Fill Those Pixels With Light!

(This is a reprint of an article written for the North American Nature Photography Association)

It’s no secret that the digital age of photography has arrived. More and more articles are being written about the pros and cons of digital equipment, digital processing, and so on…often ad nauseam. Inevitably there will continue to be the comparisons made between using film and digital media to capture and process our images.
One such comparison often referred to seems to be with regard to film ISO and digital sensor ‘speed’ ratings. As a part of these discussions many times there appears to be a direct comparison being made between film ISO and digital sensor ISO settings. In reality there are subtle differences between the two.
While film emulsions are prepared with the ability to capture light at different speed ratings digital sensors have one inherent 'speed' of capture ...and increasing the ISO on a digital camera can only increase the gain in the amplifier circuitry in the camera (just as turning up the knob on your radio will increase the volume you hear from the speakers). It does not make the digital sensor "faster", it just amplifies more the information the sensor has captured. Doing so (if the increase is high enough) also increases certain types of digital noise (just crank up the volume on that distant radio station and listen to how the static is brought up as well). When the ISO of a digital sensor is increased both the noise inherent in the circuitry and the image data captured by the sensor are amplified. Unfortunately the higher the ISO the more certain types of noise information will be increased with respect to image data. Every digital camera has a certain amount of ‘floor noise’ present all the time. Longer exposures (no matter what the ISO) are noted for increasing this ‘floor noise’ due in part to heat build-up in the sensor and associated circuitry. In a perfect world a digital image taken at high ISO and/or longer exposure would be perfectly exposed and without any noise artifact at all. The problem is that in the real world when ISO and/or exposure are increased so is the inherent noise which is present in every digital system. The end result is a lowered Signal To Noise Ratio (SNR)…more noise information compared to image data. In other words the image data is there but it is 'covered up' by noise. We see it as those pesky little specks of goofy looking pixels especially apparent in the darker portions of a digital photo. Noise in these darker areas is more obvious because in the white areas of the image the sensor received more input and the resultant SNR in these areas is higher (in effect letting the image data cover up the noise).
Fortunately there are things we can do to help keep noise levels down. As 'digitists' we are being taught to 'expose for the highlights’... in other words move the Histogram of a particular image as far to the right as possible without losing image information. Sensor pixels measure light quantity and the more light filling them the more valid image information available to the camera circuitry. This in turn means a higher SNR. Most of the higher end cameras have significant ability to record amounts of light… commonly 5-6 f/stops…some even higher! Not using that capacity to measure light levels fully will always result in an image with lower SNR…sometimes to the point of making an image useless. For several technical reasons it would appear the general consensus is that less noise is produced by increasing exposure within a particular ISO than by increasing the ISO itself. Some feel that certain types of noise inherent in all digital cameras are apparently more affected by changes in ISO than they are affected by longer exposures at a lower ISO. Either way the object remains…. ‘fill the sensor pixels with light’.
Many of today’s cameras have noise reduction software built into them. In some cases it may be possible to use specific programs to try to reduce image noise, but usually with varying degrees of success. Often these programs are better at removing certain types of image noise than others. Most people would agree I believe that having to try to remove noise in any way will result in a photograph of lesser quality than the same image would appear without the noise.
My own personal experience has me now looking at the Histogram (RGB histograms in my camera) as much as I look at the jpeg image on the camera LCD. I shoot in RAW format so if any exposure changes are necessary as a result of moving the Histogram to the right I can make them in my RAW converter. And on an image with lower noise at that! I want to make sure those sensor pixels are working to the full extent of their ability to record light!
So...if we were shooting film and found ourselves in a low light situation we could just use a faster film and adjust exposure accordingly... but ‘cranking up’ the ISO on a digital camera may not be the same thing when it comes to the resultant image and the amount of noise it contains. I realize there are many ‘digitists’ who only want to take photographs and not be bothered with the ‘techie’ side of photography. I’m sort of that way myself. But…show me a good photographer and I’ll show you someone who at least understands the basic tools of his or her craft (and probably someone who understands a great deal more of the technical side of things than he or she admits).
In a particular situation ‘exposing to the right’ may not be a big deal, but what if it turned out to be so? I believe the best rule of thumb would be to try and ‘fill those pixels’ to the max.