Home Photography 101 ISO Invariance - What is it and Why Do I Care?
ISO Invariance - What is it and Why Do I Care? PDF Print E-mail
Written by Edward Eastman   

What is ISO Invariance?

If your camera has an ISO dial, check to see if it has an H1, H2 and an L setting. If your camera does not have an ISO dial, look up ISO settings in the camera's menu and see if it lists H1, H2, L1 and/or L2 options. These are called "Extended" ISO values. These extended ISO values are not real ISO values, but are ISO values simulated in-camera by the manufacturers' in-camera firmware. I'd suggest you never use them.

Why are they there then? Great question.

It's a marketing ploy to convince the you, the buyer, to upgrade to a newer camera body. (But you didn't read that here!)

These values imply an increased sensor Dynamic Range. A lot of digital camera users have been asking camera manufacturers to design sensors that work allow them to take pictures in very low light, i.e., in side a church without using the flash.

In the last 4 or 5 years, sensor manufacturers have increased the available ISO values in an effort to meet the camera manufacturers' demands. (Did you know that not all camera manufacturers make their own sensors?) There has also been a demand to increase a camera's Dynamic Range. (Dynamic Range is the number of f-tops in a scene the camera's sensor can record.) As of this writing, Nikon D800E is the king, or queen, depending on how you look at life, with a Dynamic Range of 11.5 stops at ISO 50 - 200. (You see Dynamic range drops off as the ISO value increases, but more on that later.) The Canon 5D MKIV comes in at 11 Stops at ISO 50 - 200. (My Fuji XT-1 with a native ISO of 200 has a dynamic range of just under 10 Stops.)

Okay, so why all this fuss about Dynamic Range? This link gives you more info on Dynamic Range. And for a more in-depth discussion with sample images, click here.

As I have mentioned in many different posts, photography is all about give and take.

The best image quality is obtained by using the lowest ISO value. On my Fuji XT-1 that's ISO 200. On a Nikon or Canon DSLR it's 50.

Eventually you're going to want to take a picture where the light level is lower than 12.00pm noon on a sunny day, e.g. inside a church or some other such building where flash is not allowed. You have two options:

  • Mount your camera on your sturdy tripod

or

  • Increase the ISO value until you can take a handheld shot

(Of course, you could put your camera away and tell yourself you didn't really want that picture anyway.)

Lets assume you'll increase your camera's ISO value. So where does the "give" come in?

As you increase the ISO value, the resulting image will display more "image noise" starting in the shadows. (More info below too.) And as you increase the ISO value, the sensor's Dynamic Range is reduced. Therefore, the resulting image will not look as great as what your perceived. (To check out what happens as your increase the ISO value on your camera, click here and follow the instructions.)

Image Noise

What are the sources of noise in an image?

Noise #1

Photon noise originates from the scene you're photographing. Okay, stay with me for a bit. Without getting too technical, light is made up of photons. A scene has a range of tones from brightest to darkest (Dynamic Range). There are more photons coming from the bright areas of a scene than from the dark or shadow areas. Therefore, the stream of photons from bright areas are more steady or constant, while the photons from the shadows are more random. The more random the photons the more noise they create. So the most noise appears in the shadows.

Noise #2

Front-end read noise occurs in-camera. When photons strike a pixel site, an analogue electrical signal is created. Within that signal is dark-current noise. All electrical currents have dark-current noise, or static. The front-end noise or dark-current noise occurs at the beginning of the image capturing process.

Noise #3

Back-end read noise is the third type of image noise. Like front-end read noise, back-end read noise occurs in-camera. The image capturing process consists of two parts. Part one is the photons hitting a pixel site and creating an analogue current, which contains the front-end read noise. Part two is the analogue-to-digital conversion process and the amplification process. The back-end read noise occurs in the amplification process.

To summarize, these are the three types of noise that appear in every digital photo you take. The first two types - photon and front-end read noise - occur before the camera begins its analogue amplification process. The third type - back-end read noise - happens after the analogue amplification process.

