35mm equivalent

• Digital 35mm equivalent cameras and 35mm film cameras consider lenses in the 21mm to 35mm range wide angle. Lenses less than 21mm are considered extreme wide angle for this film size.
  
  

Medium format

• Medium format cameras have a larger image size and require a longer focal length lens to fill the image area. Medium format cameras come in three major sizes: 6x4.5cm, 6x6cm and 6x7cm. This equates to wide-angle lens sizes ranging from 28mm ultra wide angle to 65mm wide angle.
  
  

Large format

• The most common large format cameras are 4x5 and 8x10 cameras. Wide-angle lenses for 4x5 cameras start in a range from 47mm to 125mm. The focal lengths of 90mm and 75mm are the most popular wide-angle lenses for 4x5 cameras.
  
  

Perspective

• According to lens manufacturer Tamron, "As the focal length becomes shorter in a wide-angle lens, this perspective difference expands making closely located subjects even bigger and remotely located ones even smaller (exaggerated perspective)."
  
  

Perspective control

• Manufacturers like Nikon and Canon make special perspective control or tilt/shift lenses. These lenses are designed to correct the distortion inherent in wide-angle lenses.