I have loaded a wav file into PhotoSounder 1.8.3, and it displays a spectrogram, but I'd like to view the spectrogram with a linear frequency scale.

I turn the knob from the default 2 down to 1, and it now says "Linear". The frequency scale on the vertical axis changes to a linear one, but the spectrogram still looks the same. Only the waveform at the top changes.

Am I misunderstanding how this should work?

That knob only changes the way the spectrogram is synthesised, not the way it is analysed.

There is one way to do what you want and it's not very pretty, but it works. You can make a test file that only contains this line:
Analysis frequency scale 1.0
Save it as let's says "linear_analysis.pha" (the important part is that it has a .pha extension, not .txt), then open Photosounder, open linear_analysis.pha, only then load the sound file, and that should do it (just tested it here and it works). You'd have to load this script file before every time you want to load a sound if that's the way you want it. However remember that Photosounder's editing tools aren't really designed to work well with anything but the default logarithmic scale.

If you want to change more parameters that just this that are not in the config.txt file then have a look at this scripting reference http://photosounder.com/PHA_spec_v1.1.pdf

Thanks, that's what I suspected. It's becoming obvious that this program is heavily oriented towards synthesis, whereas I only want to look at the spectrograms of existing recordings.

I became interested in this program because I read somewhere that you calculate the spectrogram a special way. I can't find the reference to it now, but I got the impression that you might be varying the FFT window size with frequency to make the image sharper for a greater frequency range than normal.

Is that the case, or have I got it all wrong?

Yes, that sounds about correct (although I have doubts about the very last thing you said, it's still a tradeoff, it doesn't make everything overall sharper), that's how by default the frequency resolution increases in the lower frequencies to the detriment of time resolution. However I don't see the interest for this if you use a linear frequency scale for analysis, the lower frequencies are already quite hard to see as it is in linear frequency, I don't see how having more resolution in there would help. That's the whole advantage of using a logarithmic scale, you get more space for the lower frequencies, and using the varying window size you get more resolution to use that extra space.