Film Speed and noise

evilded777

Having a bit of a discussion elsewhere about tone mapping. There is a contention afoot on wether or not increasing film speed, in Iray, results in noise.

It is my contention there is no increase in noise when increasing film speed. Others disagree. I know what happens in the physical world with increased film speed, but I do not think that Iray mimics this.

Who's correct?

Tobor

The ISO setting in Iray only really alters the exposure value, same as shutter speed and f/stop. None have a qualitative result in Iray, at least the version used in D|S. They're just feel-good settings that in the end result in the same thing, which is changing an "Exposure" setting in the same way that Photoshop works.

Along similar lines, the f/stop setting doesn't increase DOF (that's done elsewhere in D|S), and a slower shutter speed doesn't introduce blur.

evilded777

Thanks, @Tobor, I was pretty sure I was right... not going to go crow about it; but at least I have a more confident feeling.

Tobor

Some renderers have "artistic" features that add film grain or noise at higher ISO, or even processing filters to replicate certain types of film such as Kodachrome, but the nature of CG rendering doesn't require it. 

Just to throw out some FYIs, grain in film comes from silver halide crystals that become larger when the emulsion is made to be more sensitive -- this can come from the design of the film, or by persisting the emulsion in its developer bath for longer (or using more agitation or a higher temperature or a number of other mechanical means). In a digital camera, noise comes from increasing the sensitivity of the sensor.  This is why if you choose a very high ISO in a digital camera, the images may not be as crisp and clear. 

CG rendering doesn't involve either silver halide emulsions or sensors that exhibit electrical noise, so any influence the ISO setting has in the renderer would be there by design as part of an artistic process. Since noise, grain, and film response curves are all possible as after effects, where they are more controllable, this is where they are more often applied. 

I'm not sure if there is a version of Iray that provides these effects. The Iray "camera" is a type of shader, and with appropriate internal programming, it's possible to have it do most anything you want.