Aperture & Focal Length

Check out this informative Video & take notes (click on the link below):

Choosing the Right Lens Focal Length

How Focal Length Affects Viewing Angle:

Different focal lengths produce different viewing angles, which in turn create different images. In addition, different focal lengths also produce varying levels of perspective and depth of field. As focal length changes, the amount of subject captured by the lens (viewing angle) also changes. 

Lenses are typically grouped into 5 categories (you should familiarize yourself with these):

1) Ultra-Wide Angle (a.k.a. Fish-eye) lenses range from 14mm to 20mm

2) Wide Angle lenses range from 24mm to 35mm

3) Standard lenses range from 45mm to 70mm 

    (these are similar in field of view to the human eye)

4) Telephoto lenses range from 80mm to 300mm 

    (allow you to "zoom in" close & compress the background)

5) Ultra-Telephoto lenses consist of any focal length greater than 300mm

For more detailed information...check out the link below:

Understanding Focal Length