ISO Control

Bradley Hill Spring, Chesterfield, NH

Getting It Right in the Digital Camera
My father handed down my first 35mm SLR at a time when
everyone was taking snap-shots with Instamatics. I know, I'm older than dirt, but I just got a $10 lifetime National Park Pass, so suck it!

Song in a Dark Czech Bar  : ISO 6400

Anyway, when I got the camera, I had to decide which speed of Kodachrome to use. I made the radical decision to forsake the time-honored ASA 25 and go with the new, lightening fast, ASA 64. My father worried that I was a communist, but that extra stop felt liberating. As film improved faster speeds became available, always with the trade-off of coarser grain. The increased sensitivity was great, but If you wanted to change film speed in the middle of a roll, you had to wind the old film back into the canister while trying to stop before the lead got sucked in all the way. I know what it feels like to open the camera and find that the lead is gone, and with it, the remaining exposures on the roll. The whole process was complicated, time consuming and wasteful. The result was that changing film to adjust speed for varying conditions was not a routine consideration. Exposure was almost always adjusted through the manipulation of just two variables; aperture and shutter speed and these two factors were unbreakably linked. For any specific aperture there is only one shutter speed that will result in the desired exposure, but the ease of adjustment of ISO in digital cameras has broken the chains of that strict inverse relationship.

With digital photography the adjustment of sensor sensitivity is achieved through the turn of a dial or tap of a button, making ISO the important third variable in controlling exposure. Through the adjustment of ISO, there is now a range of shutter speeds that can be used with a fixed aperture while keeping the exposure constant.

Nazca Boobie, Galapagos Ecuador;: ISO 1600

Conversely, when a fast shutter speed is needed to capture action, adequate depth of field can be preserved by stopping down on the aperture and compensating by increasing the ISO. The combination of easily adjustable ISO and the ridiculously high sensitivity of modern digital sensors has remarkably expanded our flexibility in controlling exposure. But this flexibiliy comes at a cost and that cost is noise.

Sensor Noise
As is true with film, higher ISO's lead to increased image noise. Newer sensors have greatly improved low light response making it possible to get quite serviceable images with surprisingly high ISO settings, but different cameras have varying abilities to handle noise. Even as it is essential to know how shutter and aperture affect exposure and image quality, it is also important to know how adjustments of ISO affect image noise and sharpness on your camera. The physics of shutter and aperture are essentially the same for all cameras, but digital sensors vary greatly and the only way to understand how your camera handles high ISO is to test it. I have to admit that, until recently, I have used the" shoot and pray" approach to high ISO, but after testing my Canon 5D Mark 2, I feel much more comfortable pushing the ISO when needed. The result has been better low light images and sharper action shots.

The Test

ISO Test Imag

1) First start with a scene that has a wide range of brightness. Dark areas are especially important since noise is most noticeable in these regions. If you wonder why this is true, check out my recent "Exposing to the Right" article.

2) Shoot a series of images using your full range of ISOs. Aperture preferred mode works best since it keeps the depth of field constant. The scene should be well illuminated to avoid long exposure noise from contaminating the images taken at low ISO. If possible, shoot in RAW for maximum flexibility.

3) In my test I first I brought each image through Adobe Camera Raw with identical settings for optimal exposure, but without any sharpening or noise reduction. Zooming in on a small segment of the image, it is easy to see how noise increased with higher ISO. To my eye, results were quite good up to about ISO 800, but then progressively deteriorated up to the max of 6400. 

Image Detail, No Noise reduction

4) I then went back to the images and used Camera Raw's excellent noise reduction function to clean up the images. The noise reduction function in the new Lightroom 4 has also been improved. There are many other good software solutions for noise reduction, but in all of them the best final result is a compromise between noise reduction and the inevitable softening of detail. In my test, noise reduction expanded the acceptable ISO range by a step or two. 

Noise Reduction in Camera

A test like this can be a helpful guide, but the impact of high ISO on any specific image will vary depending on its brightness, the importance of shaddow detail and the final destination of the image. A large fine art print will have a narrower range of acceptable ISOs than a picture destined to be a small, low resolution jpg on Flickr.

As is always true, photography is defined by the compromises you make. The control and expansion of ISO made possible by digital photography, provides a great new creative variable to the complicated equation that makes up each image. The art is in how you balance the competing factors to achieve your own solution to the equation. It is just one part of getting it right in the digital camera.