The fallacy of 35mm "equivalent focal lengths"

Started 3 months ago | Discussions thread
Mark Scott Abeln
Mark Scott Abeln Forum Pro • Posts: 13,433
Equivalence does not

Camera equivalence theory does not take into consideration any of these factors:

  • Megapixels
  • Lens aberrations
  • Sensor read noise
  • Camera processing or post-processing for the most part
  • Bokeh (the quality of blur)
  • Dynamic range
  • Highlight headroom
  • Shadow noise
  • Image aspect ratio
  • Sensor technology and quantum efficiency
  • Angle of view of lenses that aren't rectilinear
  • Lens transmissivity (T-stops vs f/stops)
  • Film vs. digital
  • Lens microcontrast and modulation transfer functions (MTF)
  • Changing camera positions or perspective changes
  • Color, contrast, saturation, or sharpness
  • Ergonomics

Equivalence only describes how photography changes with scale or size. It is pure geometry with a little physics and nothing else. It is simply an application of the geometric Law of Similar Triangles, or what is called isometry in the sciences. Equivalence is the stuff that's pretty much always guaranteed to be true, at least approximately, without regard to specific technologies or brands or times or practices, and it describes 19th century photography as well as 21st century photography, DSLRs, Mirrorless, large format, smartphones, etc., without bias.

Of course you can put any of the above factors into an equivalence analysis (and sometimes you have to) but then the theory becomes very specific and no longer generally applicable, and you risk "losing the forest for the trees", getting lost in specific minutiae.

 Mark Scott Abeln's gear list:Mark Scott Abeln's gear list
Nikon D200 Nikon D7000 Nikon D750 Nikon AF-S DX Nikkor 35mm F1.8G Nikon AF Nikkor 50mm f/1.8D +2 more
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