By Joseph Grosjean,
Photographer and Photograph Editor
In this issue we will discuss the final point on the exposure triangle, ISO, but first we must understand how the camera captures an image.
As discussed previously, light passes through lens elements at the front of the camera which redirects the light to a single point. When the light as this point is projected onto a surface it forms an image, which can be magnified by moving the positions of the individual lens elements. Before the light is project though it passes through an aperture which assists in controlling the amount of light which makes it to the camera body. The aperture can also further sharpen the light by shrinking the size hole. Finally, the light is projected on the back of the camera body where the shutter protects the film or image sensor is housed. When the shutter opens, and image is captured.
Camera film is essentially silver halide suspended in gelatin and spread on a plastic sheet. When the silver halide is exposed to light it darkens at a rate proportional to how bright the image is. In this way something bright such as the sun will appear dark while objects that are not as bright, such as the ground, will appear lighter. What we are left with is a negative of the image where all the brightness values are the reverse of what they should be. The photograph must now be developed to make the brightness values appear correctly. Because of the extreme light sensitivity of the silver halide it is important that the film is not prematurely exposed to light, which is why film is loaded in a dark room, the inside of the camera is kept dark by the shutter, and when the film is developed it must be done in a dark room to keep the image from further exposure. The reason for the red light is that red is a low energy light so any effect the light will have on the film will be minimal. On the other hand, a digital image sensor is a collection of digital light sensors which each capture a pixel. When assembled together the pixels form an image.
The sensitivity of the film was originally called ASA which stood for the American Standards Association. Today the sensitivity is referred to as ISO standing for the International Standards Organization. Both systems use the same scale, just different names, so ASA 100 is the same as ISO 100. ISO is a system used both on film and D-SLR’s alike. To change the sensitivity on a film camera, a new roll of film must be loaded in with the desired ISO value, whereas on a D-SLR the ISO can be selected from the camera’s shooting menu. This makes D-SLR’s far more versatile and require a lot less planning if you plan to shoot in some especially dark situations, like a club, a basketball court (they are basically dungeons), or the night sky.
An excellent place to start is ISO 100 which is used for outdoor photography. For gym’s I typically go for ISO 400-500, and it’s here that I stop for all but the strangest circumstances. As you increase the sensitivity of the film or image sensor, more and more noise is introduced into the image. You have seen this before if you have ever taken a picture in a dark room and have noticed dark specs all over your image, almost like static.
Now that we know the three points on the exposure triangle, aperture, shutter, and ISO, we can create a balanced exposure. There are a few ways to do this, one is trial and error, another is rules of thumb, such as Basic Daylight Exposure (BDE), and finally there are light meters. Next issue we will explore the viewfinder, light meters, and how to choose the settings on the camera. In the meantime, try going outside on a sunny day and using BDE, which uses the setting of ƒ16, 1/125s, and ISO 100.