Fixing the “crop-factor” issue

We use the term “cropped sensor” only due to the desire to describe a sensor in terms of the 35mm standard. It is a relative term which compares two different types of sensor, but it isn’t really that meaningful. Knowing that a 24mm MFT lens “behaves” like a 48mm full-frame lens is pointless if you don’t understand how a 48mm lens behaves on a full-frame camera. All sensors could be considered “full-frame” in the context of their environment, i.e. a MFT camera has a full-frame sensor as it relates to the MFT standard.

As mentioned in a previous post, the “35mm equivalence” is used to relate a crop-factor lens to its full-frame equivalent. The biggest problem with this is the amount of confusion it creates for novice photographers. Especially as focal lengths on lenses are always the same, yet the angle-of-view changes according to the sensor. However there is a solution to the problem, and that is to stop using the focal length to define a lens, and instead use AOV. This would allow people to pick a lens based on its angle-of view, both in degrees, but also from a descriptive point of view. For example, a wide angle lens in full-frame is 28mm – its equivalent in APS-C in 18mm, and MFT is 14mm. It would be easier just to label these by the AOV as “wide-74°”.

It would be easy to categorize lenses into six core groups based on horizontal AOV (diagonal AOV in []) :

  • Ultra-wide angle: 73-104° [84-114°]
  • Wide-angle: 54-73° [63-84°]
  • Normal (standard): 28-54° [34-63°]
  • Medium telephoto: 20-28° [24-34°]
  • Telephoto: 6-20° [8-24°]
  • Super-telephoto: 3-6° [4-8°]
Lenses could be advertised using a graphic to illustrate the AOV (horizontal) of the lens. This effectively removes the need to talk about focal length.

They are still loosely based on how AOV related to 35mm focal lengths. For example 63° relates to the AOV of a 35mm lens, however it no longer really relates to the focal length directly. A “normal-40°” lens would be 40° no matter the sensor size, even though the focal lengths would be different (see table below). The only lenses left out of this are fish-eye lenses, which in reality are not that common, and could be put into a
specialty lens category, along with tilt-shift etc.

Instead of brochures containing focal lengths they could contain the AOV’s.

I know most lens manufacturers describe AOV using diagonal AOV, but this is actually more challenging for people to perceive, likely because looking through a camera we generally look at a scene from side-to-side, not corner-to-corner.

AOV98°84°65°
MFT8mm10mm14mm
APS-C10mm14mm20mm
FF16mm20mm28mm
Wide/ultra-wide angle lenses

AOV54°49°40°
MFT17mm20mm25mm
APS-C24mm28mm35mm
FF35mm40mm50mm
Normal lenses

AOV28°15°10°
MFT35mm70mm100mm
APS-C45mm90mm135mm
FF70mm135mm200mm
Telephoto lenses

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