Why use Prime lens for Mirrorless Camera

LEICA M-SYSTEM

SIZE MATTERS
The world’s most compact professional camera

Constructed with a depth of less than 42 mm and an extremely quiet shutter, the M-Cameras are ideal partners for travel photographers and photojournalists.

source from Leica official website.

Why M-system, it is RF camera, that means the Flange focal distance (FFD) are closest to the EVIL / Mirrorless camera which is about 20mm. Then you can enjoy the best optical performance from the lens.

The Leica M mount
(incomplete list)
With the Leica M:

• 15mm f/8 Hologon
• 18mm f/3.8 Super-Elmar
• 21mm f/1.4 Summilux
• 21mm f/2.8 Elmarit
• 21mm f/3.4 Super-Angulon
• 21mm f3.4 Super Elmar
• 21mm f/4 Super-Angulon
• 24mm f/1.4 Summilux
• 24mm f/2.8 Elmarit
• 24mm f/3.8 Elmar
• 28mm f/2 Summicron
• 28mm f/2.8 Elmarit
• 35mm f/1.4 Summilux
• 35mm f/2 Summicron
• 35mm f/2.5 Summarit
• 35mm f/2.8 Summaron
• 35mm f/3.5 Summaron
• 40mm f/2 Summicron
• 50mm f/0.95 Noctilux
• 50mm f/1.0 Noctilux
• 50mm f/1.2 Noctilux
• 50mm f/1.4 Summilux
• 50mm f/1.5 Summarit
• 50mm f/2 Summicron
• 50mm f/2.5 Summarit
• 50mm f/2.8 Elmar
• 75mm f/1.4 Summilux
• 75mm f/2 Summicron
• 75mm f/2.5 Summarit
• 90mm f/2.0 Summicron
• 90mm f/2.5 Summarit
• 90mm f/2.8 Elmarit
• 90mm f/2.8 Tele-Elmarit
• 90mm f/4 Elmar
• 90mm f/4 Macro-Elmar
• 135mm f/4.5 Hektor
• 135mm f/4 Tele-Elmar
• 135mm f/2.8 Elmarit
• 135mm f/3.4 APO-Telyt

Multiple Focal Length Lenses Leica M

• 16mm,18mm,21mm f/4 Tri-Elmar
• 28mm,35mm,50mm f/4 Tri-Elmar

With the Leica CL:

• 40mm f/2 Summicron-C
• 90mm f/4 Elmar-C
• 40mm f/2.8 Elmarit-C

Elcan lenses

Ernst Leitz Canada made special military lenses under the ELCAN name:

• Elcan 50mm f/2, for the Leica KE-7a
• Elcan 66mm f/2
• Elcan 90mm f/1, apparently without helical, focused by extension rings

Konica lenses

With the Hexar RF:

• 21mm/35mm f/3.4 / f/4
• 28mm f/2.8
• 35mm f/2
• 50mm f/2 (*review of 50/2)
• 50mm f/1.2
• 90mm f/2.8 (*review of 90/2.8)

Minolta lenses

With the Leica CL:

• 40mm f/2 M-Rokkor
• 90mm f/4 M-Rokkor

With the Minolta CLE:

• 28mm f/2.8 M-Rokkor
• 40mm f/2 M-Rokkor (new version)
• 90mm f/4 M-Rokkor (new version)

Voigtländer lenses

With the Bessa T, R2, R2A, R3A, R4A, R2M, R3M and R4M:

• 12mm f/5.6 Ultra Wide Heliar Aspherical II ^[1]
• 15mm f/4.5 Super Wide Heliar II ^[2]
• 21mm f/4 Color-Skopar P type ^[3]
• 25mm f/4 Color-Skopar P type^[4]
• 28mm f/2.0 Ultron ^[5]
• 35mm f/1.2 Aspherical Nokton ^[6] with Armalite hood, the fastest ever production 35mm lens for 35mm cameras
• 35mm f/1.4 Nokton classic ^[7] (Available in MC Multicoated or SC Single Coated)
• 35mm f/2.5 Color-Skopar P type II ^[8]
• 40mm f/1.4 Nokton classic^[9] (Available in MC Multicoated or SC Single Coated), the fastest ever production 40mm lens for 35mm full-frame cameras^[10]
• 50mm f/1.1 Nokton^[11]
• 50mm f/2.0 collapsible Heliar
• 50mm f/3.5 Heliar
• 75mm f/1.8 Heliar^[12]

Other Cosina Voigtländer lenses are screwmount and need an adapter to fit on the later Bessa bodies.

Rollei lenses

With the Cosina made Rollei 35 RF:

• 40mm f/2.8 Sonnar
• 80mm f/2.8 Planar

Rollei also announced at one point that they were going to release a 28mm lens and a 50mm lens, but lost interest in the 35RF (really a Cosina Bessa R2) system.

Carl Zeiss lenses

With the Zeiss Ikon rangefinder:

• 15mm f/2.8 - Distagon T* 2,8/15 ZM
• 18mm f/4 - Distagon T* 4/18 ZM
• 21mm f/2.8 - Biogon T* 2,8/21 ZM
• 21mm f/4.5 - C Biogon T* 4,5/21 ZM
• 25mm f/2.8 - Biogon T* 2,8/25 ZM
• 28mm f/2.8 - Biogon T* 2,8/28 ZM
• 35mm f/2 - Biogon T* 2/35 ZM
• 35mm f/2.8 - C Biogon T* 2,8/35 ZM
• 50mm f/1.5 - C Sonnar T* 1,5/50 ZM
• 50mm f/2 - Planar T* 2/50 ZM
• 85mm f/2 - Sonnar T* 2/85 ZM
• 85mm f/4 - Tele-Tessar T* 4/85 ZM

Ricoh lenses

• 21mm f/3.5 GR 21