how to choose the right combination of shutter and f number

[cicopo]

See

http://www.canonoutsideofauto.ca/

[kecarlsonmi29]

I just saw this. Thanks for posting.

[ScottyP]

A CORRECT exposure is one that gives a properly illuminated image by selecting the 3variables in the "exposure triangle" of shutter speed, aperture (f/number) and ISO setting (sensor sensitivity).

There are usually about 6 correct exposures for any image.

It is up to the photographer to pick the RIGHT exposure from among all the possible CORRECT exposures to give he look he wants.

The shutter and the aperture both affect how much light the sensor gets. The Shutter does this by leaving the camera open a longer or shorter time. The aperture does it by opening up wider or pinching down narrower. You can trade one against the other and get the same amount of light in your exposure. Double the time the shutter is open and you can halve the opening size (aperture). Or switch that around and double the aperture opening size in order to let you cut the time the shutter is open in half. This ability to trade one against the other is called "equivalence".

The reason you would pick one possible correct exposure instead of another is to find the "right" one for your desired look. The shutter speed and aperture affect the look of the image in certain ways. Aperture dictates the depth of field in focus. Shutter speed either prevents or allows motion blur.

APERTURE: A low f/number is a wide aperture, and it produces a narrow depth of field in focus, to force attention on your subject and away from background or foreground elements. A high f/number is a narrow aperture opening, and it gives you a deep depth of field, where everything in your image may be in sharp focus if that is what you want.

SHUTTER SPEED: A short shutter time will freeze action. A long shutter time will allow blur, so moving water looks like smoke, or so people walking through your shot of a public building/monument don't even appear in the image because they didn't stay in one spot long though to register an image.

To make the swapping of shutter speed for aperture opening size easy to do, they measure both in units called "stops", which are doubling or halving the amount of light. You can trade one stop of shutter time for one stop of aperture size (or 2 for 2, or 5 for 5, etc., etc.) and your exposure is still correct, but you get the effect you are looking for whether it be a certain depth of field or whether it be a sharp freeze of motion or deliberately created blur.

[ScottyP]

As far as how to use this, I would use shutter priority (TV mode) when shutter speed is most critical to the look I wanted, and I would use aperture priority (AV mode) when depth of field in focus is most important to the look I am after.

In TV mode you set shutter speed and the camera then selects the aperture needed to give a correct exposure. You may need a fast shutter (1/640th, or 1/1000th of a second, for example) to freeze a football player so you set that speed. Or you may put the camera on a tripod and set a long shutter opening time (like 30 seconds for example) to turn a waterfall into a cool river of smoke.

In AV mode you pick an aperture (f/number) and the camera figures out the right shutter speed for you so you have a correct exposure. You might set a low f/number (a wide aperture) so your little girl's eyes are in sharp focus but everything else is out of focus so viewers are paying attention to her eyes. Or you may really want to shoot a city street scene where everything is in sharp focus, from the car right in front of the camera to the building 3 blocks away, so you pick a narrow aperture (high f/number) for that.