by William Lulow
Most people know these days that the light sensitivity of your camera’s CCD is now measured in numbers decided by the International Organization for Standards or ISO. In the days of film, the sensitivity to light of a film’s emulsion was decided by the American Standards Association (ASA), hence the ASA rating. You will remember that a “fast” film was rated at around 400ASA and a slow film, like Kodachrome was rated at 25ASA. You could “push” a film’s sensitivity by boosting its ASA. But you would pay for it with extra graininess caused by the underexposure and over-development.
In the digital age, CCDs are much more sensitive to light than emulsions ever were. Now, ISO ratings of 25,000 are not uncommon. The good news is that higher ISO ratings do not have to be accompanied by grainier images within certain limits. It is true that high ISOs do result in some extra “noise” in the images, but in many cases, that extra noise is tolerable depending on how the photographs will be used. One has to realize that by setting the ISO rating higher, you are still underexposing the image and you may have to alter it somewhat “after capture.” Occasionally, I have had to increase the contrast of these types of images during processing.
So, keeping all these things in mind, you are able to alter an image’s exposure by adjusting the ISO setting, the aperture or the shutter speed.
Here is an image shot with available light. The ISO setting was 2000. The shutter speed was 1/125th of a second and the aperture was f/5.6:
This exposure would produce an acceptable print at least to 8×10 inches, maybe even to 11×14.
I tried several other exposures with ISO settings of around 4000. At that level, I began to encounter a bit too much “noise.” Here is an example where just a small part of the total frame was used:
Here’s the whole shot:
Although the overall shot was acceptable as far as noise goes, there really was not enough light on the pianist’s face to make a blow up that would stand up to making an 11×14-inch print without the visible “noise.” To me, making a good print is really the goal. This image may be acceptable for viewing on a small screen, such as an IPad, but the aim of “photography” should be to produce an acceptable print.
So, my take away from this little exercise is that exposures of this nature are okay up to about ISO 2500. If you are looking to make good prints at anything more under-exposed, you simply won’t have enough light. By the way, the camera’s function menu was set to allow for the least possible noise with high ISO settings.