How the Sensor Works

When the photons start hitting a pixel site, the site emits electrons. The emitted electrons build up an electrical charge. The more photons hitting a pixel site; the more electrons are emitted and the greater the charge.

The camera converts the charge into an analogue voltage and then amplifies that voltage. The amount of amplification or gain is directly related to your camera's ISO setting. When you use any ISO value higher than your base ISO setting, e.g. 50, 100 or 200, you're telling your camera to use a larger gain in the analogue amplification process. The amplified analogue voltage goes through an analogue-to-digital conversion process to covert the analogue data to digital information (a binary number). This is where the back-end read noise occurs.

The digital information (binary number) is what the computer reads to display your image. This binary number is the total data of all the pixel sites on your sensor.

So photons and front-end read noise are amplified in the analogue voltage amplification process; whereas the back-end read noise remains constant. Whew, that was a lot of technical stuff.

Brightening a Dark Image

Lets assume you have an underexposed image on your computer screen. What happens when you use software to brighten it?

The binary number mentioned above represents your digital image. In Lightroom (LR), moving the Exposure slider to the right brightens the image. The more you move the slider to the right; the brighter the image gets. What LR is doing is simply multiplying the binary number by a factor. What the factor is doesn't really matter. It's the concept that's important.

Extended ISO Values

I'll use my Fuji XT-1 to discuss this content.

The Fuji XT-1's native ISO range is from 200 - 6400. The ISO dial indicates a H1 and a H2 ISO value and an L ISO value. The H1 would theoretically give me an ISO value of 12800; and the H2 would give me an ISO value of 25600.The L would give me a ISO value of 100. The problem with these three ISO values is they are only theoretical or simulated; they are not real ISO values.

The H1, H2 and L ISO settings only perform digital amplification of the highest real ISO value on your camera. Using my Fuji XT-1 as an example, if I set the ISO dial to H1 the camera uses the digital amplification process to multiply the binary number for the real 6400 ISO value to give me a brighter image. There is no image quality benefit from using the H1 ISO setting because the camera's digital amplification process simply multiplies the image binary number by a factor of 2. I can accomplish the same thing in LR by moving the Exposure slider to a value of 2.00.

The downside is the extended ISO values make it easier to clip highlights in a RAW image file. This further reduces Dynamic Range of your resulting digital image.

Remember, as you increase the ISO value on your camera, the sensor's dynamic range also goes down. For example, using my Fuji XT-1, the dynamic range at ISO 200 is about 9 f-stops. For each 1/3 Stop incremental increase in ISO value, the dynamic range drops until ISO 1600 is reached. At ISO 1600, the sensor's Dynamic Range is about 6.75 stops and remains close to that for the balance of the native range up to ISO 6400. The graphic below illustrates the point.

Fuji XT-1 Dynamic Range

Fuji XT-1 Dynamic Range

Practical Point of View

What does all this mean to me (and you) when actually creating images?

If I need to set the Fuji XT-1 to ISO H1, along with an appropriate Aperture and Shutter Speed setting, to get the proper image brightness, then I'll get a better quality image by using ISO 6400 and then using the LR Exposure slider to increase the image's brightness by 1 Stop. Remember, the H1, H2 and L ISO values are simulated ISO values.

I've done some testing of my Fuji XT-1. I found the image noise in an ISO 6400 image to be unacceptable to me, so the highest ISO value I use is 1600. (Ideally, the highest ISO value I should use is 800.) In post-processing in LR, I work with the Exposure slider, the Luminance - Noise Reduction slider and the Shadows slider. However, based on the fact that the XT-1s sensor's Dynamic Range decreases for each 1/3 ISO incremental increase I make, I try to use the lowest ISO value possible.

Since I mostly shoot hand-held candid photos these days, this is where I can rely on an IS (image stabilized) lens, or a fast lens (f2.8). When shooting landscape images, I use my tripod and always shoot at ISO 200 to maximize the sensor's Dynamic Range.

I've created a series of images with my Fuji XT-1 that demonstrate the above concept in greater detail. Click on the link and review the post.

 

Add comment


Security code
Refresh