The balance between ISO and shutter speed.

When photographing wildlife, sports, or anything moving, shutter speed is an important parameter to consider. Low shutter speeds risk blur from both subject and camera motion. Cooperative subjects, panning, and vibration compensation (in-body and lens) may allow for slower shutter speeds. A good strategy is to photograph the subject in the desired manner and play the resulting image on the sensor. Zooming in on the shot will confirm the viability of that choice.

Higher shutter speeds are often necessary but are difficult to achieve given the amount of available light and the minimum aperture value of long lenses. Most of us are not in the position to purchase long, fast lenses as their cost can exceed ten thousand dollars. The only alternative then is to adjust the camera's ISO to achieve the required shutter speed. Higher ISO numbers mean faster shutter speeds can be achieved. Although this seems like a simple solution, there are potential repercussions.

Altering ISO changes the amount of necessary light falling on a sensor to create a correct exposure. Higher ISOs mean that a sensor has to "work harder" to produce an image. The truth behind this is more complicated and has to do with signal-to-noise ratios; this won't be discussed in this blog. The "harder" a sensor has to work the poorer the resulting image is. However, slow shutter speeds often result in poor images because of blur; neither is desirable. So, what is the solution?

It is important to strike a balance between the two variables. The above image shows two exposures; one with an ISO of 400 (bottom) and the other with an ISO of 160,000 (top). The images on the right are enlarged sections to provide a clearer sense of how the image is affected. The bottom image has a reasonable shutter speed of 1/250th of a second. When equipped with vibration reduction technology, this is fast enough to prevent camera-related motion blur regardless of the focal length of the lens (in most cases). It is generally fast enough to freeze subject motion (I prefer to shoot at 1/500th if possible).

The top image is sot at 1/5000th of a second. This will freeze motion from all sources and, in fact, approaches the duration of a flash burst. Any blur from motion is eliminated but image quality takes a hit from artifacts introduced because of high ISO. Detail has been lost because of the presence of signal noise.

Noise is the result of electronic sensors being pushed to record faint data. Instead of pixels being accurately created to represent colour and density, the information becomes distorted. The colour and density of each pixel become more and more uncertain as ISO values rise. There are a huge number of factors involved in the level of adulteration. Suffice it to say it is important to keep ISO low enough to achieve the minimum desired shutter speed.  

Finding the balance between these parameters is a matter of trial and error. Low ISOs may negatively impact shutter speed while high ISOs may result in too much noise. Pick a pair of settings and test their effectiveness at producing a good shot. Remember to press "play" and then to "zoom" in. Thumbnails can be very deceptive alone. Notice that the left images above do not look all that different. When you find the correct balance you can shoot knowing the results will be something to look forward to.

Thanks for reading.   www.ericspix.com