Exploring Low F-Number Camera Lenses: A Deep Dive into Optics and a Custom F0.38 Lens
0:00:04 Introduction to F-Number: The F-number of a lens is defined as the ratio of the entrance pupil diameter to the focal length. It determines the brightness of the image produced by the lens.
0:00:35 Fastest Commercial Lens: The fastest commercially used lens is the f/0.7 lens used by Stanley Kubrick in the movie "Barry Lyndon," which was famously lit only by candlelight.
0:00:56 Debunking the f/0.33 Lens: The f/0.33 lens by Zeiss is revealed to be a non-functional joke lens, created for marketing purposes.
0:01:10 Building an f/0.38 Lens: The presenter built a working f/0.38 lens using an Ethernet-based single-board camera from IDS.
0:01:33 Camera Setup: The camera is mounted in a 3D-printed carbon PLA frame with a fine-pitch slide adjuster and a kinematic mount with a 40x oil immersion microscope objective (numerical aperture 1.3).
0:01:45 Oil Immersion Objective: This type of objective typically uses oil to optically couple the objective to the slide. Here, it's used to couple the objective to the camera sensor.
0:02:20 Sensor Modification: A critical step involved removing the protective glass cover on the camera sensor without damaging it, allowing the oil to directly contact the sensor for optical coupling.
0:02:49 Challenges and Solutions: Removing the glass cover was a low-yield process. The presenter went through multiple cameras and sensors, eventually succeeding using heat and careful manipulation.
0:04:14 Test Footage Comparison: The custom f/0.38 lens was compared to a micro 4/3 camera with an f/1.4 lens, demonstrating that the f/0.38 lens is significantly brighter, especially in shadowed areas.
0:04:49 Unexpected Depth of Field: Despite the low F-number, the lens exhibited a surprisingly large depth of field due to its very short focal length (around 4mm).
0:05:56 Fresnel Lens Discussion: The presenter discusses a previous experiment with an f/0.5 Fresnel lens, which led to a debate about the theoretical minimum F-number in air (0.5).
0:06:29 Aberration and F-Number: The relationship between F-number and spherical/coma aberration is explored. A lens with significant aberration may have a less meaningful F-number.
0:08:52 Adding More Lens Elements: Adding more lens elements can increase light-gathering but also increases focal length, not necessarily lowering the F-number.
0:09:17 Limiting Factor: The minimum achievable F-number is limited by the distance between the final lens element and the focal plane.
0:09:45 Numerical Aperture and F-Number: The relationship between numerical aperture (NA) and F-number is discussed. The precise formula is debated, but the approximation NA = 1 / (2 * F-number) is generally accepted.
0:13:34 Aplanatic Lens: A lens corrected for spherical and coma aberration is called aplanatic. In such lenses, the principal plane is curved, affecting the relationship between NA and F-number.
0:14:51 Role of Coupling Oil: The coupling oil matches the refractive index of the final lens element, allowing light to enter at steeper angles and maximizing light collection on the sensor.
0:15:24 Conclusion and Future Plans: The presenter plans to build a larger oil-coupled lens for a micro 4/3 camera to achieve a shallow depth of field with high light-gathering ability.
0:15:38 Complexity of Optics: The presenter highlights the difficulty in understanding seemingly simple optical questions without sufficient background knowledge.