• Background
• Instructions
• Illustration
• Quiz

## Background

One difference between photopic and scotopic vision is the relative acuity of the two systems. Acuity refers to the ability to see or resolve fine details. The reason for the acuity difference is partially in that rods and cones have different spatial summation characteristics. Spatial summation means the ability to pool light across different regions of space. At night, you want to detect as much light as possible, even if the spatial summation leads things to be a bit blurry. The scotopic system pools responses across many rods in order to maximize the sensitivity to light. However, by pooling across rods, the scotopic system loses some ability to resolve light to particular sources in the visual world. This pooling of information is called convergence, and it is much greater for rods than for cones.

To illustrate this concept with an example, consider admiring the fine coat of a Bengal cat (CAN I COPY THE BOOK IMAGE HERE?). You want to be able to make out the intricate patterns on the coat and see them in fine detail. To do this, you cannot confuse one location along the catâ€™s fur with another. Thus, you want as little spatial summation as possible. As you increase convergence, you lose the ability to distinguish one part of the fur from another part.

In this illustration, you can explore how increasing and decreasing converges alters your ability to see fine details.

## Instructions

### Full Screen Mode

To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.

### Settings

Below is a list of the ways that you can alter the illustration. The settings include the following:

Alter the Convergence: press the More button to increase the convergence and the Less button to reduce the convergence. Convergence is demonstrated by averaging the light over a given area. You are increasing or decreasing the size of that area.
How Display: Single refers to showing only one image which has all your current settings; Compare will put the original photograph on the left and the image in the current state of convergence on the right.

### Reset

Pressing this button restores the settings to their default values.