1. Form Factor and Design:

• Sony FX3:
  • The FX3 is designed with a compact and ergonomic form factor, resembling a traditional camcorder. It features a top handle with dedicated controls for ease of use in handheld shooting scenarios. The compact design is well-suited for solo operators or those working in tight spaces.
  • Fan for active cooling for longer shooting times
  • Button layout optimized for video
  • Multiple tally lights on front and back
  • 
• Sony A7S III:
  • The A7S III follows the design principles of Sony’s Alpha series mirrorless cameras. It has a more familiar camera form factor with a built-in electronic viewfinder and a tilting touchscreen. The design is geared towards versatility, catering to both photographers and videographers.
  • Requires cage/rig for mounting accessories
  • Optional XLR Adapter needed for advanced audio
  • No active cooling, overheating potential 

Sony FX3 is clearly a winner in this category! 

2. Sensor and Resolution:

• Sony FX3:
  • The FX3 sports a 12.1-megapixel full-frame sensor. The lower resolution is a deliberate choice to enhance low-light performance and provide larger pixels for improved sensitivity.
• Sony A7S III:
  • The A7S III features a 12.1-megapixel full-frame sensor, also optimized for low-light conditions. The slightly higher resolution may be advantageous for photographers looking to capture still images with more detail.

3. ISO Performance:

• Sony FX3:
  • With an extended ISO range of up to 409,600, the FX3 excels in low-light shooting conditions, making it ideal for professional video production in challenging environments.
• Sony A7S III:
  • Similar to the FX3, the A7S III boasts an impressive ISO range, providing excellent low-light performance for both video and still photography.

4. Video Capabilities:

• Sony FX3:
  • The FX3 is designed primarily as a cinema camera, supporting 4K video recording up to 120fps. It offers 10-bit 4:2:2 internal recording and includes the S-Cinetone color profile, providing a cinematic look out of the box.
• Sony A7S III:
  • The A7S III is known for its exceptional video capabilities, offering 4K recording up to 120fps. It supports 10-bit 4:2:2 internal recording, and its extensive range of picture profiles, including S-Log2 and S-Log3, caters to professional filmmakers.

5. Autofocus:

• Sony FX3:
  • The FX3 features Fast Hybrid AF with 627 phase-detection points and 89% coverage, providing reliable autofocus for video and stills. However, it may not match the advanced Real-time Eye AF found in the A7S III.
• Sony A7S III:
  • The A7S III incorporates an advanced autofocus system with 759 phase-detection points and Real-time Eye AF for both humans and animals. This makes it an excellent choice for solo shooters or those capturing dynamic scenes with moving subjects.

6. In-Body Image Stabilization (IBIS):

• Sony FX3:
  • The FX3 does not have in-body image stabilization. Stability relies on lens stabilization or external stabilization solutions.
• Sony A7S III:
  • The A7S III features 5-axis in-body image stabilization (IBIS), providing enhanced stability for handheld shooting. This is beneficial for both video and still photography, especially when using lenses without built-in stabilization.

7. Audio Inputs:

• Sony FX3:
  • The FX3 includes two XLR/TRS combo jacks for professional audio input. This is advantageous for users who require high-quality audio recording in their video production.
• Sony A7S III:
  • The A7S III has a standard 3.5mm microphone input and a headphone jack. While it supports external microphones, it does not have built-in XLR inputs like the FX3.

8. Built-in ND Filters:

• Sony FX3:
  • One standout feature of the FX3 is the built-in electronic variable ND filter. This allows for seamless and precise control of exposure without adjusting aperture or shutter speed.
• Sony A7S III:
  • The A7S III does not have a built-in ND filter. Users need to rely on external ND filters to control exposure in bright conditions.

9. Build and Durability:

• Sony FX3:
  • The FX3 is constructed with a durable magnesium alloy body, providing robustness and effective heat dissipation during prolonged shooting.
• Sony A7S III:
  • The A7S III also features a robust build with weather-sealing for protection against dust and moisture. It is well-suited for various shooting conditions.

10. Storage:

• Sony FX3:
  • The FX3 uses CFexpress Type A and SD UHS-II memory cards for storage.
• Sony A7S III:
  • The A7S III supports dual card slots for CFexpress Type A and SD UHS-II memory cards, providing redundancy and flexibility in storage options.

Conclusion:

The choice between the Sony FX3 and Sony A7S III depends on the specific needs and preferences of the user. The FX3 is tailored for professional video production, offering a compact cinema camera design with features like built-in ND filters and XLR inputs. On the other hand, the A7S III provides a more versatile solution, excelling in both video and still photography with advanced autofocus, in-body image stabilization, and extensive video capabilities. Users should consider their primary use case, whether it be dedicated filmmaking or a combination of photo and video work, to make an informed decision. Always check for the latest updates and user reviews for the most current information.