How low, should I go, on the max ISO?

Started Oct 24, 2014 | Discussions thread
Christof21 Veteran Member • Posts: 4,052
Re: How low, should I go, on the max ISO?

JimKasson wrote:

Christof21 wrote:

ISOLess is a property of a sensor. It has only one definition and I don't change this definition.

It must be wonderful to be so self-confident. However, I beg to differ.

Reading your post, I don't see where we differ.
But to be honest, I have been too positive in my first posts in this thread about the implementation, it certainly differs from the Fuji one in most cameras. My fault, I admit, it was an hypothesis
If the D4 was implemented like the Fuji, then the op would not have to care about shooting at a lower ISO.

Saying that ISO-less-ness is a property only of the sensor makes the concept of little use to a photographer trying to get the most out of her camera, due to the wide variety of digital camera implementations.

For example, I own two cameras, the Hasselblad H2D-39, and the Leica M9, that have sensor chips that have neither variable gain nor ADCs. It offers no utility to a working photographer to say that those cameras are ISOless because the ISO knob doesn't affect the sensor. The ISO knob in both cameras affects the pre-ADC signal level, and thus the values in the raw file. That's what the photographer cares about.

What's important to the photographer is that turning up the ISO knob of the M9 past 640 actually creates more image noise than pushing in post:

http://blog.kasson.com/?p=2824

You make a confusion because you include in your ISOLess definition the way the raw data is saved (you say:"the values in the raw files are proportional to the raw knob").

The values in the raw file are what is important to the photographer.

There are two main ways the values in the raw file can change when you turn the ISO knob. The first is if that knob affects the gain of the pre-ADC amps. In that case, the input-referred read noise may vary with the knob setting. The second is that the ISO knob may control the application of digital gain. In that case, to a first approximation, the input referred read noise will not vary with ISO setting. Some Canons employ both techniques throughout most of the ISO range.

The definition of an ISOless sensor is not at all ambiguous (the read noise is constant when you increase ISO), I don't understand why you want to make things confusing.

Actually, when you put it that way, your definition is confusing (or maybe incomplete). I would say that in an ISOless sensor, the input-referred read noise is constant with ISO. If all the gain is analog, the read noise measured at the ADC input (or the output, modulo quantizing noise) will increase as the gain increases.

I fully agree with this definition..

Jim

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