The captures now show histograms indicating that there is no "crush" of black levels:

Attachment 14667

Attachment 14668

Attachment 14669

The best capture in my opinion is the capture with edit=on; for example the details on the face of the girl are better in this mode:
Attachment 14670

Here some image comparison:
edit vs norm
https://imgsli.com/OTM0Nzc
edit is better

edit vs norm_calib
https://imgsli.com/OTM0Nzg
edit is better

norm vs norm_calib
https://imgsli.com/OTM0Nzk
very similar

In my captures the edit=off mode is better, in yours edit=on is better. Another evidence that we should not blindly follow any general guideline but always experiment with our own tapes and hardware!

Comparing the old capture with crushed blacks and the new capture I see an improvement. However is difficult to understand if details in the range Y<16 are now present in the clip with adjusted levels because the crush is avoided, or if they are visible just because thr higher brightness. You have to experiment yourself and to judge yourself (for example, increase brightness in the "crushed" clip and see if they appear or not):
Attachment 14671

clipped_edit vs edit
https://imgsli.com/OTM0ODE

-- merged --

Addendum: your "brightness increase" was probably able to bring some more details in the dark areas (not sure, as I said before, it must be checked). But as you can see, the video now is too bright imo. It was ok to (probably) avoid the crushed blacks, but now in postprocessing, you should bring the levels back to the "original" look. It can easily be done in AviSynth with Levels or ColorYUV without clipping anything.

Brightness +3, Contrast -16. All others at 0.

Though I'm under the impression that I'll need to change these settings for every tape.

-- merged --

Lollo, thank you for the histograms and the image sliders. These are helpful!

I have no real way of brightening the older clip to compare it with the new clip/check for missing details. (I imagine there's a way in AviSynth, but it's still very much a foreign language to me.)

Regarding my new, "too bright" clips: should I have lowered the Brightness slider by one before capturing? Or were my settings correct for capture, and post-processing is the accepted method for fixing this?

So -- thanks to you, I now have a better understanding of setting the levels before capture, and of the best settings for my VCR. I still have two problems left:

1) according to your screengrabs above, I'm still showing gaps in my histograms, correct? But this can be fixed in post-processing. (You suggested denoising.)

2) the videos are too bright. But this can also be fixed in post-processing. (And maybe with slider levels beforehand?)

Could this mean that I'm ready to stop doing tests and start capturing real videos?!

Yes to both questions.
Now you have all the elements for "good" captures.
Enjoy your captures!

Hooray! Thank you, this is exciting. ;)

Do you have any thoughts on this question of mine?

"Regarding my new, "too bright" clips: should I have lowered the Brightness slider by one before capturing? Or were my settings correct for capture, and post-processing is the accepted method for fixing this?"

I do not know. Use the brightness slider at its minimum change able to stay inside 16 Y range for a dark sequence.

With post processing you can correct the parameters. Of course, is better to have the clip at its "best" before postprocessing.

Start using AviSynth or VapourSynth, you'll be surprised by its power. If needed, ask here ;)

You're saying that, in general, I should only make the *minimum* changes necessary to the sliders in order to get the preview histogram fully inside the 16-235 safe zone. And nothing else. Correct? That makes sense.

In your screengrabs of the histograms from my most recent clip, I see that most of the graph is clumped over on the left. That's okay? I don't need to play with the levels to try to "center" that curve?

I'll get around to AviSynth at some point. One mystery at a time... ;)

Correct. If your card can capture 16-254 (for example) I would stay inside that range, rather than 16-235.

That's the "nature" of your video. You could offset the Y channel (-> increas/decrease brightness) at your preference to shift left or right the histogram, but in general the original pattern is the best. There is nothing to "center"!

Yes and no.

You need to establish a baseline that works with most tapes. The deviate from there. Don't start from 0 at all times if you know 0 is bad.

So is +3 / -16 the baseline?

You're asking me? Heck if I know.

I've only ever tried this on two tapes. The first (in my OP) worked fine at 0/0. The second one worked fine at +3/-16 (though given Lollo's comments it sounds like +2/-16 would have been better). So I don't have enough data yet to establish a baseline.

Sounds like I should start keeping track of which settings work best for each tape going forward, and trying to eventually glean a baseline that way.

I'll look at mine sometime soon, share my settings. I've always pre-adjusted with the AVT-8710. If the tape needed better proc amp, I'd use another workflow with another capture card.