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I would like to print something on A1 size paper, and just opened my Photoshop to edit the picture and I am confused about somethings. I found online which size to choose, but other than that all the dpi/resolution is confusing me. I just need to print a full colored picture picture on a poster so, I'm confused if I am doing it the right way. enter image description here

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You are in the wrong Photoshop menu. You are in the menu to create a new blank image, but you already have an image. Open your image. Then you should be in Photoshop menu Image - Image Size. Uncheck the Resample box at bottom, you only want to scale dpi for printing. Your A1 paper is 841x595 mm, select mm and enter that (Document Size). For your given image size, whatever size it is, it will show the resulting dpi calculation (image dimension in pixels / paper size in inches) The tool will convert from mm if you select mm. A1 paper likely will not be near 300 dpi, maybe 150 dpi (no clue what size your image is). But that is how it is, that is how large your image is, and large wall posters do not need to be as much as 300 dpi.

You are probably not printing posters at home. If you are sending the image out for printing, you don't need to do this, just give them the image and tell them paper size and they will do it. Except you should verify the calculation first to ensure you have sufficient pixels to print this large. Or to do that, you may want to first tell your print service the size of your image (dimensions in pixels) and discuss printing A1 paper for their advice.

There is more to it, the image must also be cropped to be the same shape as the paper shape. The A1 paper is metric aspect 1.414:1, and the image is some different shape. See http://www.scantips.com/lights/resize.html

  • Actually as per my comment to the previous answer, I don't have one image, I have multiple images. And I need to combine them together, so for that I have to create a blank image first. If I had one big image, and only wanted to print that, I would just go to photoshop and just open it. My question is since I have to put a couple of images, what should the initial size of the blank page should page. – samayo Aug 31 '16 at 14:44
  • If you have two images 4000x3000 pixels, then the size of the two will be 4000x6000 pixels, which is the size of your new image. But it is not that simple, the dpi numbers all have to match too. Note again, a very huge deal, is the 4000x6000 is NOT the same shape as the paper, metric A1 is 1.414 to 1. Saying, size is of course important, but SHAPE is also extremely important. The image shape should match the paper shape – WayneF Aug 31 '16 at 14:55
  • This is getting a lot more difficult than I thought it would be at first. How are two images 4000x3000px (w&h) amount to 4000x6000? Did you mean 8000x6000? Also the most important question I have is: what if I created a blank image with sizes seen the image and just fill it will little images and print it in A1 paper? What would go wrong? I'm thinking nothing since it is like basically opening word and putting images and printing it in a4 paper. – samayo Aug 31 '16 at 15:58
  • No clue about your image sizes and shapes, which is all important, both regarding shape and also regarding dpi. Assuming two images 4000x3000. Stacked one above the other, is still 4000 wide, but now 6000 tall (3000 + 3000 = 6000, two images). But 4000x6000 is shape 2:3. A1 paper is shape 1:1.414. So 6% of your combined image simply will not fit the paper shape, some portion will be cut off. If you crop it first yourself, then it is your choice what part is cropped off, normally not much problem if you choose it, but the paper randomly cropping it can be a big issue. – WayneF Aug 31 '16 at 17:06
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    Yes, it would have been good to have said what you were doing. :) OK, sorry, I made wrong assumptions for your actual goal. I'd suggest first asking the place that will do the printing what dpi they want for A1. I suspect 300 dpi is excessive, probably 250 is maximum useful. Ask them first. A1 at 250 dpi is about 5846x8278 pixels. If you put a 3000 pixel wide image into that, it will be 3000 / 5846 or 51% of the full width. Fill in your real numbers. – WayneF Aug 31 '16 at 19:18
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This is a dialogue that creates a new image and the DPI shown here is only for reference if you want to set the resolution of the document according to how large you want to print it. However, since you already have an image (that you're not creating from scratch), you can open it directly (you don't need to create a new document for that) and only be concerned about DPI when you are ready to print it.

DPI is a physical measure meaning how many pixels of the image will go into one linear inch of paper; it is a measure of density. 72 PPI is a somewhat antiquated value that many displays were assumed to have some decades ago. Usual print settings today are 150 or 300 DPI.

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    Thanks, but the problem is that I don't have one big image, I'm going to combine a couple of pictures. I guess if I had just one picture it would just fit. So, first I have to create the right size for an A1 space. I just the settings are fine, but second thing I don't understand is that is the 72 resolution the same thing as 72 PPI? If so does that mean I can change the 72 to just 150 or 300? – samayo Aug 31 '16 at 11:59
  • yes, resolution in this dialog means DPI (or rather PPI, a fine distinction which doesn't need to concern you here). (the dropdownbox ion the right literally says "pixels per inch") Set it to the value your printer uses ideally, but if not, your picture will be scaled when printing anyway. – ths Aug 31 '16 at 12:07

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