ISO has become a third variable in the exposure equation
Once upon a time, a high ISO was used as an absolute last resort in low-light situations. When the only choice a photographer had was to take a hit from excessive noise or to lose image sharpness, then photographers went with the noise. An image with noise, after all, is still better than an image with blur. This is no longer the case, plain and simple.
ISO ratings on a DSLR are mathematical expressions of how sensitive your sensor is to light, given in equivalence to film ISO ratings. The visual impact of noise, however, historically associated as a characteristic of each increasing ISO stop, has been mitigated in the last couple of years. Practical digital photography is maturing into its teenage years, and more advanced technology has led to more sophisticated hardware and better image-processing techniques and algorithms.
The Canon EOS-1D Mark IV and the Nikon D3S are both offering workable images at an astounding ISO 102,400 equivalent, for instance. While shooting at levels like these is hardly the ideal for practical photography, the concept of image capture above ISO 100K exemplifies the leaps and bounds that manufacturers have made. Shooting above 100K is a worse-case scenario, but the worst-case scenario used to be ISO 1600, and then 3200, and then 6400.
At this point, even mid-range DSLRs are offering expandable ISO settings into levels previously reserved for the pros, and as the price of the camera goes up, the levels of noise go down. Bigger sensors provide higher signal-to-noise ratios, and when you add in more megapixels, noise reduction at the analog level and CMOS sensors with lower power consumption and better heat dissipation, plus quality noise-reduction software from Adobe, Imagenomic, Nik Software, PictureCode and others, we’re getting into science-fiction levels of clean imagery. There’s plenty of controversy about when exactly noise is noticeable in an image, but there’s no denying that current ISOs at 6400 and above have produced truly stellar results.
So, if noiseless images are possible at fairly high ISOs, why take images at low ISO at all? Choosing aperture will change depth of field, and choosing shutter speed will determine sharpness in an image, but the advantages of variable ISOs are a little more abstract. Without visible noise, changing ISO won’t alter the visual aspects of an image, but it does free you to play more with shutter speed and aperture. Ramping up ISO means that you can capture fast action and low-light shots without sacrificing extended depth of field. Shooting low ISO is also a way to expand shutter times, ideal for lengthening exposures for motion blur when it’s a desirable effect.
High ISO is also a great advantage for photographers who use large telephotos, as it helps to gain faster shutter speeds for hand held shooting with lenses that often have a large minimum aperture. Freeing yourself from a tripod is an incredible boon for photojournalists and photographers who must respond quickly to rapidly moving subjects. What’s more, ambient lighting often can be the appeal of a scene, and a strobe can kill ambient light easily, not to mention lugging artificial lighting sources around is cumbersome.
Freezing fast action in a shot and shooting in low light used to be a trade-off between shutter and aperture. Because high ISO has become relatively free of side effects, it also has become a consideration for making an exposure, just as the speed of film used to be. Obviously, digital is even better, as you can freely change your settings shot by shot, and who knows what may happen in the future? Longer exposures produce noise from a heating sensor and pixels. Ironically, it’s conceivable that someday using a higher ISO actually may result in less noise.