Mark wrote:
dpi vs. ppi–what is the relationship exactly? When I open photos that I intend to submit to publications, I change the ppi from 72 (camera result) to 300 (without resampling). Recently I started saving those as tif files for submittal (but previously saved as jpg’s which worked out well enough for publication–I’ve been relatively successful). Am I on the right track or is there something else I can do to enhance my photographs?
I had one editor tell me I could submit the photos just as they came off the camera and that my 5 megapixel size was adequate for their needs (they require 300dpi [not necessarily ppi] for the magazine)–thus my question on the relationship of dpi/ppi. Are 300 dpi and 300 ppi relatively similar?
I would greatly appreciate any enlightenment you could provide.
PPI and DPI are often used (incorrectly) for the same thing. PPI stands for ‘pixels per inch’, and since digital images only have pixels, that is the only term you can use for images. DPI stands for ‘dots per inch’, but images do not have dots. Therefore, you cannot use ‘DPI’ for an image, but many people do when they mean PPI.
Printers use dots to simulate colors, because they can only print those colors that are in their inkset. So, if a printer needs to print a red pixel, and it only has CMYK inks, it uses several magenta dots and several yellow dots to simulate one red pixel. That also means that the DPI value of a printer needs to be much higher than the PPI value of the image, because more than one dot is used for one pixel. On a printing press, it’s usually a matrix of 8 by 8 dots used for one pixel. Inkjet printers often have 2400 DPI or more, but I trust you now understand that this doesn’t mean your image has to be 2400 PPI. 300 PPI is enough for the human eye not to see the individual pixels anymore.
Forget about DPI. As a photographer you do not need to know the DPI of the printing press. In principle, you also do not need to know the PPI value either, because any editor can simply change that. That one editor is right when he says he can use the image straight from the camera: only the total number of pixels determines whether an image can be used at a certain size or not.
Unfortunately, editors are also only human. There are many editors who are not computer specialists and also do not really know the difference between PPI and DPI. Those are the ones that tell you they need "300 DPI" without telling you the image size in inches to go with that. True, they don’t know what they are talking about. So just send them the full camera image, that you set at 300 ppi in Photoshop (without resampling). That will keep them happy and then they will keep you happy.
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Johan W. Elzenga johan<<at>>johanfoto.nl Editor / Photographer
http://www.johanfoto.nl/