2. Plain Exposure
Ok, now we are back to the basics. And now it gets
much easier, because
ISO sensitivity will simply remain fixed at the value selected by YOU. Period.
(That is, of course, until you yourself decide to change it for some good reason).
Let's just pretend we want to stay at the base ISO 200. So for now we can just set it there and then
forget all about ISO. Your camera will
not change or adjust your ISO setting.
Mode
A: Aperture Priority
You choose the Aperture to control depth of field (dof), and the camera automatically adjusts the Shutter speed. Bingo. That's it. See
p. 82 in your D90 Manual.
Try it out: Put your camera on a tripod and point it at a fixed subject. Rotate the Mode Dial to A: Aperture Priority.
Press the shutter button halfway and watch the aperture and shutter values in either the viewfinder or the exposure display.
Now rotate the
sub-command dial (on the front) to another aperture and watch how the shutter speed is automatically adjusted.
You don't need to take any pictures, just watch the values.
Mode
S: Shutter Priority
You choose the Shutter in order to either blur
or freeze moving subjects, and the camera automatically adjusts the Aperture. That's all. See
p. 81 in your Manual.
Try it out: Rotate the Mode Dial to S: Shutter Priority.
Press the shutter button halfway and watch the shutter and aperture values in the viewfinder.
Now rotate the
main command dial (on the back) to another shutter speed and watch how the aperture is automatically adjusted.
Don't shoot, just watch the values.
Mode
M: Manual Exposure
In Manual mode
you are completely in charge. The camera won't save you from underexposing or overexposing by adjusting
anything for you. See
p. 83.
Try it out: Rotate the Mode Dial to M: Manual mode.
Press the shutter button halfway and watch the shutter and aperture values in the viewfinder.
Rotate the
sub-command dial to select an aperture, and the
main command dial to select a shutter speed.
At least you can on a scale in the viewfinder
see whether you are underexposing or overexposing, but you have to do all necessary adjustments yourself.
In a way you now know what you need to know and understand. Because the rest is similar to this.