f/stop, shutter speed and ISO

At the 27th January session, I talked about the three settings on the camera that manage the light that is used to make the picture.

Each of them is divided into “stops”:

f/stop (Aperture)

f/2.8 (Widest); f/4; f/5.6; f/8; f/11; f/16; f/32 (Smallest)

Each of those, as you step through them, halves the amount of light reaching the sensor at the back of the camera.

So you step Down from f/5.6 to f/8. “Step down” is one full stop, as shown above. Or you could “Step up” or “Open up” from f/5.6 to f/4 – letting in twice as much light.
DSLRs (Digital Single Lens Reflex) cameras usually also subdivide those into halves and thirds. But a stop is a full stop, not a fraction.

Aperture priority on a camera is set manually by turning to “Av” or “M”. Once set, the aperture can be selected – say f/16.
On “Av”, the camera will work out the shutter speed for you. On “M” you will need to enter that too, though the camera helps.

Shutter Speed

Shutter speed is a measure of the time the shutter is open. A change of a full stop again doubles or halves the amount of light reaching the sensor. Values include:

30 seconds; 15s; 8s; 4s; 2s; 1s; 1/2s; 1/4s; 1/8s; 1/15s; 1/30s; 1/60s; 1/125s; 1/250s; 1/500s; 1/1000s; 1/2000s; 1/4000s etc.

So you stop Down from 1/60s to 1/125s. The shutter is open for half the time, so half the light reaches the sensor.

Shutter speed is set on the camera by selecting “Tv” (Canon), “Sv” (Nikon) or “M”. Set Sv or Tv and the camera will work out and set the f/stop. “M” is set by you, as in f/stop, above.

ISO

ISO (International Standards Organisation) is a measure of the sensitivity of the sensor. It will be no surprise that a full stop change of ISO doubles or halves the sensitivity. Values include:

100; 200; 400; 800; 1600; 3200; 6400; 12800; etc.

So you would stop Down from ISO 400 to 200 – halving the sensitivity of the sensor.

ISO will have its own setting, or “A” – Automatic

Balancing Av; Tv (Sv) and ISO

Because a change to each of the controls results in halving or doubling the light used to create the picture, a corresponding change in any of them will leave the overall exposure the same. So changing Aperture from f/16 to f/11 AND Shutter Speed from 1/60s to 1/125s. Will leave overall exposure the same.

The creative use of that ability will be covered in a different note

Bob Hallsworth
Sandhurst Camera Club

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