Thanks to some really useful comments from Eileen I now understand why I was so disappointed with the first attempts at this exercise. I had assumed, wrongly, without checking the manual, that every change in aperture setting equated to 1/2 a stop on my camera whereas it equates to 1/3rd stop. What I should have done was check the manual and also the table so that I knew by how much the settings were being changed between shots. I’ve had another go at this, this time taking the first shot on aperture priority at f/8, then changing to manual and taking further photos at f/11, f/16, f/5.6 and oops, this lens only goes to f/4.5…another lesson learnt there, should have started at f/11 or used a different lens. The reason I chose this one is that there is a macro setting on it so it allows me to go in closer. I can see much more clearly the difference between each of the pictures this time, not just in terms of the effect of under and over exposure, but also the fact that the over exposed images are brighter and there is more saturation of the colours in the under exposed ones. Eileen has also pointed my in the direction of some materials related to exposure and colour which I haven’t accessed yet but I will.