An online book of Common Sense Photography, by Rhett Stuart

Camera Sensors in SLR Digital Cameras


The camera sensor is where the film used to be. Remember that room with the small hole in one wall discussed in Camera History that would let the light in? Well, they figured out how to make a small box with a lens to focus the light onto a ground glass at the other end and would trace the image that was projected on the ground glass. That was an advance in technology! They didn't need a room with an entrance in the floor. Then somebody figured out how to make light sensitive film that would record the light without having to trace the image on paper. Then they made better quality film so the film size could be made a smaller size at 35mm and still look sharp. After that some people figured out how to record an image on a light sensitive electronic sensor. Now the film industry is being replaced by the sensor industry.

Your eye has light sensitive sensors in it. The rod and cone cells are on the back of the eye in the retina and act as electronic light sensors. The 120 million rod cells can just tell light from dark, and the 6 million cone cells can tell what color it is. Most of the cone cells are located in the center of the retina. Your vision is much more acute in the center of what you are looking at. Try to read a road sign without looking directly at it. If you really think about it a lot, it can be a little disturbing.

Okay, this is getting complicated, but hang in there. Film was rated on how "fast" it was. The higher the number of the film rating, then the faster the film was. Remember how everything doubles or cuts in half the amount of light in photography? Well, film speeds work the same way. Film speeds start at 25, then 50, 100, 200, 400, 800, 1600, and 3200. For example, 400 ISO speed film is twice as sensitive to light as 200 ISO speed film. ISO stands for the International Standards Organization, and they are responsible for international quality standards.

25 ISO film had a lot of detail, but the problem was 25 ISO speed film wasn't that sensitive to light so a slower shutter speed or aperture was needed to allow more light onto the film. Sometimes there wasn't enough light, so using a faster speed film overcame that. Using 400 ISO speed film would allow more pictures with less light, and 1600 speed film was so sensitive it would allow a picture of somebody in candlelight! However, the film would be "grainy" at 1600 ISO due to it being so sensitive to light.

Sensors get "grainy" also at higher speed ISO's, but it is now called "noise". Also, "grainy" and "noise" have different textures. The only problem was hardly anybody used 1600 ISO speed film, it was expensive and it was too much hassle to change the film. Digital cameras make it a lot easier. Just change the ISO setting.

Digital sensors are designed to mimic film since all the photographers were used to how film was rated with the ISO speeds. Most SLR (single lens reflex) camera sensors have about the same noise level from 100 to 400 ISO. Most SLR cameras go up to 1600 and sometimes even 3200 or 6400 with out too much noise. So it isn't that big of deal what ISO you are using with digital SLR cameras. Compact digital cameras don't handle the noise nearly as well; at 800-1600 ISO speeds they have a lot more noise then digital SLR cameras.

Digital SLR sensors are 1.3, 1.6, or full size. For example, a 1.6 sensor is smaller then a full size sensor (or 35mm analog film) so to make the image the same size as a full size sensor; it has to be enlarged by 60%. The full size lenses are not fully used on 1.6 sensors, just the center part of the lens is needed to project the images onto the sensor. Since the image is enlarged, it has a telephoto effect. A 100mm lens now has the same angle of view as a 160mm lens. Wider angle lenses are not as wide on a 1.6 size sensor, a 17mm lens looks like a 27mm lens.

Do you think the current ISO standards to rate digital camera sensor sensitivity will be used now that digital has taken over? What's the horse power of your car? Ahem.