Effect of accommodation on the retinal image.

These two statements, to which we shall refer as Statement 1 and Statement 2, while seemingly contradictory, are each partly true, though neither is complete. The first says that accommodation increases the size of the retinal image, that is it makes the image " larger ", but larger than what? The second says that in accommodation the retinal image is decreased in size, that is the image becomes " smaller ", but smaller than what? Let us first note that there are several kinds of images in the eye. The two we will consider here are: (i) the optical image (ii) the retinal image. The optical image is the clear focused image formed by the optical system of the eye. In emmetropia, for a distant object, this image falls on the retina and becomes also the retinal image. In myopia it falls in front of the retina. In hyperopia it falls (or tends to fall) behind the retina. The retinal image is that which falls on the retina. It may be clear or blurred. Its size when blurred may be taken from the point in the diffusion circle where the ray along a secondary axis (through the nodal point) strikes the retina. There are other methods of calculating the size of the blurred retinal image, but they are not pertinent to our present discussion. We will ignore for the present the enlargement due to the diffusion circles.