Uebelhart . Net

Okay, unless you want to know about f/stops and exposure time, this post will not interest you. For you other photogs:

A camera (whether film or digital) depends on one main thing for a picture: the amount of light that gets to the film or sensor.
Two major things control the amount of light that gets there: the f/stop and the shutter speed. One other thing plays a roll as well once the light is there: the ISO rating.

The f/stop (or aperature, which is the size of the opening the light passes through) is a number like f/2.8 or f/8. The larger the “number” than the smaller the opening (buckshot and medical needles are rated the same way). Thus an f/8 is a smaller opening than an f/2.8.
The sizes (from largest to smallest) generally go: 1.0 1.1 1.2 1.4 1.6 1.8 2.0 2.2 2.5 2.8 3.2 3.5 4.0 4.5 5.0 5.6 6.3 7.1 8.0 9.0 10 11 13 14 16 18 20 22 25 29 32 (and more smaller)
Each of these steps are actualy a 1/2 factor from each other: i.e. an f/stop of 5.0 actually lets in half the light of 4.5. (the actual math, and numbers, have to do with circle areas, inverses, pi, and square roots)

The shutter speed obviously reflects how long the shutter is open to let the light through.
General Shutter speeds (from fastest to slowest): 1/4000 1/3200 1/2500 1/2000 1/1600 1/1250 1/1000 1/800 1/640 1/500 1/400 1/320 1/250 1/200 1/160 1/125 1/100 1/80 1/60 1/50 1/40 1/30 1/25 1/20 1/15 1/13 1/10 1/8 1/6 1/5 1/4 0.30 0.40 0.50 0.60 0.80 1 1.3 1.6 2 (note there can be faster or slower speeds depending on your camera)
Like f/stops, each step in the shutter speed results in a 1/2 increment of light: i.e 1/2000 of a second of exposure is half as much light as 1/1600ths of a second. Don’t ask me why.

The ISO rating depicts the sensitivity of the film to light.
Normal ISO ratings are : 100, 200, 400, 800, 1600, 3200
The larger the number, the more sensitive it is to light. And like the other 2 number systems, they are in 1/2 step increments: ISO 100 is 1/2 as sensitive to light as ISO 200.

Got all that down? Good
But why are you telling me this?
Well….

You need an optimum amount of light to reach the film/sensor to get a good picture. Too much light and it becomes overexposed (washed out, too bright, a large white blob, etc). Too little light and it’s underexposed (too shadowed, dark, a grey/black blob, etc).
So you adjust the amount of light getting to the sensor (or read by the sensor with ISO) by adjusting the f/stop and shutter speed. Once you get that perfect balance, then knowing the numbers above can help you adjust it for other desires.
If the conditions of the light and day make an image perfect with a setting of f/5.6 and shutter speed 1/1250 great. But what if you wanted a smaller f/stop (i.e larger number) to get a better Depth of Field (DoF: that’s another lesson)? Say like an f/11? That’s 6 f/stop steps (say that fast!) smaller, thus alot less light will get through and you’ll have a darker picture. So to get the same amount of light you determined was best earlier, you’ll have to slow the shutter down by 6 steps, so 1/320.
That’s an easy concept, eh?
An to through ISO in, you can move it a steps in either direction to compensate for one of the other factors.
So if your original f/5.6, 1/1250 shot was done at ISO 100, and you wanted to adjust the f/stop to f/11 like above (6 stops), you can insteas slow your shutter down 4 stops to 1/500, and raise your ISO 2 stops to ISO 400.
Got it?
I hope so.

Now still, you may be asking where all this leads? Well there is an old and tried formula used by photographers for centuries, well decades anyways, called the “Sunny 16 Rule”.
This rule basically says that the normal exposure for a normal picture taken on a bright and sunny day is f/16 with a shutter speed of 1/ISO (1 over the ISO rating - so for ISO 100 than the shutter is 1/100).
This gives you the starting point to adjust all the numbers in the directions needed for more light, less light, or equalization.

Did you learn something?

I hope so, since this took awhile to write out!