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I order prints of my pictures from an online place that has a number of scaling options. I upload JPGs. They have a number of paper sizes, 17x22, 18x27 etc. I do not understand the "no scaling" option. A jpg does not have a physical size, does it? I mean, if I created a photoshop document at 24"x24" and save it as a jpg, that jpg is not 24"x24" is it?

And the "use smaller image" confuses me too. In order for that to make sense, I would need to know the pixel dimensions of the print job, right?

enter image description here

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No, an image has no physical size, you are correct (and ahead of many askers here). However, the file can contain a metadata info tag that contains a instruction what the designed print pitch of pixels should be. This tag is slightly confusingly called resolution, not to be confused with amount of pixels which is also called resolution (in the os, games and cameras. This setting defines size and is measured in PPI/DPI.

Pixel dimensions of image is irrelevant for scaling though. All the printer needs to know is asingle conversion factor, width/height or if nonuniform size is needed width and height. Rest can be calculated.

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I would assume the no scaling option means printing at the size determined by a standard 300dpi setting, or quite possibly with respect to the DPI you have chosen. You may need to ask them exactly what they mean. This is only a guess.

Generally the DPI/PPI setting determines the output size of a print. For example if you have an image that is 4000px x 3000px, divide each dimension by the DPI to get the size in inches.

4000px/300dpi = 13.3"

3000px/300dpi = 10"

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And the "use smaller image" confuses me too. In order for that to make sense, I would need to know the pixel dimensions of the print job, right?

Yes, you would - some online printing services make it easy to find the minimum and recommended image sizes in pixels, you should check if the one you're using has them in a non-obvious place.

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