The DoMeBo Photography Class
Lesson #3 – “How Big and How Fast?”
OK, boys and girls, this will be my feeble attempt to explain exposures, shutter speeds and apertures. If I fail to get the point across (and I just know I’ll fail to get the point across), you are free to peruse the many, many other attempts at the same thing. A Google search will get many hits, and on these very boards there have been previous posts on the subject.
Now… to action!
“Exposure” refers to the act of letting light hit light-sensitive film for a brief amount of time. For us digital photographers, the film is replaced with a light-sensitive chip, but the idea is the same. Exposure is used to refer to the amount of light captured by the chip and the lightness or darkness of the resulting image. This is controlled by three things: ISO (film speed), shutter speed, and aperture. Aperture also controls how much of the picture is in focus, called “depth of field.” More on that later.
Here are some examples of different exposures:
Papa Bear took this picture of mrs.gnu. It’s underexposed. Aperture at f/2.8, shutter speed at 8 (1/8sec.).
Mama Bear took this picture of a rock stack on a beach. It’s overexposed. Aperture at f/7.1, shutter speed at 125.
Baby Bear took this picture of Jiminy Cricket. It’s just right. Aperture at f/7.1, shutter speed at 320.
OK, OK. I took all three pictures. Ssshhhhhhhh.
- - - - - - - - - - - - - - - - - - - - - - - - -ISO (Film Speed)
This actually stand for “International Organization for Standardization,” not anything to do with film speed directly. The ISO merely standardized the numbers. In them olden days of film, there was still something kind of like pixels used, call “grain.” The size of the individual grains determined their sensitivity to light; the bigger they were, the faster they could be exposed. The flip side of the coin is resolution; faster film lost detail and if you embiggened faster film you could see individual grains.
In digital photography, the ISO setting on your camera is an arbitrarily-generated speed setting on the image chip that does a fairly good approximation of the film speed numbers it’s emulating. You still get the trade offs of resolution and speed, but the graininess of digital is pretty ugly.
For a ridiculously detailed description of film speeds, get thee to Wikipedia.
What the numbers mean:
The higher the number the faster you can take a picture, the lower the number, the better your resolution. Each increase is a doubling of the speed, meaning 800 film takes half the time to expose than 400 film. 100, 200, 400, and in some cases 800 are the most common for normal people, like you and me. I recommend keeping you ISO set in this range. If you have all the time in the world, set it lower, but you probably won’t see much of a difference unless you print it really big.
To be continued...