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Old Mar 23, 2006, 2:00 PM   #21
me2
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Great thread, guys.

So... lets say I want to test how big I can blow an image up or how many DPI I need to make a good image. How would I do this ? I'm thinking something along the lines of take an image (a special one?) , put it in photoshop or similar and save it as various sized jpeg files and send it off to print on 4x6 paper ? One could then compare 100, 150, 200, 250 DPI ?

Would this be a good test ? By good I mean representative ?
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Old Mar 23, 2006, 6:08 PM   #22
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me2 wrote:
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Great thread, guys.

So... lets say I want to test how big I can blow an image up or how many DPI I need to make a good image. How would I do this ? I'm thinking something along the lines of take an image (a special one?) , put it in photoshop or similar and save it as various sized jpeg files and send it off to print on 4x6 paper ? One could then compare 100, 150, 200, 250 DPI ?

Would this be a good test ? By good I mean representative ?
A better test would be to crop different sized 4 X 6 sections so you end up with the various PPIs. They don't have to be exact. I use 4 X 5.33 images because I can fit four on a sheet of 8.5 X 11 photo paper. If you have Photoshop you can get them exact with the rectangular marquee tool in fixed size mode. To get 100 PPI you would set 600 X 400 pixels. For 150 PPI set 900 X 600 etc. Inverse the selection and hit the Delete key. Select > Deselect and crop out the visible part left.

Another interesting exercise is to crop out small sections that are say 100 and 150 PPI at print size and upsample both to 300 PPI. Print both the original crops and the upsampled images. You don't gain as much by an upsample as some people suppose.

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Old Mar 23, 2006, 7:15 PM   #23
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[Please delete - ghost post due to file upload server error]
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Old Mar 23, 2006, 7:19 PM   #24
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One possibly useful exercise to determine what resolution your printer can usefully resolve is attatched. Open it in an image editing program and print out exactly as-is (don't change the size or 'fit-to-page'). Look closely at the result (with a magnifying glass if necessary). If you can see distinct vertical lines at any particular resolution, it's worth printing at that resolution (interpolating if necessary); if the printer can't resolve the lines, it will appear as a grey block.

For comparison, mine can resolve 300 dpi -- wait, sorry, ppi -- on photo paper, and 200 ppi on normal paper.

N.B. The server gives an 'invalid file extension' error when you try to upload a TIF; but saving it as a GIF (which it will upload) makes it default to a resolution of 72ppi. So, whilst the file below is a TIF, I have renamed the extension to GIF to enable it to upload. If your image editing program won't open it as-is, rename it from ...test.gif to ...test.tif
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Old Apr 20, 2006, 2:43 PM   #25
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At my website I have a comparision that shows a 600PPI image (viewed at 400%) and a 1200 DPI print (scaned in at 2400 PPI). This shows the difference between a digital image viewd on a monitor and a printed image from an inkjet.

The direct URL is www.mattspinelli.com/ppidpi.html

Below is a downsided version to fit here on this post:


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Old Apr 20, 2006, 4:15 PM   #26
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Ugh, I just read this whole thread again! Something continues to really irritate me, though.

If your image is 3000 x 2000 pixels, there will be a DPI tag in the exif (or someplace in the jpg file) which is visible in photoshop, often set to 72 DPI. What does this tag DO? It. Does. Nothing.

Nothing at all.

If the picture file has a 72DPI parameter, your program will look and say "Hmm, 3000 pixels divided by 72 means this file is 41.6 inches wide". What, pray tell, does it do with THAT information? Nothing.

Nothing at all.

If our monitor is actually 72 dpi (mine is 14.875" wide and 1280 x 1024, so its 86dpi) then looking at the image 1:1 would display it 41.6 inches wide (or 14 inches of that, anyway). This is a silly thing to consider since every monitor is a different resolution, we rarely view the pictures at 100%, and who considers the physical display size of an image when creating a webpage?

The only time we care about actual/physical image size and its pixels per inch is when its printed. When that happens, the embeded DPI tag is.... NOT USED. Gawd, I wish it wasnt there in the first place!

For prints that your going to get close to, aim for 200-300 PPI. If you'll be farther away, less and less pixels per inch are needed. I've seen HUGE photos glued to the wall at malls, they looked good but when I walked up close the pixels were the size of DIMES with enough white space between them to fit my little finger.

Its all relative.
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