Chromatic and monochromatic optical resolution in the rainbow trout

Area of Science:

• Comparative physiology
• Vision science
• Aquatic optics

Background:

• Fish vision is adapted to aquatic environments, often with large pupils and specific optical challenges.
• Longitudinal chromatic aberration (LCA) can significantly impact image quality in animal eyes.
• Understanding optical limitations is crucial for interpreting animal visual capabilities.

Purpose of the Study:

• To calculate the modulation transfer function (MTF) for rainbow trout (Oncorhynchus mykiss) eyes, specifically considering longitudinal chromatic aberration (LCA).
• To assess how LCA limits visual resolution in trout compared to the diffraction limit.
• To investigate the relationship between optical aberrations and visual acuity in fish.

Main Methods:

• Calculated the MTF for adult rainbow trout eyes under daylight conditions, accounting for LCA and cone pigment absorption.
• Assumed unaberrated optics apart from LCA.
• Modeled the impact of receptor length, pigment density, water depth, and pupil size on chromatic resolution.

Main Results:

• LCA significantly limits the resolution of the adult rainbow trout eye to levels far below the diffraction limit.
• The measured monochromatic MTF closely matches the MTF calculated from LCA alone, indicating a balance of aberrations.
• Low visual acuity in fish is consistent with optical limitations imposed by LCA, potentially a trade-off for a brighter image.

Conclusions:

• Longitudinal chromatic aberration is a primary factor limiting visual resolution in the rainbow trout eye.
• The optical design of the trout eye represents a trade-off between image brightness and resolution.
• The findings on chromatic aberration and MTF are likely applicable to other teleost fish with similar eye sizes (lens > 1 mm).