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Old Jun 23, 2010, 1:58 AM   #1
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Default Camera default ppi

I've noticed that my husband's DSLR Nikon D3000 takes images at 300 x 300 ppi. My Ultra Zoom Olympus SP 590, however, takes them at 72 x 72 ppi. Is it possible to alter this? (I think I know the answer - but just hoping).

Thanks for any information.

Bernice
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Old Jun 23, 2010, 2:38 AM   #2
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They're just numbers in the image header, and they're 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

Those numbers are just fields in the image header. Some cameras plug in 72 ppi, 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 72 pixels per inch. :-) Again, it's only a number placed in the image header and the actual pixels per inch 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

If you have a unique software application that uses the PPI information for the initial size the image is displayed on a page at (freestyle, etc.), there are ways to edit the number stored in the header. Many EXIF editors can change the numbers stored for you.

For example, you can click on the ppi numbers for image resolution in height and width using PhotoME (free) and change them to anything you want (you'll find them in the Image section of the EXIF info with that software).

But, for most applications, the ppi numbers are not used at all.
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Old Jun 23, 2010, 2:52 AM   #3
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Thanks Jim. I'll print out your explanation and mull over it at home. I had been told it's preferable to have image resolution between 180 - 300 ppi if I wanted to print 25 cm x 30-35 cm (that's about 10 x 12 - 14 inch isn't it?) and because all the exif data on my photos tell me it's 72 x 72 I didn't think I'd be able to get good prints (allowing that the photos were sharp enough of course). But from what I understand of my first reading of your explanation I might be OK to try this size.

Thank you.

Bernice
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Old Jun 23, 2010, 3:13 AM   #4
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Again, the numbers in the header have *nothing* to do with the dpi you're printing at for a specific print size with most software if you're sending the full resolution image to the printer. ;-)

You determine that by the resolution of the image in pixels, and the size of the print you want to make. See the examples in my first post above.

For example, if you wanted a 10 x 12 inch print at 180dpi (about the minimum you want to use with most subject types, with higher dpi being better), you'd need an image with a resolution of 1800 x 2160 pixels (10 inches x 180 = 1800; 12 inches x 180 = 2160). IOW, you'd want a 3.9 Megapixel Image (1800 x 2160 = 3,888,000 pixels, or approximately 3.9 Megapixels) to insure you have at least 180 pixels per inch for that print size.

You should have more resolution than that after cropping for the correct aspect ratio for that print size, since your camera produces an image size of 3968 x 2976 pixels (approximately 11.8 Megapixels), so you should end up with a higher pixel density for that size print.

Ignore the 72ppi numbers you see. They don't have any importance at all for most applications. The total resolution of the image in pixels is what matters to insure adequate pixel density when printing.
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Old Jun 23, 2010, 6:42 PM   #5
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Thanks Jim for your time and patience in explaining this. Yes, my camera says it produces 11.8 Megapixels for a single shot image and 4.9 when on Hi1 drive continuous mode.

Your help is much appreciated.

Cheers

Bernice
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