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Old Jul 20, 2007, 10:18 AM   #1
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Perhaps someone can help clarify the relationship between resolution and dpi.

Now I am needing to take photos and have them at 300dpi for printing purposes but taking pictures with my Canon EOS 350d seems to have them at 72dpi where as the Canon PS A410 has the photos I take at 180dpi.

Now would I take a photo taken with the EOS and drop its resolution (it's at 8MP) and then increase the dpi via photoshop? Or am I missing something?

72dpi is usually for web applications so I am curious how I would get the higher pixel depth for print.
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Old Jul 20, 2007, 10:40 AM   #2
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It's just a number in the image header, and it's ignored by most software entirely.

The actual pixels per inch you are sending to a printer will depend on the image size in resolution and the print size you need (since you're spreading the existing pixels thinner for larger prints, resulting in less dpi).

For example, if you wanted an 8x10" image at 300dpi, you'd need to send an image size of 3000 x 2400 pixels (7.2 Megapixels) to the printer.

8 inches x 300 dpi = 2400 pixels
10 inches x 300 dpi = 3000 pixels.

But, if you tried to take that same 3000 x 2400 pixel image and print it at 16x20 inches, you'd end up with only 150 dpi:

2400 pixels / 16 inches = 150
3000 pixels / 20 inches = 150

It's just a number in the image header. Some cameras plug in 72 dpi, some plug in 240, some plug in 300, etc. But, it doesn't mean a thing to most software.

Hardly a month goes by when someone doesn't get upset because their new camera is taking images at only 72dpi. :-) Again, it's only a number placed in the image header and the dpi you get from a given image size will depend on the resolution of the image in pixels (width x height), and the print size you need.

Here is an article that may help to explain it:

http://www.photo.co.nz/faq/resolution.htm

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Old Jul 20, 2007, 11:39 AM   #3
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Jim explained it pretty well. As for the Photoshop part of your question, if you want to convert an image to 300 dpi you would open the image up, go to image - image size, uncheck the resample checkbox, and type in the new dpi. Notice that with resample unchecked the pixel dimensions don't change when you change the dpi, only the size in inches (or cm, etc...).

The way I think of it, dpi tells the printer how many dots to print in every inch. Every dot represents 1 pixel. If my image is 300 pixels wide and printed 300 dots per inch, that's 1 inch. If it's 150 dots per inch, it's printed 2 inches wide, and so on.

Something else to understand is that if you don't change the dpi value and then print the image, and in the printer settings you have it shrink the image to fit a certain size, that's going to increase the dpi value automatically to suit the size you specify. If you're just having your photos printed as 4x6s or 8x10s, whether at home or at some photo lab, you don't really need to do any kind of manual resizing.
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Old Jul 20, 2007, 11:45 AM   #4
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Wow that totally makes sense and the link helps too!

Thank you so much, i love learning!
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Old Feb 13, 2010, 7:27 PM   #5
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Good link JimC. It explained what confused me. This is my first post. I was about to ask this question you answer here when I did a search and ended up here.
Great Board. I'm looking forward to hanging out.
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Old Feb 13, 2010, 8:35 PM   #6
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Welcome to the forums. I'm glad that helped.
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