Help with dpi/resolution

[kelleyrie]

So, I was browsing around on the web today and I looked at several photography contest type things. I got to reading all the specs for entering and they all specify 300dpi resolution. All my images say 72dpi, even the larger ones. So my question is, can I not achieve a 300dpi image? My camera is set to Large/Fine. Please someone help me

[RoyL]

Ok, looking at your images they are all shot using jpeg and as such the camera is setting the resolution to 72 (used for postscript, screen, and web). to achieve the 300dpi you can do one of 2 things, either use software to increase the dpi or shoot Raw (and either of them require you to post process so time to learn software manipulation)

[ttosifa]

This makes no sense as a requirement. First off, is it dpi (dots per inch) or ppi (pixels per inch)?

Second, practically speaking, the dpi is an uninformative number by itself. It is a rate. It is "per inch." It seems you also need the number of inches.

So one of my cameras outputs a file that is 2736x3648 pixels. That's the file size. That can be 180 ppi, 72 ppi, or 300 ppi. Whatever I want, and the file will NOT change (subject to a technicality I describe in a moment). What WILL change are dimensions of the file. If I want a 180 ppi file, that will support dimensions of 15.2 x 20.267 inches. If I want a 72 ppi file, that will support dimensions of 50.667 x 72 inches. If I want a 300 ppi file, that will support dimensions of 9.12 x 12.16 inches.

The technicality is that Photoshop's default is to have resample image turned on. This means that changing the dpi will change the file by increasing or decreasing the total number of pixels so that the dimensions are fixed (300 ppi FOR THE SAME DIMENSIONS requires more pixels than 72 ppi).

So if the contest does not provide information about the dimensions, I would say to just open the image in Photoshop, click Image-> Image Size, unclick the Resample Image button, and enter 300 in the pixels/inch box. Then save the file. The file will be exactly the same (except that the metadata will say that it now has 300 ppi and correspondingly smaller dimensions). Practically speaking, any picture you already have can be converted to 300 ppi without changing the picture even the tiniest bit (pixel)!

[kelleyrie]

Originally Posted by ttosifa

So if the contest does not provide information about the dimensions, I would say to just open the image in Photoshop, click Image-> Image Size, unclick the Resample Image button, and enter 300 in the pixels/inch box. Then save the file. The file will be exactly the same (except that the metadata will say that it now has 300 ppi and correspondingly smaller dimensions). Practically speaking, any picture you already have can be converted to 300 ppi without changing the picture even the tiniest bit (pixel)!

This is what I was thinking, when I did attempt it brought my dimension down to around an 8x10 and I thought that was bad And yes, they are asking for 300 DPI.
RoyL: I already use pp software, both gimp and elements. I don't shoot in RAW because I don't want to deal with the extra work I am working on that though .

Thanks y'all, for giving me some insight on this !

[smc1377]

Just wanted to second ttosifa's post about unchecking the resample box when changing the ppi. Not doing so will cause photoshop to split or combine pixels and you don't want to do that unless you really have to.

[kelleyrie]

ok So I understand a bit better about dpi but now I have another question. Since the amount of pixels do not change when I go 72 dpi to 300 dpi, how are my prints gonna be affected if I were to print one at 72 dpi... will I have a pixelated print? One of the contest websites showed an example to explain the difference and in the photo they used, the 72 dpi was looking like an old school atari game while the 300 dpi one was very crisp and clear. I have printed some of my things and they looked normal but now I'm wondering if I am missing something!