One of the basic questions of myopia development is why it does not inhibit itself but rather tends to progress. This is in apparent contradiction to results of animal models, where myopia can be simulated by positive lenses and, in fact, inhibits eye growth, leading to shorter eyes and hyperopia. Together with Barbara Swiatczak at IOB, we found that also the human retina can detect myopic defocus and trigger shorter eyes, but this was possible only in emmetropic eyes while myopic eyes had apparently lost this ability. A big question is then how the emmetropic retina can detect positive defocus (note that we don't need such a function for vision). A great signal would emerge from longitudinal chromatic aberration which makes image focus more myopic in the blue, compared to the red. Comparison of focus in both planes could provide the sign of defocus. We have developed software to present movies with simulated chromatic defocus. We found that the emmetropic retina responded exactly as expected. When blue was calculated unsharp, eyes became shorter but when red was calculated unsharp, eyes became longer. Again, myopic eyes had lost this ability and only became longer. There appears to be a fundamental functional change in the myopic retina – we need to find out what it is and when it happens. An emerging question is then why various novel spectacle designs can slow myopia progression – as we found that the myopic retina does not respond to positive defocus, these glasses must stimulate the retina in a different way, other than imposing positive defocus.