Slow ISO's mean more latitude for opening up shadows without noise.
In the race for speed and glory we may have forgotten an overriding consideration in photography. The ultimate image quality. The pursuit of high ISO performance seems to have clouded the judgement of both manufacturers and practitioners as to what the end clients of commercial photographers really want, and have always wanted: good, clean, sharp images of the product, service or people being presented in the advertising and promotion. Getting an "acceptable" image at 1600 ISO is not the same thing as getting a "perfect" image using ISO 100. Or ISO 50.
Every camera seems to have a base ISO at which the sensor is able to achieve both the lowest noise floor and the highest dynamic range. In fact, the two parameters go hand in hand. As the noise floor rises the dynamic range declines. The race to get to higher and higher ISO's has led camera makers to do strange things. Many sensors (such as the one in the Olympus OM-D and EP-3) now seem to be optimized to deliver their best performance at ISO 200. It's ultimately counter-intuitive as most of the lenses designed for the smaller sensor format seem to be near diffraction limited at nearly their maximum apertures. When used outdoors or in any favorable light (which is the predominant environment for the vast majority of picture taking) these cameras struggle with too much light. When shooting with an optical system like the Leica Summilux 25mm 1.4 I find that I struggle to use it at its real business end (aperture-wise) because I am limited by the highest shutter speeds of many cameras. Pity, since the lens really performs well around f2. And it has a wonderful aesthetic look at f2....
One maker who seemed to understand that low ISO's aided in getting quality images was, ironically, Kodak. They had a wonderful mechanism in their SLR/n camera with which you could set the camera to ISO settings of 12, 25, and 50 and the camera would do long exposures which were really a series of exposures stacked on one another with all noise anomalies cancelled out. The end result was files that could be printed to enormous sizes with high sharpness (partially lens dependent) and with absolutely no electronic or sensor noise. While the cameras had to be used on tripods and the exposures could run into the seconds the process was as easy as shooting with a view camera and that was something many pros did right up until their ultimate conversion to high res digital capture.
When I switched camera systems to the Sony SLT cameras I tested them at various settings. At first I was a bit disturbed by the high amount of noise present at ISO 3200 and beyond. I chalked it up to the price I had to pay to get 24 megapixels on an APS-C sensor. What I have now come to understand is that the engineers at Sony were/are working with a sensor, the performance of which peaks at ISO 50. Rather than being a design fault it is, in fact, a chance for us to go back to the practice of wringing ultimate quality out of our files instead of ambling down the path of least resistance and handholding our cameras at ISO 12,000 and wondering why the saturation and integrity of our files is....mediocre.
People say that the Sony sensor is really optimized at ISO 100 but they don't have any more objective information at their fingertips than me. I tested the camera at all the lower settings, up to 200, and I looked at them at 100%. I also looked at the DXO DR curve that showed 50 ISO as the highest differential between noise floor and signal. Doesn't matter if I'm 100% correct or if Sony has massaged the ISO 50 setting in a way that's similar to the way Canon and Nikon massage their high ISO's with software processing. All that matters are the resulting images.
I spent a large part of my professional career shooting black and white films like Agfapan APX 25 (ISO 25) and Ilford Pan-X (ISO 50) as well as color films like Kodachrome 25 and 64, Ektachrome 64 and 100 and various slow Fuji films because, when blown up big, they showed more detail, more sharpness and less grain than their faster brothers.
With the new Sony cameras I've started to hew back to the old ways. I'm finding that using a tripod and medium apertures will get you a very high impression of image sharpness. The Sony a77 also incorporates a unique setting called Multiple Frame Noise Reduction. The camera shoots six frames in a row at whatever ISO you want then micro-aligns each frame, kicks out all spurious and random noise, blends the files together and presents you with a noiseless image.
In studio portrait situations I'm pulling out bigger strobes or using longer exposures with continuous lighting. It's a different look.
I know there are times when a high ISO is useful. If you are shooting fast action in the low lights of the UT Swim Center you'll need to start at 1600 and go up to freeze fast action. To freeze a dive might require you to go into the 6400 ISO area. When you go to the summer Olympic this year you might need fast ISO for the indoor venues. And if you shoot NFL Football for a living you certainly don't need to confer with me about which lenses and what ISOs you'll need to use in some God foresaken taxpayer funded arena somewhere in the midwest. But I'm guessing it will be fast, long lenses and a high ISOs.
I use high ISO settings on cameras when I shoot theatrical dress rehearsals. But I don't need high ISO in the street, on the beach, in the mountains, at the outdoor pool or, with fast lenses, in most of the restaurants and coffee shops I patronize.
But lately I've seen just how different ultimate low ISO performance can look. We moved away from top quality as a compromise between camera handling and convenience. But it is a different look. Perfection is a different look as well. (not that we'll ever achieve it...).
I'm starting a movement. It may end up having only one full time member (me) but it's antithetical to the pervasive practice of photography today. I'm going to try to shoot every possible digital photograph at the optimum ISO of my cameras. In the case of the Sony a77's I've decided that the right number is 50. In a pinch I'll go to 64 or 80 or even 100. I'll practice my steadiest camera hold and try to optimize all the other parameters. That means shooting at f4 and f5.6 with most of my lenses. It means shooting the Leica lens on the m4:3rd cameras at f2 and f2.8.
I already try to shoot as much as I can on a tripod and I keep one in my car. I won't use it for street shooting because I'm more interested in being highly mobile than perfect but in genres where rapid response is secondary to vision I'll continue to press a good tripod into service.
For generations we worked easily with slower films and made images that stand the test of time (perhaps better than digital). There's no reason why we can't migrate these best practices back into our digital efforts. The end result might just be much better imaging. A slower and more thoughtful pace. Fewer reasons to join the seasonal hunt for the newest and greatest cameras.
And most importantly, prints and electronic images that you can be proud of. Not proud because you were able to pull something out of a bad situation with a compromise...but actually proud because you came closer to achieving the potential of your vision.
So, for me it's about good tripods and good glass. It's about letting subjects take time to settle. It's about finding and using optimum apertures and shutter speeds. Amazingly, there is a reason why the camera makers put all those different ISO settings on every camera they sell. While most people will race to the high ISO extreme the engineers know that most things look better at the other end of the "dial." Slow down and shoot better.
Finally, in marketing and advertising and art, if everyone is rushing to do things one way it's always more interesting to take the opposite point of view from the pack. Herds don't make art.