1.4 Focal Length

Focal length is how much the lens is zoomed in or out. The reason this factors in to exposure calculation is twofold:

1) When you zoom in, both the image and your hand shakiness are amplified. And so when you’re in Program mode (or either Auto mode), the camera will tend to choose faster shutter speeds when you’re zoomed in all the way to try to counteract the shakiness.

2) Most zoom lenses change their maximum f/stop slightly as it zooms in, necessitating a longer shutter speed. (Kind of counteracts number 1), above.)

Shutter Speed and f/Stop tradeoff – there are many different possible f/stop / shutter speed combinations that will let in an identical amount of light for any given scene. You can use a combination that emphasizes frozen subjects (fast shutter speeds), or everything in focus (small lens openings).

1.5 Tradeoffs

• When you increase the shutter speed:

–You freeze motion

–You let in less light (the picture looks darker)

• When you make the f/stop smaller (i.e., increase the number):

–You increase what’s in focus

–You let in less light (the picture looks darker)

 

The relationship between these variables can be seen graphically on the previous page.

1.6 Program Shift

When evaluating the appropriate combination of f/stop and shutter speed to use for a given amount of light, you will find that more than one combination of these variables will produce a correct exposure. For example, any of these combinations below will provide the same amount of light for the sensor:

Shutter speed	f/stop
1/1000 (fast)	f/2.8 (wide open)
1/500	f/4
1/250	f/5.6
1/125	f/8
1/60	f/11
1/30 (slow)	f/16 (small)

 

Which combination to choose depends upon the kind of picture you’re taking – for example, if this were a sporting event, you would tend to use 1/1000^th at f/2.8 (because the faster shutter speed stops the action), whereas if you were doing a cityscape you would tend toward smaller f/stops and use 1/30^th at f/11, because the smaller aperture produces the greatest depth of field. But when you’re in “P” Program mode, the camera will choose the combination somewhere in the middle, like 1/250^th at f/5.6 (the yellow row in the table above), because it has no idea what kind of picture you're trying to take. Average is safe.

Another way to tell the camera to “use the faster shutter speed combination” or “use the smaller f/stop” is the Program Shift. It’s not a mode; it’s what the rear control dial does by default when the exposure mode dial is set to “P”.

To use it, just rotate the rear control wheel (or the top Control Dial if you haven’t reassigned it as I have) while in Program mode. This will tell the camera to choose another equivalent f/stop and shutter speed combination that will let in the same amount of light.

Program Shift is a very handy function which lets you change to a different f/stop / shutter speed combination from what the camera recommended very quickly, without having to press a lot of buttons or turn a lot of dials. One control dial rotation is all it takes.

TIP: Program Shift doesn’t work when you’re using flash.