Generally, Hi-Key and Lo-Key images are conceived as a part of a shoot process rather than being part of the development in post processing. Optimally images for such styles are exposed specifically for such a look.
Recently I decided to explore the more extreme use of mid-gray values along with conventional exposures and was pleasantly surprised by the results.
I selected 2 street scenes … the first with a longer tonal scale and slightly contrasty, this was converted (mid-gray=1.5) into a pseudo hi-key; the second with a lower tonal range (mid-gray=36) to low-key. In both cases the exposure remained ‘as shot’ and as normally processed.
These images may not fit with everybody’s concept of those styles but I found this was an interesting use of moving the ‘pivotal’ mid-gray.