ISO - And related side effects

In a simple conversation, there are three elements a photographer controls to capturing the correct exposure.  There is shutter speed that controls how long light is exposed to the sensor.  There is the aperture setting that controls how much light is exposed to the sensor through the lens.  And you have ISO that controls how sensitive the sensor is.  The combination or balance of these elements determine how successful the photographer is with capturing the scene.

Before I get a mass revolt to this post about motion blur and Depth of Field (DOF), this is going to stay with the subject of ISO (and related side effects).  I will say that motion blur and DOF are artistic elements that factor in to a shot.  I just want to stay a bit more on the the techie side in the hopes that if you understand how ISO works in a digital environment, the artsy part will come easier.

What is ISO?

ISO is a very technical term that is used in a lot of engineering environments.  International Standardization Organization.  Kid you not.  For photography: https://www.iso.org/committee/48420.html

What this covers:
Standardization primarily, but not exclusively in the field of still picture imaging - chemical and electronic - including, but not limited to:

Definitions for still imaging systems; 

Methods for measuring, testing, rating, packaging, labelling, specifying and classifying the dimensions, physical properties and performance characteristics of media, materials and devices used in chemical and electronic still imaging;

Specifications and recommendations of logical and physical characteristics, practices, interfaces formats and metadata for still imaging capture, processing, and output systems;

Methods, measurements, specifications, and recommended practices for storage, permanence, integrity and security of imaging media and materials, and imaging materials disposition.

What this means:  Simply, when a photographer sets 100 ISO in the camera the expectation that the sensor will perform the same in Nikon, Sony, Canon, Pentax, etc.  Sounds more like a consumer protection value?  Yep.  The sensor is rated by its base ISO setting.  The base ISO setting is the lowest numerical value before you start getting to the LO settings if your camera has them.  For the prosumer class - it is usually 100.  That being said, my D90 base ISO setting is 200, the D500 is 100 and the D850 is 64.  As a consumer, this ISO value protects your buying decision.

What you really need to know is the value of your base ISO of your camera.  Have it set correctly on your camera's menus where it is needed.  All other ISO settings are really manufacturer stuff.

That's all well and good, but how does that help me in the field?  Why does it have a setting?  It does help in the sense that if you set a ISO setting of 200 in your camera that has a base ISO setting of 100, you are adjusting the sensor in a way that is expected and designed.

Moving away from the technical descriptions, how does this work in my camera?  And why does a higher ISO have all those squiggly lines in the image?

Hold the squiggles for a moment, we'll get there.  For dialing in higher ISO values you are making the sensor more sensitive to light.  These adjustments have the same effect as slowing the shutter speed or opening the aperture for controlling the light to the sensor.  

The light that is hitting the sensor at the base ISO is the highest quality light.  The sharpest image.  This is what the design engineers set as a target.  Colors are the best.  Tones are the best.  For each ISO setting above the base ISO, you trade quality for quantity.  More light for a lesser quality.  Images can be a bit softer with higher ISO.

Let's not get carried away with lesser quality light.  Each camera is different.  On my D500, ISO 1600 is very usable without post processing.  The D90 was about ISO 800.  Still looking for the D850 value.   With post processing software, you can add to the ISO value ceiling.

But, again, what are all those squiggly lines associated with higher ISO?  Simple answer - Noise.

For now, lets separate noise from ISO.  Noise is everywhere in digital photography.  Can't escape it.  For now, let's understand it.

First we have Shot or Photon noise.  Most of us common folk don't think of light at the molecular level.  To that extent, think of light interacting with the camera's sensor as tap water filling a cup.  Place the empty cup below the tap and start to fill.  Pretty violent, eh?  That is light hitting one pixel.  Think of many taps hitting many cups.  A little more violence.  Violence in this case equals noise.  Light does not hit the pixels evenly.  And what's more, the less light that is available to the sensor, the more violent the light on the sensor.  Add more light, decrease the light violence on the sensor.  You can lower the ISO.

Another source of noise comes from the camera.  Electronics.  With each new generation of camera, the wayward electronic fields are controlled or decreased.  This is the noise that can cause 'hot' pixels.  This is more prevalent in longer exposures.  A bright cluster of pixels in a sea of cool pixels.  Think nighttime shots.

In the end, what happens is the camera and the photographer need to add as much light into the photo to overcome noise.  

I've had discussions with some in the club who think that any shot over the base ISO is no good.  I, of course, beg to differ.  In any situation with less than perfect light, you have to choose your poison.  Trade a little of this for a little of that.  I'm always willing to give more ISO to get better speed and aperture.  Usually when I get in these discussions, my opposite number assumes that I start as ISO 1600 or so.  And that would be incorrect.  I would try to get as close to base as I can.  But.  In the case of shooting at an air show, I need (would like to get) speeds of greater than 1/2500 th of a second for shutter speed.  I like my aperture to be as wide open as possible.  If the photo is fuzzy, I can't fix it.  To me, speed and aperture will get me the the sharpest image to work with.  If there is a little noise or a little softness, I can work with it.  If I'm working with a train - and I need more DOF I may not be able to sacrifice that much shutter speed.  So I'm happy to come off the base ISO if I need to.  For most situations I find myself in.

The trick is to find the combination of shutter speed and aperture setting where the ISO overruns the noise, or can be easily corrected.   Practice, Practice, Practice.