Does this mean the stack from the small JPGs has less diffraction and is a "better" image?
Or at this mag. and at 800px to be viewed on a monitor maybe it makes no difference which JPG size one uses (small, medium, large; in Nikon's terminology)
It's the second one.
The effective f-number for both images is (2.1+1)*6.7 = 20.77, hence f/21 is a good number. Effective f-number is not affected by pixel count, only by the magnification and the lens setting.
I presume the confusion is caused by my statement
HERE that "
the pixels in the stacked image [from the small jpegs] are effectively twice as large as we've been assuming, and therefore the effects of diffraction are only half as large."
By "half as large", I meant only that the Airy disk would cover half as many pixels in the "small" images. This is important when we're trying to make sense of pixel-peeps. But when you're looking at images that have been converted to constant size after stacking, then diffraction effects do not depend on the resolution of the images being stacked. Smaller pixel counts mean that the Airy disk will spread over fewer pixels in those intermediate images, but when the result is then scaled to constant size, the Airy disk ends up being the same size in the final image.
There are other effects, however, that do depend on the pixel counts of the images being stacked.
In brief, the more pixels the better. This is because having more numerous but smaller pixels reduces any degradations caused by interpolation. It may also improve the quality of alignment.
One caveat, however, is that if whatever process generates the small images also sharpens them, then that sharpening will propagate right through the stacking software and reappear in the final constant-size output. To really tease apart these effects could require some careful study of the original images, not just looking at final images that have been run through the whole pipeline all the way to posting.
Of the two images that are posted here, layer-and-flash shows that the first image is clearly sharper than the second. Per the discussion above, I'm guessing that the first image comes from large images, the second from small. If that's not the case, then I'd be very interested to get the original stacks so I can figure out what's going on.
By the way, it's clear that this was a fresh specimen. Layer-and-flash shows significant shrinkage of the abdomen and increased bending of the tarsi from the first picture to the second.
--Rik
