All they can tell is if the environment around them is light or dark.
The argument is that the morphologic gaps are so narrow that it would be a very simple process to step from one gradation in visual acuity to the next with no more than one or two genetic mutations.
In fact, it is often argued that these gradations already exist in a population that expresses one of the above listed steps.
This "graded index" significantly improves image quality in that it is able to correct for distortion.
There are just a few problems with this "theory" of eye evolution however.
(The exact size that is "perfect" depends on the brightness of the lighting in a particular environment.) An example of a narrow aperture lensless eye is found in the chambered nautilus. To get a lens, a ball-shaped mass of clear cells with a slight increase in the refractive index is needed.
Once this mass is formed, it can be refined with very slight increases in the refractive index to produce greater and greater visual acuity.
will be more acute if the lens moves towards the center of curvature of the light-sensitive surface.
So, over time, the lens not only moves, but increases in refractive index with a great index in the center of the lens vs. This is possible because the lens is made from a mixture of proteins.
This feature allows for the direction of the source of light to be determined so that the creature can deliberately head toward or away from the source of light. This increases visual acuity by allowing the eye to sense the direction the light is coming from better than a flat eyespot. Around this point the pit begins to fill with a clear jelly-like material.
It is thought that producing this jelly would be rather simple for most creatures - probably no more than one or two mutations.
For example, a population that has flat eyespots is said to have at least some individuals within the population that have slightly dimpled eyespots.