Inside Photographic Lenses

The filmmaker's YouTube channel, Filmmaker IQ, has an excellent introduction to lens properties for still and motion picture use. Note that while they talk of 35mm and smaller formats, the same principles to larger film formats, also. For example, the approximate normal focal length for the following film sizes are roughly: Medium Format

• 6x4.5: 75mm
• 6x6: 85mm
• 6x9: 110mm

Large Format

• 4x5: 160mm
• 5x7: 220mm
• 8x10: 325mm

These values are simply derived from the nominal size of the format and Pythagoras.

This video on how lenses work is well worth the 20 minutes to view.