Field-of-view:

Almost all frog species possess large, protruding eyes positioned on top of the head. This orientation provides almost a full 360 view of the world with considerable bifocal overlap in the central 90 degrees or so. In contrast to humans with forward-facing, bifocal vision, bifocal vision in frogs is oriented upward and extends all the way to the rear of the animal. This type of vision is especially helpful for frogs that sit at the water’s edge, allowing them to constantly monitor in all directions for predators or food.

Image: Fig. 1. from Fite 1973: Perimetric maps of anterior and posterior fields-of-view as measured for Rana pipiens and Rana palustris, showing right and left eye's visual field and region of binocular overlap.

Depth-of-field:

Image: Microscopy image of the eye of a wood frog (Rana sylvatica) tadpole.

In general, frog vision is limited to close range. Ranid frogs, like bull frogs and green frogs, have a difficult time distinguishing between size and distance of objects farther than 6 inches away (Ingle and Cook, 1977).

Interestingly frog vision changes remarkably over time. Frog species often exhibit two-part life-stages, beginning life as an aquatic larva before metamorphosing into a frog. Thus, frogs have evolved to live in, and see in, two very different environments. As tadpoles, the lens is round much like a fish. During metamorphosis, the lens distorts into an oval shape more similar to the the lens of other terrestrial animals.

However, many adult frogs regularly live in both terrestrial and aquatic habitats and must see in air and water. But, the refractive properties of water are much different from those of air. Frogs have evolved a wide range of accommodation, but even that is not enough to see well in both environments. As such, above water, frogs are myopic while below the surface, they are hyperoptic. For humans, the opposite is true. All humans are extremely near-sighted under water. We are so nearsighted that we cannot focus clearly on anything in water unless we wear goggles, which allow us to see through the air trapped behind the goggle lenses.

In addition, recent research suggests that bullfrogs produce a special enzyme in just half of the retina that may allow for better perception of red and infrared light in the water, without impairing above-water vision (Enright et al., 2015).

Focal field:

Frogs lack a fovea; instead, clarity is fairly constant across the visual field (Schwab, 2004).

Spectral sensitivity:

Frogs, along with other amphibians, are trichromats, with three color receptive cone-types tuned to perceive shorter and longer frequency wavelengths with less discrimination in the mid-range. In daylight, ranid frogs perceive about the same frequency range as humans (Govardovskiĭ and Zueva, 1974; Kennedy and Milkman, 1956). However, in low-light, frogs exhibit incredible vision, even in color. In fact, a recent study found that frogs can discriminate color differences at lower light thresholds (especially in the blue range) than any other animal thus far reported (Yovanovich et al., 2017).

Comparison of night vision:

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Next: Fish Vision