best way to customize DPI for photoshop generator?

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@vanfam wrote:

ie. When I look at the images in finder etc it says 72dpi so will the printer read it as 72dpi and therefore affect the way it prints or will it not matter and print as per the quality of the file anyway?

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Just stating ppi doesn’t say what the effective print resolution is. You might say 72 ppi, but that has no meaning unless you say both the height and width in pixels and in inches. If you don’t, you have only one half of the math; to see what the effective ppi resolution is, you have to state both the pixel dimensions and the physical dimensions (inches).

For example, if you have a 2400 x 3000 pixel image, and the printing company sets it up as 8 x 10 inches, you get:

2400 pixels wide divided by 8 inches = 300 pixels per inch

3000 pixels wide divided by 10 inches = 300 pixels per inch

So you see, even if the ppi value in the file is missing or not 300 ppi, it must end up at 300 ppi just because there are 2400 pixels wide divided by 8 inches. It doesn't matter whether it says 72 ppi or 6000 ppi in the file, as long as the printing company specifically lays out those pixel dimensions as 8 inches wide by 10 inches tall, the math dictates that number of pixels has to end up at 300 ppi at that physical size in inches.

Look at the pixel dimensions produced by Generator, and divide them by the print size you want in inches. If the resulting ppi value is enough, you’re OK no matter what ppi it says in the file.

Where it can be confusing is when the image does say 72 ppi, and the print layout software uses that ppi value to set the initial physical size on import. For example, if the 2400 x 3000 pixel file says 72 ppi, many print-oriented applications will say “OK, so 2400 x 3000 pixels divided by 72 ppi means the default print size is 33.33 x 41.66 inches.” Well, that size isn’t what a customer usually wants. But if the printing company is doing their job, they should say “That’s not the size the customer asked for, let’s set the physical size to the 8 x 10 inches they want,” and as we saw earlier,  if they set 2400 x 3000 pixels at 8 x 10 inches, that results in 300 ppi, regardless of the ppi metadata in the file.

Because ppi has no meaning until the pixel dimensions and the print size in inches are both stated, (assuming resampling isn’t used) a low ppi value in the file can be OK if the pixel dimensions are there to support the ppi value at the print size you actually want. An 8 x 10 inch image at 300 ppi can also be 4 x 5 inches at 600 ppi, or 16 x 20 inches at 150 ppi…because those are all the same 2400 x 3000 pixels, just stretched across different sized areas. Put another way, if you have 2400 x 3000 pixels at 72 ppi but you require 300 ppi, the math says you are OK as long as it will be printed at 8 x 10 inches or below.