This image is an attempt to use the technique described starting with an old 35 mm colour slide. It shows Chamberlain House in Birmingham and how it might look on a moonlit night. The conversion to B & W was done using Macphun and some further processing with Photoshop. Your comments would be welcome.
1. To get the best effect (like we have been discussing), you need (a) a clean, noise-free sky; (b) clear distinctions between well saturated primary colors (ie, cyan-blue sky, green trees, and orange-red bricks; and, (c) smooth gradations between the various tonal (ie, brightness) levels. In fact, I already commented on this in post #9: "...As an example, I'm going to use one of my images that has strong colors: a red fire truck, a nice blue crisp September sky, a yellow line on the street, green street sign & trees, etc. ...". The results of the procedure we discussed are very sensitive to these factors.
I'm not sure how you processed your scan after the version you posted, so perhaps you have already done these things, but the version that you posted has lots of noise in the sky. It is a very high contrast image with really murky shadow areas; and, there are some funky non-blue colors in the sky, as well as a slight blue cast to the trees.
If it was me, I would try to adjust that image to look something like the version attached below before I tried to apply the procedure we had discussed. In fact, whenever I intend to apply almost any effect to a photo, I almost always first try to bring it a standard, conventional, good-looking state that any photographer would be proud to show, ie, good tonality, good color correction, lighting problems corrected, etc..
2. To enhance the visual suggestion of moonlight / night, I would (a) put a blue cast over the result; and, (b) show some contrasting yellow or orange lights coming from inside a few of the rooms/windows.
Tom, I really appreciate your comments and agree with much of what you write. However, please bear in mind that we are starting with not only a poor photograph but an emulsion which has had lots of time to deteriorate. For instance, the noise to which you refer, is most probably inherent in the film and is not digital noise. There is also a blue cast due to changes in the dyes. My intention was to finish with a B & W image and so I did not pay as much attention as I would have done if the intention had ben colour for the final image.
Thank you again for taking the trouble to help, it gives me something to do to occupy my time which is all I can ask for.
I certainly undestand being constrained to work with old emulsions containing faded, color-shifted, grainy images. I've been down that road too many times, myself, LOL.
I guess that what I neglected to explicitly say in my last post was that if, for some reason, I had no choice but to work with such an image, and I wanted to try to do a day-to-night "conversion", I would never use the technique we've been discussing to darken the sky. Essentially, what it is doing is a sky "selection" step that is integrated with a darkening step (ie, sandwiching the blue-sensitive B&W negative with the transparency). The technique you asked about is just too sensitive to flaws in the original.
Instead, I would use other, very conventional techniques to select the sky (eg, pen tool, quick mask, alpha channels, etc.), followed by other, much more conventional and straighttfforward techniques (eg, levels, curves, etc.) to darken the previously selected areas. I showed an example of this towards the end of Post #15 in this thread.
Of course you are correct, Tom, and it was only because I was seeing what might happen using the technique you devised that I stumbled upon the idea of a day to night conversion. As you pointed out there are several ways of making a selection all of which require a certain amount of experience and skill. These look so simple when demonstrated by a professional but quite difficult to do by such as myself.
I will keep trying to find images which could benefit from this technique and I may continue posting them for comment.