IV.2 IV.3 IV.4 Analyze the effect of reducing the aperture: As you slowly stop down the lens, what happens to the brightness of the image? Does the image distance change? That is, does the focal plane (where the in-focus image is formed) shift when the lens diameter is decreased? Does the image size shrink with smaller aperture? Does a portion of the image get “chopped off” because of the smaller lens opening? Although you may be surprised at these findings, they merely point out the fact that each tiny portion of a lens forms a complete image; the final observed image is simply the sum of all of the independent contributions! The brightness of an image formed by a lens (amount of light per unit area) is actually determined by the ratio (called the f-ratio) of the lens focal length to its effective diameter. As you stop down the aperture, are you increasing or decreasing the f-ratio of the lens? Do larger f-ratios produce brighter or darker images?