If you find a combination of settings that gives you the magic "0" reading on your meter (which means the photo should be exposed properly), and then you want to change the aperture value to get more depth of field, it's fine -all you have to do is some elementary math:
You may have heard of a "stop" of light? That's what those numbers on your exposure meter are -"stops" of light.
(Your exposure meter looks like this: [-2...-1...0..+1..+2] although Nikon flips the numbers around.)
The handy thing about measuring light in "stops" is the fact that when you half or double the number you use for ISO, aperture, or shutter speed -that's a "stop". So, in this example, if you remove a "stop" of light by using f/8 instead of f/4 your photo will come out too dark (at -1 on your exposure meter). But, by doubling the shutter speed to ADD a stop, you'll be back at "0" (correct exposure)! But if we want f/16, then you're doubling the number twice, so we have to half the shutter speed twice, which gives us 1/250 at f/16.
All you have to remember is how the settings effect the light that hits your film or sensor:
* Shutter speed: you gain a stop of light by halving the shutter speed.
* Aperture: you gain TWO stops of light by halving the number.
* ISO: you gain a stop of light by doubling your ISO.
So, as an example assuming your ISO is set to 100 and doesn't change, 1/1000th at f/4 would be the same as all of the following:
* 1/500 at f/5.6
* 1/250 at f/8
* 1/125 at f/11
* 1/60 at f/16
* 1/30 at f/22
...if you wanted a shutter speed of 1/15 you can't double your aperture to 1/32 -that setting isn't available on most lenses. So, you'd have to get that extra stop of light by doubling your ISO instead.
So, assuming your aperture was set to f/4 and doesn't change, 1/1000 at ISO 800 would be the same as all of the following:
* 1/500 at ISO 400
* 1/250 at ISO 200
* 1/125 at ISO 100
If you're not one for math you could even just count the clicks up from f/4 to f/16, then change the shutter speed down by the same number of clicks to get back to correct exposure. ...the math really isn't hard though.
This may be hard to read and understand, but it's really easy once you do it a few times. A great way to see this for yourself is with this camera simulator:
One reason that you may want to know how to do this is night shots. For exposures longer than 30 seconds, you need to calculate the settings yourself. More info on this can be read in Part II of this article here.