For a well exposed photo the film needs to see a certain amount of light, not too much or too little. You control the light reaching the film by selecting a combination of aperture and shutter speed that permit the correct amount of light to reach the film.(posted 8675 days ago)Usually, if the lighting conditions are not too extreme, several combinations of f-stop and shutter speed are available to choose from. Each increase in shutter speed halves the amount of light. Each increase in aperture size (that is, moving to a lower number f-stop) doubles the amount of light let in. So: f8 at 1/125 sec lets in the same amount of light as f11 at 1/60th, and so forth up and down the scale.
Due to the laws of Physics, smaller apertures (the larger f-numbers) produce greater depth of focus. Larger f-stops produce shallower depth of focus (also called depth of field).
You get to choose which way you want to expose your film. If you want the photo of your friend to be sharp but the background to be blurry so as not to distract the viewer's eye, choose a large aperture and focus on your friend. If you want everything from six feet away to the horizon to be sharp you will need to use a small aperture. Either choice requires that you adjust the shutter speed to keep the light delivered to the film constant.
Most camera lenses have a scale on the top of the lens that lets you estimate the depth of focus for each f-stop. It is a very handy tool, one that I use frequently, and it's right there, built-in, and free.
OK?