Engineers would describe the camera as a “black box.”  You put a signal, in this case the object you are photographing, into the black box.  The black box transforms the input signal into an output signal, in this case the image.   The black box is a combination of the lens, the sensor, the electronics, as well as any intentional image processing that the camera does.  In general, we refer to this as “the system,” a fancy word for “the camera.”  There are many ways that the camera transforms the object.  Let’s consider some of them.”  The camera:

• takes a three dimensional object and flattens it to form the image
• typically makes the object smaller in the image
• pixelates the object
• distorts the object via its point spread function
• distorts the object due to various lens aberrations, such as fish-eye distortion
• if set to do so, may
  • sharpen the image
  • set the brightness and contrast
  • set the white balance
  • set the color or convert to monochrome

There is a lot going on, and we’ve already discussed some of it.  This engineer’s description can be a very useful one, as it enables the basis for a systematic language to describe what the camera does.

We have, in fact, without knowing it been using the engineer’ signal-processing by a black box” perspective in our discussion of image sharpness.  In creating this uniform language, engineers typically look at how a “black box” modifies one of three types of signal; each of which totally describes how the system transforms an object and forms an image.

• The first is to input an impulse.  This is our point of light
• The second is to input a so-called square wave.  This is our set of black and white lines
• The third is to input sine waves.   These are the spatial frequencies shown in Figure 2 of the last blog.

Each of these definitions are essentially equivalent.  Each will be useful as we further consider resolution and other camera properties.