How do different apertures affect photographs? [duplicate]

Question

Possible Duplicate:
What is aperture, and how does it affect my photographs?

As an amateur photographer, can someone explain to me how different apertures affect photographs?

Answer 1

At the intro level, aperture changes effect primarily two things:

1. Amount of light. Aperture as expressed in f-stops is the ratio of the optical diameter of the lens to its focal length. It so happens that this ratio tells you how much light the lens lets thru regardless of the particular lens. Not counting close-up photography, f/8 on one lens will give you the same exposure as f/8 with another lens.

   F-stops go in powers of the square root of 2 because that is the multiple that causes a change of 2 in the light. Common f-stop sequences are 2.0, 2.8, 4.0, 5.6, 8, 11, 16, 22. Each of these lets thru half the light the previous one does. In photography, sometimes "f-stop" is used when really just a factor of 2 in brightness is meant as a result.

2. Depth of field. This is a measure of how far front and back of the distance you focus at the objects will still be sharp. Some lenses have this marked for whatever they think "sharp" means. However, regardless of what your limit for sharpness is, smaller apertures (higher f-stop numbers) will allow for higher depth of field (wider range within minimum sharpness threshold). Depth of field is also a function of the focal length. A long lens will have less depth of field at the same f-stop as a shorter lens.

There are other effects of aperture, like diffusion at very high f-stops, and the f-stop brightness rule breaks down as you get towards macro, but the above basics should get you going well enough.

It can be educational to learn to judge exposure on your own. This is rarely necessary with today's automatic cameras, but the experience will give you some insight to what the camera has to do when you give it different situations. Put your camera on full manual and take some practise shots. The basic rule of thumb is that a normal brightness subject in normal sunlight is exposed well at the shutter speed set to the ISO value and f/16. Adjust from there mentally. Hazy sky is 1 f-stop down, cloudy bright another 2 f-stops. You'll be surprised how dim indoor light is. Try it.