
Contents
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Fundamentals Fundamentals
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Resolution Resolution
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The retinal sampling frequency The retinal sampling frequency
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The optical cut-off The optical cut-off
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The diffraction limit The diffraction limit
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Other optical defects Other optical defects
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Photoreceptor optics Photoreceptor optics
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Light absorption by photoreceptors Light absorption by photoreceptors
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Resolution and eye design Resolution and eye design
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Sensitivity Sensitivity
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The consequences of low photon numbers The consequences of low photon numbers
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Available photon numbers Available photon numbers
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Making eyes more sensitive Making eyes more sensitive
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Conclusions Conclusions
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Summary Summary
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Cite
Abstract
Eyes can be characterized by their resolution and sensitivity. Resolution is the fineness, in angular terms, with which the optical environment is sampled. Sensitivity is quantifiable as the number of photons a receptor receives when the eye is viewing a scene of standard luminance. Resolution depends on the sampling density of the retinal receptors and also on the quality of the optical image. This quality can be affected by defects of focus, and by spherical and chromatic aberration. It is ultimately limited by diffraction (interference of light waves in the image). The larger the aperture of an eye, the smaller the effect of diffraction. Because of waveguide effects, photoreceptors cannot be made narrower than 1–2 μm without compromising resolution, which means that improved resolution can only be achieved by increasing the focal length of the optical system. In dim light the ability to detect contrast is limited by the numbers of photons that receptors can obtain. The smaller the number of photons caught, the worse the statistical quality of the image. Photon numbers are maximized in high sensitivity eyes by the use of high relative apertures (aperture diameter/focal length) and wide receptors. However, wider receptors will compromise resolution. In general, either an improvement in resolution or an increase in sensitivity requires an increase in the size of the eye.
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