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So would it be correct to say that the retina would prabably evolve first and then the rest of the eye would, through mutational trial and error build upon that foundation (each step being helpfull) until it is an eye?
Do you know of any resources that would go through the process of how a retina would evolve from the light-sensitive chemicals?
The retina would be the most common-sensical first step since it is the photosensitive part of the eye. All other additions make "seeing" better, such as a concave retina and a lens.
All you need is for a light sensitive chemical to start a nerve impulse. You mentioned earlier that you need a "brain" and nervous system before the signal can be used. This isn't necessarily true. If nerve cells previously used for touch became light sensitive, then light would be analogous to being touched. This would make the organism shy away from light, a very advantageous feature for hiding in dark areas and for foraging on food that is only found in dark places. This may have been what happened with planaria, a very simple flatworm with a ladder like nervous system (ie no brain). It has two photosensitive eyespots that allow the planarian to stay in dark places where it is both protected and where it can find suitable food. The eyespot is also concave which give the planaria an idea of which direction the light is coming from.
Planaria:
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