Are there theoretical limits to a lens’s maximum and minimum aperture?
Asked 8/27/2013
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Is there a theoretical widest aperture a lens can have (smallest f-number), and a theoretical smallest aperture it can stop down to (largest f-number)? Do these limits make physical sense, and are there any notable examples of extremely fast lenses?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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Physics plays a role in answering your question and that information is out there. The basics from that linked discussion are that the index of refraction of the lens material will affect the maximum aperture you can achieve, so for pure glass that has an index of refraction of 1.5, the maximum aperture would be f/0.5 or thereabouts. Better substances, such as diamonds, with an index of refraction of 2.417 can give you an aperture of f/0.235 with a corresponding insane cost of ownership (consider just how much a lens of pure diamond might cost). The lensmaker's equation is the basis for the numbers.
As for minimum aperture, you could basically get down to what amounts to an atomic size hole, large enough for one photon to pass through, but that's useless for, well, anything. For a lot of lenses, getting to some place around f/11 or higher results in loss of sharpness as a function of diffraction, so f/32 is about the top out point for 35mm lenses though they can get smaller for larger formats and do so. Pinhole lenses are often in the smaller range, as much as f/177 (Lensbaby has one like this). Still, even if the optics were perfectly able to handle something like this, consider what the ISO and shutter speeds would need to be to get an image, so at some point, the value of this is pretty much zero unless you're into blurry abstracts.
Originally by user472. Source · Licensed CC BY-SA 4.0
user472
12y ago
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Generated from our catalog & community — verify before relying on it.
Yes—these ideas make sense.
Aperture is usually described by the f-number, N = f / D, where f is focal length and D is the entrance pupil diameter.
- Minimum aperture (largest f-number): In theory, if you close the aperture completely, no light passes through, which is effectively f/∞. A tiny pinhole-like opening is possible, but it quickly becomes impractical because diffraction and lack of light make the image unusable.
- Maximum aperture (smallest f-number): In principle, making the entrance pupil larger relative to the focal length lowers the f-number, so lenses can go below f/1. In practice, physics, lens materials, optical design, and losses through large amounts of glass impose limits.
There have been extremely fast lenses. The often-cited f/0.33 Super-Q-Gigantar 40mm is generally regarded as a nonfunctional marketing stunt. A real, functional example is the famous Zeiss f/0.7 lens made in very small numbers.
So: the smallest practical f-number is constrained by optical physics and engineering, while the largest f-number trends toward infinity as the aperture closes completely.
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