ISO Numbers & Digital Sensors
by William Lulow
Since I do a fair amount of concert photography, I was curious to see what effect higher ISO numbers would have on the images. So, I began to experiment. In the past, when I did concert photographs with film, I would purposely underexpose the film by two or three stops and then make up for it with high speed, extra fine-grain developers which would bring out some extra detail in both the shadows and the highlights. Remember, when you use higher ISO (or ASA in the old days) numbers, you are essentially UNDEREXPOSING your images. You are trying to add light where there really is none by assuming that the CCD “sees” more light than it normally does. So, something has to give.
With digital sensors, I have found that underexposing the sensors by as many as five stops (ISO100 to ISO1000) doesn’t seem to add appreciably to the amount of “noise” or “grain” visible in the images. It’s not until you begin to approach ISO2000 that you begin to see the image exhibit more noise than I normally like in my photographs. What I’m looking for is to be able to produce an 8×10 print that has an acceptable degree of sharpness and detail in both highlights and shadows.
Here are a couple of images shot at ISO1000:
Both made acceptable 8×10 enlargements. I used to make 16×20″ enlargements of my concert photographs and they were grainy when shot with film, but contained what we used to call “sharpness in grain.” That is, they were sharp even though a bit grainy.
Here are a couple of images shot at ISO100:
You certainly can’t tell the difference on the screen, but you can when the images are enlarged to 16×20″.
This brings me to my next point about exposures and ISO numbers. If you are shooting primarily for use on websites (a computer), you may just as well shoot at ISO1000 or ISO2000 if you don’t require any prints!
Here are two images of singer/songwriter Iris DeMent, shot in concert a few days ago:
They are the exact same shot, but the one on the right was made from an 8×10 print! It’s not a perfect copy shot, but it was pretty close to the original and was made using my iPhone! I was fairly impressed with the quality of the print given that the image was shot at ISO2500 at 1/100th of a second and f/4.5. (All my concert shots are made with aid of a monopod for extra stability).
My point to all this is that ISO numbers are pretty forgiving these days. Sensors are much more sensitive to light than any film was and it is a lot easier to make a decent exposure for almost any purpose. My rule of thumb is that if there is plenty of light, I always use ISO100. If you are shooting in any kind of low light situation, begin with ISO1000 and then see what you need to make an acceptable image. Keep in mind, of course, that the higher the ISO setting, the more “noise” you will have in your image. Also, remember that you shouldn’t really make images with most digital lenses wide open. In order to obtain maximum sharpness, you need to stop down at least two stops.