less noise than the capture shot at ISO 100. Thanks to ETTR as well as the path of signal amplification.

[NikoJorj]

Oh geez!  Don't do that to the poor guy. 

He shouldn't have asked.
And as wisely said Andrew, this http://www.luminous-landscape.com/forum/index.php?topic=56906.msg460277#msg460277 sums it very well.

[Luminosity]

Say Guys
Thanks for all the reply's but....
It's not enough information!

I'm sure I manage to extract a question of two though.

I found this Podcast very informative.
http://www.photo101podcast.com/phone/lesson10.mp4

In the meantime I'm away have a look at that snippet of information to figure out my questions!
Have your answers ready!