I bought the A7sii on the grounds that it might produce better results than the A7ii because of its smaller number of larger pixels. The suggestion had been that post processing involves floating point arithmetic and this involves errors that can accumulate along a chain of operations. Starting with better data for individual pixels might cause the calculation errors to be smaller and accumulate slower, giving a better final result.
In this post at photomicrography.net Rik Littlefield agreed that starting with better pixel level data might be advantageous for some post processing operations, although not mainly for reasons of floating point error accumulation.
So it doesn't seem like an intrinsically silly idea that my heavily processed images might end up better if starting from better pixel level data.
Within 10 days of getting the A7sii I had had seven sessions with it, and I was pleased with the results I was getting. I was curious as to whether these were better than I could get with the A7ii; had my purchase been worthwhile in terms of image quality? I know from experience that doing effective comparisons can be very difficult, even if you can reduce a lot of the variability by doing like for like tests in a controlled environment. However, I also know how difficult it is to draw real world conclusions from highly controlled comparisons. And it was real world results that mattered to me. So I tried a real world comparison.
Since image by image like for like comparisons were impractical out in the field, I decided to try comparisons at a session level. In the previous four days I had had three sessions with the A7sii with the 100mm Laowa 2X macro lens with 2X plus 1.4X teleconverters, using f/40 for the whole of two of the sessions, f/45 for one session and f/36 and f/32 for the other session. All four sessions were in the church grounds opposite our house.
I decided to do another session in the church grounds, using the same setup apart from using the A7ii rather than the A7sii. I would use f/40 throughout the session. The images I kept from the session are in this album at Flickr. The images from the four sessions with the A7sii are here, here, here and here at Flickr.
And my conclusion?
I can't convince myself that, at the session level, what I got from the A7ii session is any worse (or any better) than from the four A7sii sessions. There may be differences that someone more observant than I am might notice, but as it stands, I'm not seeing it.
So do I regret getting the A7sii? No, I'm comfortable using it, I like the results I'm getting from it, I think it is giving me stronger focus peaking signals than the A7ii, although even there I'm not 100% sure about this; it is something else that is rather difficult to test in real world use. But if it is giving better focus peaking signals that will probably be improving my focus success rate. Having the A7sii means that the A7ii is freed up for other things. And if I hadn't got the A7sii I would have had a permanent niggle in the back of my mind that perhaps I could do better by using a low pixel count camera.
And I have this feeling, which may of course be no more than wishful thinking, that it may in fact be giving me better results, at least some of the time, but I just can't put my finger on it. Anyway, I'm entirely content to go forward with the A7sii as my primary camera for photographing invertebrates.