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If you open an image in Preview, change the size, and change the dpi from, say, 300, to 72, it shows a calculation about how this will affect filesize, but it neither resizes the image to a new dimension (which...i guess is fine), but it also never actually reduces the filesize of the image upon save. does it actually change the dpi/ppi? is there any workaround to this beside doing it in another app?
MacBook Pro 13″, macOS 10.15
i should add that this page: https://support.apple.com/guide/preview/resize-rotate-or-flip-an-image-prvw2015/mac
specifies that you can change an image's filesize by changing the dpi/resolution setting.
but - it's not actually saving. therefore...it seems to me there's a bug, yeah?
Again: changing the dpi of an image does nothing on the screen.
A 1000x1000 image is a 1000x1000 image.
It contains 1 million pixels, regardless of dpi.
If you resize it say, to 50%, it will become 500x500, effectively one quarter of the number of pixels.
But if you change the dpi you are just changing a number in the file metadata.
The file data does not change.
This is the information on a test png file:
And this is the same file, after changing the resolution to 300dpi
so, dpi/ppi is more than just a field in metadata.
i understand that some photo apps let you change it as just a change to metadata. that's great. but not all apps work that way. if you're working in photoshop, and you change the dpi, it will change the dimensions of the image.
but back to Preview.
see how just changing the resolution suggests the filesize will change?
that's the problem i'm talking about.
because the change is never saved. the file stays at 626 kb, not 26kb.
now, perhaps that's because it doesn't force a change in dimensions, which would change the filesize,
but, regardless, does this not seem like a problem to you?
what is this "resulting size" section if not a communication of the new size of the file once the change is complete?
The number of dpi does not affect the file size.
A 1920x1080 file at 72 dpi has exactly the same content as the same file at 300dpi.
This is just meaningful for printing.
If you print at 300dpi it will be printed at a smaller size, that's all.
Put it like this: the size of each pixel on paper will be 1/300 of an inch, instead of 1/72 of an inch, but still it is the same content.
let me see if i can rephrase this. when resizing an image in Preview, the option to change resolution (ppi/dpi) is given.
when you change the dpi from, say 300 to 72, Preview tells you what the new filesize will be.
why would it change the filesize? because the dimensions have actually changed as well. if you had a fixed width of, say, 1k px and your resolution had been at 300dpi, well, on a monitor with 100ppi resolution (most of them are 72, right, but math is easier at 100) that image actually appears in a web browser as closer to 3k px wide. but now that you've change the dpi, you've actually reduced the dimensions of the file and thus the filesize.
regardless, these changes aren't reflected in the file once you save it. it's as if it saves the exif info of resoution/dpi, but doesn't actually modify the image and it never changes the filesize to be what it calculated the change to be.
Try changing the width/height to "inches" (a dimension) instead of pixels.
I can see where the confusion is coming from. As it happens the word resolution is often informally used as a synonym for image or frame size. It is unfortunate that such an informal use happens in an official support document, in a context where the word also appears with its formal meaning.
Keeping the inches and cutting resolution by half is the same as cutting the dimensions in pixels by half.
Inches multiplied by pixel per inch = pixels.
that worked, thank you!
You're welcome.
Preview doesn't save dpi changes
