Day Two, Learning Two

 

Yesterday I wrote about Sunny 16 and the relationship between f-stop, shutter speed, and ISO.  If you shoot with a DSLR you probably already know this next bit (I understood it “generally”).  But it’s really the basis of so much of photography that it bears re-visiting.  Because you really must have this concept at your fingertips.

Reciprocals.

A reciprocal in photography is how the aperture, shutter, and ISO relate to each other in terms of f-stops.  An f-stop is the doubling or halving of the amount of light recorded on your camera’s sensor.  With shutter speed it’s simple:  double the shutter speed and you’ll half the amount of light.  So a shutter speed of 400 allows half as much light to reach sensor as a shutter speed of 200.  ISO is equally simple, only in reverse:  an ISO of 400 allows twice as much light to be collected on the sensor as an ISO of 200.  So:

Shutter speed of 200 + ISO 100 is exactly the same exposure as Shutter Speed 400 = ISO 200.  The shutter allowed half as much light to reach the sensor, but the ISO allowed double the amount of light to be recorded.  Apertures work on the same principle, only the numbers are a bit whacky.  In order to double the light:

Aperture f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32.

Each one is a doubling of the amount of light that reaches the sensor.  Each step is called a “stop” of light.  Notice that every other number is double: f/2 becomes f/4 becomes f/8 and so on.  Likewise, f/1.4 becomes f/2.8 and so on. 

Which brings us to the answer to yesterday’s quiz:  On a sunny day where f/16 at Shutter Speed 100 and ISO 100 will produce a perfectly exposed picture... in order to shoot at f/5.6 you’ll need to increase your shutter speed to 800.

Exercise:  Snap a picture in any mode you like (Manual, Aperture Priority, Shutter, etc.).  Check the settings and switch to Manual Mode.  Play around with adjusting our aperture while compensating with the shutter.  Do it without checking the exposure meter in the viewfinder.

Reciprocals are the basic building blocks for mastering photography.  But I promise the rest of my 100 learnings won’t be so technical.