It is common for images to be enlarged automatically when they are printed borderless. This ensures that white edges are not left due to inaccuracies in the positioning or angle of the paper as it passes through the printer; or variations in the precise trimmed size of the paper (which can sometimes be significant); etc. So part of the picture content that you have seen previewed as occurring inside the paper boundary, winds up as "overspray" instead and the rest of the picture is a little bigger correspondingly. (This can perhaps make a bit of a nonsense, of adopting a specific output PPI resolution which has an exact relationship to the printer's own hardware resolution).
One answer is to not print borderless, but accept a definite white page margin all round. However especially on small paper sizes, the inaccuracies mentioned above may still result in noticeably unequal or un-straight borders, which can look dreadful.
Another answer is that some printers offer a choice of borderless printing modes, including one which forbids this extra enlargement step, and takes its chances with leaving some (more or less straight) "sliver" borders.
Another is to deliberately crop your image a little "looser" than it otherwise would have been, to make allowance for the above effects. One repeatable, controllable, reversible way to do that is to use the Scaling option in Lens Corrections / Manual. This will scale the image down within its crop, to a known percentage - causing more subject content to appear. Resetting this back to 100% later, will restore the scaling of the picture content exactly. So having caused more of the picture to appear, you can then afford to lose some when printing. It'll be a matter of working out what scaling percentage works best. Perhaps make a virtual copy or proofing copy, make whatever printing specific adjustments are needed, / set the aspect ratio and fine-tune your crop for the preferred composition, then apply this extra "compensation" scaling as the final step before printing.
Another way is of course, to print on a bigger piece of paper instead - perhaps, laying several photos out together - and then trim to a just-so result either with or without a margin. (In that case, not printing borderless - to keep everything size-exact.)
So it's "normal" to see noticeably unequal borders -- even if you're comparing just top and bottom?
I totally understand why you might have different top/bottom vs. left/right borders -- but I would have expected top border to exactly equal bottom border -- so I should give up on that expectation?
And any thoughts on how the image seems to be slightly stretched? I understand the need to crop some of the photo -- but it seems like my printed photo is somehow a bit wider than I expected it to be. Meaning: if i adjust the border so just a sliver shows at top/bottom, I'm seeing more cropped on the sides than I would expect -- it's almost as if the pixel size changes a little (but I'm starting with a 2x3 image, and printing to 4x6 paper).
Of course I don't know what printer you have, nor would I know the exact details of how borderless mode works internally in that printer, even if I did.
I suppose the physical situation is a little different in practice, printing across the leading vs the trailing edges of a piece of paper - given that the printer still needs to keep hold of that last part of the paper, wet with ink as it is. This business of the borderless mode deliberately printing across the edge, is a fix for a basic inaccuracy which will be hard to eliminate altogether.
But it's also enough for the paper to have been manufactured not quite the size that is stated / that the printer driver is expecting, in order to get an obvious margin / edge mismatch on the print. An error of as much as one or two millimetres, will clearly show. That back edge's location is not actually verified by the machinery AFAIK; certainly it cannot do so when it starts to print. So it can only plan for, but not ensure, even margins.
Thanks -- so these things just going to be a bit unpredictable.
Given that I do see this slight horizontal stretching -- is there something I can do in Lightroom to compensate for it?
For example, "when printing, apply a 1% horizontal compression, then print" -- so to speak?
I've never seen (well, noticed) such an effect myself. And it seems to me like a poor implementation by Epson, if so. Which printer?
But if you wanted to correct for it, the same Manual tab within the Lens Corrections panel has another slider ("Aspect") which adjusts the appearance of the image content slightly skinnier / squatter.
(despite using the same word, this is completely unrelated to the "Aspect Ratio" setting found in the Crop tool, which solely affects the proportions of the crop boundary)
It's an Epson XP-620.
When I set it up, I just opened the Printer panel, and it found the printer, and I've been using it like that (I didn't install anything from Epson).
Regarding compensating for it -- I didn't really want to adjust the actual picture, but rather just add a process as part of the printing (kind of how I add border then too, but not to the actual image).
Thanks for sharing time to help me understand this... -s