Doubt with digital photography properties (size, dpi)


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Oct 3, 2017
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I'm an amateur photographer with very little experience, but today I saw a notice calling for photo submissions for an exhibition in my town and I'd like to send some photos I took.
But they have some requirements regarding the file's properties, and I know nothing about it.
The requirements for the file are:
300 dpi
The final size of printing must be 60x40cm
The dpi of all my pics was originally 72. I found a dpi converting website and converted it to 300. Is this it?
Now, about size, the pics I'd like to send are all 5184x3456(x16M)
I see the ratio is equal for 60x40 and 5184x3456, but I don't understand what does it mean when they say that I have to send it with the "final printing size of 60x40". The pics would have to be smallers or something?
They want the picture to be 16x24 and 300dpi. So they want 7,200 pixels on the long side, and 4,800 pixels on the short side. Basically convert 60cm to inches = 24 approx. 40cm = approx 16. Then figure your 300 ppi (pixels per inch). 24x300, and 16x300. 7,200 x 4,800 final file size.
The pixel dimensions (5184x3456(x16M)) of a digital photo define the Image Resolution.
Print Resolution (size) is determined by the Pixels Per Inch - PPI, not dpi.
Print size is determined by the print resolution & the image resolution.
Whoever makes the print sets the print resolution so the print can be made at the appropriate size - if the image file size is sufficient (see below).

60 cm x 40 cm (3:2) defines the Aspect Ratio (shape) of the image or print.

The math:
5184 pixels (image resolution-long side) 5184 / 23.6 inches (60 cm)= 219.66 ppi
5184 px / 300 ppi = 17.28 inches (43.9 cm)
Note that manipulating the equations using basic algebra allows us to solve for ppi, print size, and image resolution.

The image File Size (Mb) usually also has to be considered when making larger than desk size prints.
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Some places require the specified dimensions. Usually I have had good images at larger sizes without doing any upscaling, but I do have the On1 Resize program and it can give a bit better detail when printing larger than the above calculations would recommend. You just tell the program you want 24" x 16" and 300ppi (you can mix the units so you can put in 60cm x 40cm and 300ppi). It will upscale and give you a new file with dimensions of 7200 x 4800 (if using inches).

For an exhibition I would rather have my own printing and mounting done and not send off an image file to someone.
For an exhibition I want to chose who does the print making and mounting.
If they're asking for "dpi" (300 dpi) are they asking you to send physical PRINTS?

DPI is meaningless in an digital image -- it's just a meta-data tag that is only meant to apply to physical printing of the image. I could set the print resolution to "airplane" and it would be just as meaningful in the impact it would have on the image when viewed on a computer monitor.

If I have an image which is 3000 x 2000 pixels and the meta-data tag specifies 200 DPI then that means each 200 of my pixels should be crammed into each 1" of the physical output. So 3000 x 2000 means the image would be 15" x 10" ... but only when it's actually printed. (because 3000 / 200 = 15 and 2000 / 200 = 10). If you do not print it, then the DPI value in the file is ignored. In fact even if you do print it, since the DPI is just a meta-data tag (it's doesn't actually change the real data in the file) you could set the properties of the print on-the-fly. If you send an image to someone with some DPI value set (say it's 72) and that person would rather have it be 300, they can easily change it on a whim. Often times printing software doesn't even use the DPI value... you might set the output dimensions of the print and the software scales it to that size as it prints regardless of the DPI value.

And yet... for some reasons... "experts" in publishing and apparently also in these contests set such meaningless rules that scream "We have no idea how any of this works but we're qualified to judge your photos."
I don't understand what does it mean when they say that I have to send it with the "final printing size of 60x40". The pics would have to be smaller or something?
It's either centimeters or inches. "The final size of printing must be 60x40cm" If your printer driver will recognize centimeters, then set it to those dimensions. If not, convert to inches.

The important thing is to get the proportions correct and the print resolution as per contest requirements.

If you are not printing these yourself, take the digital files to a print shop, such as Staples, and tell them the finished dimensions. They can do this easily.

Some photo printers will give you the printer profile settings so you can get the colors correct before you take/send the digital files to them.

Good luck!

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