There are variations between units, but EInk does have an intrinsically lower contrast than (say) a paperback book. In low light conditions this can be mitigated by increasing the font size, but the PRS-505 only has 3 font sizes. Another strategy is to use a heavier font, see
Font Family should help Contrast. This is again not as easy to do on the Sony Readers as some other devices. On the other hand, if you produce your own ebooks the Sony's give you complete control over the font and its size.
What I do on my EZ Reader using OpenInkPot is turn on "always bold", which significantly increases the apparent contrast between the letters and the background at the expense of no bold for emphasis.