Several Basic concepts of photography

Angle of View

The same concept as field of view

35mm equivalent focal length

Crop factor

The termsCrop factorAndFocal length multiplierWere coined in recent years in an attempt to help 35mm film format

Slr photographers understand how their existing ranges of lenses wocould perform on newly introduced
DSLR cameras which had sensors smaller than the 35mm film format, but often utilized existing 35mm film format SLR lens mounts.

Ultra-wide lens designs become merely wide;
Wide-angle lenses become 'normal '.

Three basic Photographic Lens types (classified by relative focal length)

• Normal Lens
• Wide-angle_lens
• Long-Focus Lens

Long-Focus Lens

• Long-focus lenses are best known for making distant objects appear
  Magnified. This effect is similar to moving closer to the object, but is not the same, since
  
  Perspective is a function solely of viewing location.
• Two images taken from the same location, one with
  Wide angle lens and the other with a long-focus lens, will show identical perspective, in that near and far objects appear the same relative size to each other.
• Long lenses also make it easier to blur the background more, even when
  Depth of field is the same (???)

Four Thirds System