Fine Grain at a High ISO? It Can Happen!

As a digital photographer I am constantly concerned about the horrendous havoc of digital noise. Because I don’t shoot with a six thousand dollar camera, there is a distinct threshold at which I could risk ruining a picture because I am at an ISO that lends itself to the distractingly ugly mish-mash of oddly coloured pixels in my shadow areas. For a long time I would stay clear of this threshold, no matter the circumstances, and aim to shoot under 800 ISO.

Recently, however, a friend gave me some very good advice that proved the contrary.

He said that it is better to shoot at a high ISO and get a perfect exposure then to underexpose at a low ISO and brighten the image in post production. Anyone who shoots in ambient light must often battle to stay above hand held shutter speed, but I was never prepared to sacrifice my ISO sensitivity and, because of this, often ended up with underexposed images. I had presumed that because sensors at low sensitivity have lots of time to gather light information, Images could be adjusted as needed without falling apart.

So I tried it out.

Here are two pictures I shot of the same scene. One is shot at ISO 200 and is two stops underexposed and the other is shot at ISO 1600 and is two stops over.

Now I adjusted both images in post processing to be of equal density and zoomed in on a portion of the shadow areas. Even brought down two stops, the ISO 1600 image at close detail looks better in terms of digital noise.

The noise is greatly decreased and the pixels are smoother, they don’t have those chunky blue-ish splotches.

The reason for this is that digital sensors are designed to be more sensitive to highlights than it is to shadows. The human eye itself is always more concerned with what is going on in the lighter parts of a scene than the darker. Engineers design sensors to have 50% of their pixel’s sensitivity at the brightest stop of its latitude, 25% at the second brightest, and so on, leaving a whopping 3% sensitivity in the darkest stop. All this equates to more information in the highlights, more information to recover, and an image that is easier to save than one that is underexposed… even at the ‘worst’ ISO I have.

The rule that was drilled into my head in school was to ‘push your whites as far to the right of the histogram as you can without going over’. Now I can add the part ‘even at the expense of your ISO’.

In the race for resolution of today’s digital world, it is much more important to know your camera and how it operates best than it is to criticize it for poor performance. I have been reminded once again that, though my camera isn’t the best on the market, it still can do everything I need it to.