#10. "RE: Aperture, Shutter, ISO, and Illumination Relationships" In response to In response to 9 Thu 15-Sep-11 02:29 AM by HBB
Brian and Darrell:
Thank you for the critique. Always nice to have divergent views.
Following comments are not meant as a refutation of your individual philosophies, merely an attempt to clarify mine.
How much simpler our photography world would be if illumination were a constant; everywhere, all the time, much like the constant velocity of light itself. Then, we could ignore it completely. Fortunately, such is not the case.
Agreed: Illumination is not a camera function, per se. Illumination is, however, a variable that is measured by the camera's metering system and included in the solution of the reflected light exposure equation described earlier in this thread, any time it is in one of its automatic modes. Illumination is also measured by the camera and included in the calculation any time a photographer is in full manual mode, and attempting to "zero" the reflected light meter, using combinations of the aperture, shutter speed, and ISO variables. In this latter case, the metering system may also be used to select deliberate under or over exposure combinations.
From a slightly different perspective, the three camera variables: Aperture, Shutter Speed, and ISO are used to control what ... ? Illumination, the fourth variable of the exposure equation, as follows:
Aperture: Amount of light passing through the lens per unit of time. Small aperture = less light. Large aperture = more light.
Shutter Speed: The amount of time light is allowed to reach the film/sensor. Fast shutter speed = less light. Slow shutter speed = more light.
ISO: The relative sensitivity of the film/sensor used to capture light. Slow ISO = more light required and generally higher image quality (Higher signal to noise ratio). Fast ISO = less light required and generally lower image quality (Lower signal to noise ratio).
Given the integral, intimate relationship between the three camera variables and illumination, it is logical to me to include it in the above chart.
Outdoor ambient only illumination is varying constantly, with time of year, time of day, latitude, sun angle, cloud cover, fog, mist, etc. It can be varied/modified as needed with scrims, reflectors, and other devices. Illumination in the studio is also variable: number and power level of strobes, use of umbrellas, soft boxes, grids, reflectors, gobos, etc.
As stated earlier, in my personal view, illumination (light) is the sine qua non of photography: Without illumination, there would be no photography. Thus, I teach the the three basic camera functions and illumination at the same time, which integrates illumination into the thought process from the beginning, instead of introducing it later. Is this that much more difficult than adding illumination later? I've had no complaints to date, either from individuals or groups of thirty or more in classroom settings.
A conscious awareness of illumination as a variable in the exposure equation has made me a better photographer. I view illumination from a variety of perspectives, including; quantity, quality, color temperature, and others. One simple example: I learned that, with practice, I could suspend the physiological chromatic adaptation process that occurs when viewing a white shirt or sheet of white paper under different color temperature lights. Most people say that they both look white under all illumination sources. To me, they both look off-white or tan under tungsten, and blue/white under daylight. Simple example, but useful when considering white balance and other illumination issues.
From my admittedly peculiar perspective, I have a deep-seated need to understand things at their most fundamental, comprehensive, yes even theoretical, level. I am aware that this approach is not for everyone. We all have our own, individual, preferred learning style ... which is just as it should be.
Thanks again to both of you for your comments.
HBB in Phoenix, Arizona Nikonian Team Member
Photography is a journey with no conceivable destination.