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12-20-2006, 06:30 PM   #1
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DPI on K100D images

I can't find the answer to this one anywhere.

When viewing EXIF data on my images (such as on Flickr: http://www.flickr.com/photos/technosavant/), it shows as being 72dpi. My brother's Nikon D50 shows as producing 300dpi images (via in-camera processing). I was browsing through other K100D images, and saw some variation; most were 72dpi, with a few 300dpi mixed in. I can't find one single thing about it in the manual.

So, I wonder:
1) Exactly WHAT does the dpi of a given image matter if the resolution is the same? It seems like it is a spec without a context, especially if the resolution is identical.
2) Is there an in-camera setting I just can't find? Admittedly, I am shooting JPEG in best quality, but if the answer is to shoot RAW, I guess more memory cards are in my future.
3) Should I just get over it and tell my brother that it doesn't matter?

12-20-2006, 06:42 PM   #2
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>>>>>3-it doesn't matter.

Terms ppi and dpi are actually two different things; but it really doesn't matter unless you have hardware level control over printing. And printing doesn't actually, require, exactly 300 ppi. Nor is it an optimum or standard.

Screen resolutions haven't been 72ppi , exactly, for more than 25 years. Even then it was never a standard. Todays monitor vary widely from about 80ppi to 120 or more ppi.

The number in the EXIF is arbitrary, typically set by manufacturor-not user changeable, easy to change in photoshop. It has no effect on the image.
12-20-2006, 08:54 PM   #3
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DPI, PPI and physical size

Short answer:

If you just want to view on screen or print to a specified paper format letting your software scale your photographs up or down to fit required size, it absolutely does not matter to you.

Long answer:

PPI (not DPI, see clarification below) parameter does only one thing: it tells image editing programs how to translate pixels from virtual world to physical dimensions. It is only relevant if you want to prepare your photograph for printing in SPECIFIC size with SPECIFIC PPI setting. Notice that physical size and printing PPI are input parameters. For example, if the requirement is to print 4x4" size with output at 300 PPI, you will need 1200x1200 pixels for that: input parameters are 4" @ 300 PPI, output parameter is 1200 pixels of required resolution (4 x 300).

One clarification: Embedded DPI should in fact be called PPI (Pixel Per Inch). But since monitor dot is the same as pixel, PPI = DPI.

It becomes interesting when it comes to printing: you render physical output at 300 PPI and print at 1200 DPI. That means your pixels will be spaced at 1/300" (placing 300 pixels for one inch) while printer will use smaller "dots" to render each individual pixel which is necessary for good color rendition. In fact, printing at 150 PPI @ 1200 DPI improves color accuracy since each pixel gets represented by 8x8=64 dots -- enough for excellent tonal rendering, better that 300 PPI @ 1200 DPI where you have 4x4=16 dots per pixel (still enough in most cases though).

BTW, load your image to Photoshop and select Image -> Image Size. You will see that you can change "Resolution" in pixels/inch (PPI), not Dots Per Inch (DPI). You set DPI in printer driver, not image editing programs.

Was this helpful, or you are even more confused now?
12-21-2006, 10:28 AM   #4
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It is helpful. I haven't played with Photoshop yet; I am planning to tool around in GIMP and see if it meets my needs before potentially going for Photoshop Elements (Photoshop CS, at $600, is right out).

So, let me see if I am getting it:
1) For display on a monitor, there is nothing more useless than this spec (well, except for a dead fish on the monitor, but that speaks more to housekeeping skill).
2) For printing, as long as I stick to "accepted" norms (such as a 4x6 or 8x10 print size), the software will do all the scaling behind the scenes, and the spec is still not a big deal.
3) The only time it really becomes an issue is when printing to a specific size, where the scaling isn't automatic (such as for inclusion in a magazine or such; not an issue for me now). For that, a competent image editing program can do the scaling without breaking a sweat.

Thanks for the help.

12-21-2006, 11:55 AM   #5
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That's it!

QuoteQuote:
So, let me see if I am getting it:
1) For display on a monitor, there is nothing more useless than this spec
Correct.

QuoteQuote:
2) For printing, as long as I stick to "accepted" norms (such as a 4x6 or 8x10 print size), the software will do all the scaling behind the scenes, and the spec is still not a big deal.
Correct.

QuoteQuote:
3) The only time it really becomes an issue is when printing to a specific size, where the scaling isn't automatic (such as for inclusion in a magazine or such; not an issue for me now). For that, a competent image editing program can do the scaling without breaking a sweat.
Exactly!

For example, if several people collaborate on a marketing brochure or a magazine, designer may provide space for a photograph of, say, 3x4". Knowing that it will be printed at 150 PPI, you as a photographer must deliver to a designer finalized photograph (including sharpening and all other corrections) of exactly 450x600 pixels, with 150 PPI embedded parameter. Once designer selects your photograph to insert into a 3x4" space DTP software will automatically load and place your photograph without ANY changes. Although every DTP system can rescale photographs, resulting quality may not match the rest of the publication. Sometimes you can see that in cheaper magazines: some images are not so sharp, or they look scaled up with obvious loss in resolution. But if you work on next Cosmopolitan, Vogue or National Geographic issue rescaling an arbitrary input size with DTP won't be acceptable at all.
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