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 Post subject: DPI ?
PostPosted: Sun Feb 15, 2009 6:27 pm 
Hi all

Thanks to those who helped me with my previous query.

I have another question: this time about dpi - what exactly is it? I know it stands for "dots per inch" but what does this imply in practice?

I have been shooting in RAW and am converting some of the files to JPEGs in order to print them. Canon's DPP software asks me to choose an "Output resolution" at the point of saving the JPEG. However, which ever number I choose, it doesn't seem to affect the converted JPG. The file size is the same as is the Image width and height.

So what exactly does the dpi number mean?

Thanks for any help you can give.
Rimsky


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 Post subject:
PostPosted: Sun Feb 15, 2009 6:57 pm 
DPI doesn't mean anything when viewed on screen, since each pixel of the image is mapped to a pixel on the monitor when viewed at 100% zoom. On the other hand, DPI is a useful calculation for when you make physical prints. As a general rule, you want to print at 300 DPI which means a 3000x2000 photo will give you 10"x6.67" before quality severely degrades.


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 Post subject:
PostPosted: Sun Feb 15, 2009 8:21 pm 
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Weell, DPI actually does mean something on a computer screen. For instance,if you go to the XP Pro "Display Properties/Settings/Advanced/General" tab you can set up the DPI with default values of 96 or 120 provided and the ability to specify a custom setting. For example, if your screen is just over 13" across and it has a horizontal resolution of 1280 pixels then 96DPI is an appropriate setting. This allows programs to display objects at realistic sizes.

But it's absolutely correct that associating a DPI setting for an image in a program like Photoshop is really to do with printing.

Bob.

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 Post subject:
PostPosted: Sun Feb 15, 2009 9:43 pm 
Thanks everyone.

I'm still a little confused. Surely, if I take my 3800 x 2500 JPEG file to a photo printing company and say I want it printed on 7 x 5 paper, it shouldn't matter what dpi it's saved at. The printers will just scale it accordingly.

Rimsky


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 Post subject:
PostPosted: Sun Feb 15, 2009 10:39 pm 
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Rimsky wrote:
The printers will just scale it accordingly.


Ever printed a 72 dpi picture?


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 Post subject:
PostPosted: Sun Feb 15, 2009 11:23 pm 
DPI is just a way to compress raster files, 72 DPI is common for websites 300. DPI is common for printing. maybe you aren't seeing a big difference because your printer just isn't that great. I'm sure if you had them printed from a professional lab you would see a difference from 72 to 300 DPI.


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 Post subject:
PostPosted: Mon Feb 16, 2009 12:17 am 
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Rimsky wrote:
...if I take my 3800 x 2500 JPEG file to a photo printing company and say I want it printed on 7 x 5 paper, it shouldn't matter what dpi it's saved at. The printers will just scale it accordingly...

Have a read of Understanding Digital Pixels: PPI, Dithering and Print Size. :idea:

Bob.

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Olympus OM-D E-M1 + M.Zuiko Digital ED 12-40mm f/2.8, Lumix 7-14mm f/4, Leica DG Summilux 15mm f/1.7 ASPH, M.Zuiko Digital 45mm 1:1.8, M.Zuiko Digital ED 75mm 1:1.8.
Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


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 Post subject:
PostPosted: Mon Feb 16, 2009 11:24 pm 
Thanks again.

I think I'm being really dumb as I still don't understand. :?

As an experiment, I just converted the same RAW file as three JPEGs with varying DPIs: 10 DPI, 350 DPI and 50,000 DPI. In all cases, the files created are the same size as each other. As they appear to have the same amount of information, how can this affect the resolution?

Surely the printer just works out the DPI when you instruct it to print say a 3800x2500 image to 7x5 inch paper. It would be 3800/7 by 2500/5. Why should it be necessary to specify the DPI at the point of creating the JPG?

Very sorry to labour this but I want to understand it before I get a load of my shots printed.


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 Post subject:
PostPosted: Mon Feb 16, 2009 11:37 pm 
The same question came up in my mind as well.. I mean after all, isn't DPI just a result of having X pixels in Y print size?


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 Post subject:
PostPosted: Tue Feb 17, 2009 7:54 am 
Rimsky wrote:
Surely the printer just works out the DPI when you instruct it to print say a 3800x2500 image to 7x5 inch paper. It would be 3800/7 by 2500/5. Why should it be necessary to specify the DPI at the point of creating the JPG?


Unless I am thoroughly mistaken, I believe that this is correct. DPI is just a way of guiding you so that you can determine the physical size of the image you're working on (i.e. the size in inches/cms).

If you send a 3800x2500 image to be printed on 7x5" paper, it shouldn't matter what DPI it's saved in as I would hope that the printer would be smart enough to work out how best to print and not automatically downscale the image to 504x360 pixels because it thought you requested a 72 dpi print.


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 Post subject:
PostPosted: Tue Feb 17, 2009 8:25 am 
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Another way to look at it is DPI only means something if you want it to. Since I don't work in print, it and related physical dimensions are meaningless to me and even annoys me when it appears in photoshop. I just want the pixels!

For professional printing, I believe they do use DPI figures so you can send any pixel size file with DPI value, and get the correct size print output. This wouldn't apply to your average consumer photo printing service.

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 Post subject:
PostPosted: Wed Feb 18, 2009 11:44 am 
I don't think DPI is that important when saving the JPEGs. I've always thought of the DPI measurement as a means of connecting the resolution of the digital image with the size of the physical image (the printout).


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 Post subject:
PostPosted: Wed Feb 18, 2009 8:41 pm 
OK, I think I get it now.

Thanks for your help guys.


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 Post subject:
PostPosted: Wed Feb 25, 2009 8:17 pm 
the software i use lets you set the PPI (pixels per inch) fom 150 upto 600. for both print and poster images


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