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Just like a ~40mm lens (35mm FOV equiv.) combines somehow the 35 and the 50mm lenses, a ~30mm lens combines a 28 and a 35mm. The lens that pops into my mind in tis regard is Pentax’s Limited 31mm f:1.8 lens.
Two years ago, Panasonic launched the 15mm Leica DG lens. On a micro four thirds camera would cover about ~30mm FOV (35mm equiv.). Yes, using degrees would be more precise but in most cases using the 35mm film (full frame) equivalent is more easy to figure it.
The 15mm f:1,7 Summilux sports, unlike other Zuiko or Lumix lenses (or other m4/3 dedicated AF lenses), an aperture ring and of course the Leica badge and a higher (about ~25%) price like its most closest competitor, Olympus’ M.Zuiko 17mm f:1.8 lens. Nevertheless it is a good price for the quality it delivers 😉