What would the human eye’s T-stop be compared with its f-number?
Asked 10/1/2016
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If the human eye is modeled like a camera lens, its maximum aperture is often estimated around f/2.1 to f/2.2. Since T-stop accounts for light transmission losses, is there a reasonable estimate for the eye’s T-stop as well? In particular, does the eye’s cornea and internal fluid reduce transmission enough to make its T-stop meaningfully slower than its f-number?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
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Your eye can be treated exactly like a camera lens, and its maximum f/# really is around f/2.2 or so. The cornea has an index of refraction of around 1.33, so the fresnel reflections are around 2% or so. The reflections off of other surfaces are very small because of minimal index breaks.
T# = F# / T_avg over some bandwidth, so we have f/2.1 / 0.98. The T/# is around 2.15.
Originally by user40937. Source · Licensed CC BY-SA 4.0
user40937
9y ago
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Generated from our catalog & community — verify before relying on it.
A reasonable estimate is that the eye’s maximum T-stop is only slightly slower than its f-number: roughly T/2.15 if you assume an f-number around f/2.1.
Why: T-stop accounts for transmission loss. In the eye, the main loss is at the cornea/air interface. With the cornea’s refractive index around 1.33, Fresnel reflection is about 2%. Internal reflections are small because the refractive-index changes inside the eye are relatively small.
Using the approximation from the community answer: T ≈ f / transmission So with f/2.1 and about 98% transmission: 2.1 / 0.98 ≈ 2.15
So the eye’s T-stop is very close to its f-number, not dramatically different.
That said, comparing the eye directly to a camera is limited. Perceived low-light performance also depends on the retina’s sensitivity and the fact that camera sensors can integrate light over an exposure time, while the eye works differently. Age-related changes such as impurities or floaters can also reduce transmission somewhat.
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