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I'd like to resize an old image in order to print it for hanging on the wall with a frame. I already researched how to do resizing by itself in using Affinity Photo.

I have a 1500x630px image at 96dpi, resizing it to roughly double the size with 300dpi using Lanczos 3 non-separable). Works just fine and looks, at least to my untrained eye, pretty good.

My question now is, will this result in a good print? I mean like, hypothetically, if we the printer itself is not the limiting factor here: Would the resized image look better/decent?

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    Does this answer your question? Is there a general formula for image size vs. print size? – Saaru Lindestøkke Jun 23 '20 at 9:05
  • Your image is 1500 x 630 pixels. Doubling the size gives you 3000 x 1260 pixels. Sending this to a printer with the instruction to print at 300 dpi (or, more correctly, 300 ppi) will output a print that is 5 inches x 4.2 inches. Is this what you want to achieve? – osullic Jun 23 '20 at 9:07
  • @osullic How do you come up with these measurements? I'm not an expert or anything, so I have no idea of such stuff. But in Affinity it says, in order to print an A3 I'd need about 3500px at 300dpi. – DrDecane Jun 23 '20 at 9:19
  • The dpi/ppi setting is just an instruction. It tells the computer/printer how to convert the pixels in a digital image to printed dimensions on the page. It literally is "pixels per inch". Your image exists as a rectangle made up of pixels. So you need to think in terms of pixels really. If you want to print at A3 size (11.7" × 16.8") at 300 pixels per inch, then how many pixels do you need? About 5000 x 3500. – osullic Jun 23 '20 at 9:43
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    @osullic But you're saying above my 3000px image will result in 5" and and now you're telling me 3500px results in 11", which is more than double the size. Where's the logic there? – DrDecane Jun 23 '20 at 9:54
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Your image is not 1500px at 96DPI. It is either 1500px wide and whatever that means depends on the screen resolution in use, or it is 15.6" wide at 96PPI.

The best way to determine if an image will look good in print at a given size is to first determine what the DPI resolution of your screen is. For instance, my MacbookPro Retina is running at the default "pixel doubling" resolution of 102DPI (1440x and it is 14" wide).

Then view the image at the largest size where it looks good to you, say that is 100% zoom. If you have 1500px long edge and it looks good, then it is 1500รท102DPI=14.7" print at 102PPI. If it looks good at 200% then you can print it 2x as large. If the image begins to pixelate (e.g.300% zoom) then you have reached the limits of the PPI resolution. Technically, if the image is going to be viewed from a greater distance then you can get away with a lower print resolution; E.g. a billboard @ 10DPI.

Personally, I try to stay at/above 150PPI for quality prints, but I have printed at lower with decent results... this all really depends on the physical size and quality of "the negative" you are starting with more than anything else. I.e. you can have a 50MP image of crap, and it will print as crap regardless of the output size.

Depending on the printing method/medium (e.g. inkjet on canvas) you may be able to print a little larger at a lower PPI. And if you are not knowledgable about sharpening for the particular printing method/medium, you might want to print a little smaller. But in either case, it should be very close to what you see on screen.

The 300PPI recommendation most reference is because that closely matches the approx 300dpi maximum capability of most printers; but that does not mean you will be able to see it (you won't, not w/o magnification). The printer will use its' maximum DPI regardless of what you send.

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It will not look good by definition. When increasing the resolution you are essentially trying to add information to the image where there is none. There are some AI assisted methods and smart enlarging filters, but all they do is just an educated guesses.

However... looking at a print is a different experience than looking at a photo on a screen. I have printed things in B2 format from instagram resolution photos (1350x1080 @ 72dpi) and they look just fine.... as long as there is not to much detail in them. If there is a shallow depth of field (a lot of blur) and not to much detail (like small type and many small sharp edges) it can be just fine.

To try the AI assisted enlarging try imglarger.com. There are other services out there, but this one somehow seems to be to provide the best results, however that all depends on the use case. For other services just google "ai image upscale".

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  • Thanks for the insight, that's excactly what seemed so counterintuitive to me and why I tried to upscale the image for a better result. – DrDecane Jun 23 '20 at 9:29

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