How much does ISO matter?

Started Jul 27, 2017 | Discussions thread
beatboxa Senior Member • Posts: 5,827
Re: I should know better

Lanidrac wrote:

I can't agree more. But when you see stuff posted that is so bogus and wrong, it's annoying.

I didn't follow that thread, but what was posted that is so bogus and wrong?

Here are my 2 cents on the topic.

There is a big difference between film & digital process flow and how ISO plays a part. In the film world, ISO is effectively set per roll--it is not a variable; so exposure must accommodate for the ISO. In the digital world, the opposite can be true: the ISO can accommodate the exposure.

So the first thing to determine is "is available light a constraint?" ie. Are we using 'exposure priority' or 'ISO priority'?

  • "Exposure Priority": When light is limited, ISO setting is not a variable we can control.
    The total light, a combo of sensor area x exposure (scene lighting + t-stop + shutter speed) is the primary constraint;
    and this total light combined with desired brightness determines ISO. In this sense, the ISO is like "brightness balance," conceptually similar to "white balance". It's just compensating for the difference.
  • "ISO priority": When light is not limited (eg. shooting still landscapes on a tripod), ISO can be controlled. The max SNR is the primary constraint;
    and setting the lowest ISO will subsequently determine the exposure.

That's step 1. On film, you are 'always' ISO-priority, while digital gives the option. This also implies that if you have limited lighting & a desired image brightness, you simply cannot control ISO.

For a given exposure (ie. light limit) & rendered brightness:

  • On some cameras, ISO does not matter. "ISO Invariance" Either setting the highest ISO or brightening a lower ISO in post will result in the same image.
  • On other cameras, the highest ISO that doesn't clip will be the best to shoot at. Brightening in post has a markedly different (and worse) result.
  • I am not aware of any camera where brightening the lowest ISO will provide better IQ than using a highest ISO.

'ISO rendering' is roughly the same per total amount of light, as determined by the combo of photosensitive area x exposure.

As I mentioned earlier, people often confuse causality and don't control variables--and this is where many misconceptions can come from. Here are examples:

2 stop ISO difference (1600 vs. 6400), from an ISO-invariant camera, with limited lighting. Lower ISO brightened in post:

Here is a difference frame: subtracting one of the above from the other:

Almost identical.

Here's another example, illustrating ISO invariance; and the effect of higher ISO vs. lower ISO. All of these images have almost identical exposure & sensor area.

(Was one of the most extreme examples I could find). ISO 6400 looks pretty much the same between the cameras. On the D750 (ISO Invariant), the ISO 100 additionally looks similar to ISO 6400 except a bit of color cast. But on the 5DIII, the ISO 100 looks the the same exposure. It is not ISO invariant.

Similarly, if we look across sensor areas, in this example, the Olympus has 1/4th the sensor area of the Canon. This is 2 stops.

We see that ISO 12800 is better on the Canon than Olympus. But the Olympus has 2 stops less total light at the same exposure.

And so (predictably), when we give the Olympus 2 stops more exposure so that the total light between the Canon @ 12800 is the same as the Olympus @ 3200, the image quality is almost identical.

EDIT:  I just wanted to add in one more point for emphasis:  In all examples & both 'priorities' above, the common element is that you are maximizing exposure for your scene, equipment, and scenario.  A vast majority of the time (except when I have 'unlimited' light), I cannot control ISO.  It is determined for me.  So why bother?  Control the exposure and shoot at the ISO you need.

And so (no offense to the OP), I think the ISO question gets asked too much and is too often misunderstood; while a much more impactful question is:

"How much does total light matter?"

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