Why is there an air gap between the lens and a digital sensor instead of index-matched glass or fluid?
Asked 10/24/2020
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If filling the space between a lens and a digital sensor with glass or an index-matching fluid could help very fast lenses, why do cameras still leave an air gap? Is it mainly because of optical design, focusing requirements, shutters/mirrors, or practical issues like temperature and sealing?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
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Since lenses work based on the difference of index of refraction across the interface as well as the curvature of the lens surface (the more the difference in I.R., the more the light rays are bent) putting a higher I.R material between the rear element and the sensor would reduce the effectiveness of the lens.
You could incorporate that into the design of the lens, but you'd need to use a substance with a very low coefficient of expansion across the temperature ranges the system would be used in. Can't have the fluid expanding and forcing its way into the lens proper, or contracting and either creating a vacuum bubble or suction on the lens mount/barrel. And a solid that tried to expand could conceivably crack the lens' rear element or the sensor.
Originally by user93023. Source · Licensed CC BY-SA 4.0
user93023
5y ago
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Mainly because camera lenses are designed to work with air between the rear element and the sensor. Changing that space to glass or fluid changes the refractive-index transitions, which changes how strongly the lens bends light. You could redesign a lens for that, but it would be a different optical system, not a simple improvement.
There are also major practical problems. Many cameras have traditionally needed empty space for a mirror and/or mechanical shutter. Focusing often also depends on changing the lens-to-sensor distance or moving internal groups, and a filled gap would complicate that.
A fluid or solid coupling would also create engineering issues: it would need excellent sealing, stable optical properties, and very low thermal expansion. Otherwise temperature changes could cause pressure, bubbles, suction, leaks, or even damage to the rear element or sensor stack.
So the air gap remains because it is compatible with lens design, focusing, and camera mechanics, while a filled gap would add complexity and cost for uncertain benefit.
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