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I have canon 400D. my photos (jpgs) are usually from about 3.5 MB to 6 MB (3888 x 2592 in pixels). How can I resize them to get good 10cmx15cm photos & faster upload?

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Please see this thread for detailed information on generating quality prints: photo.stackexchange.com/questions/1715/… –  jrista Aug 26 '10 at 1:13
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4 Answers

up vote 6 down vote accepted

JPEG Quality of 9 ~ 10 out of 12 (or 70 ~ 84 out of 100) is pretty indistinguishable from uncompressed. See this article for an in-depth comparison. In short, if you have less color gradients, you can get away with higher compression (lower quality values).

For PPI (what you care about), in general, 240 to 360 PPI is high quality. This depends on typical viewing distances and your audience. For example, with posters where people won't be walking up to and scrutinizing, you can get away with lower PPI because the viewing distance is further.

Ideally, you should find out what the printer's native PPI (not DPI) is and use a quality program and algorithm to resize (including upscaling) to that resolution, as opposed to letting their software or printer do the resizing.

To calculate the number of pixels, simply take your desired physical output size, convert to inches if necessary, and multiply by the PPI:

10 cm by 15 cm
x 1 inch / 2.54 cm
x 250 pixels / inch
= 985 pixels by 1477 pixels

10 in by 15 in
x 250 pixels / inch
= 2500 pixels by 3750 pixels

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Quick note...200 PPI (note, DPI and PPI are not interchangeable terms, and PPI is what Eruditass is talking about here) is a fairly low resolution. A 10x15 inch print would need to be viewed at a distance of about 17-18" to appear appropriately clear at a PPI of 200. A PPI of 240 would be viewable at 14 inches, and a PPI of 300 would be viewable at about 10 inches. I would avoid using a PPI of 200 unless the image will be viewed by standing a couple feet away, otherwise I would use 240 or 300 for prints viewed within a foot. –  jrista Aug 26 '10 at 16:27
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Thanks, updated. Your thesis on the other question was quite informative! –  Eruditass Aug 26 '10 at 17:22
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Guys, rkrass asked about 10x15 centimeters, not inches:) Divide the number of pixels per 2.54. –  decasteljau Aug 26 '10 at 19:47
    
It didn't say centimeters at first... –  jrista Aug 27 '10 at 1:09
    
@Eruditass: your units don't add up (or is it just me?). Inch*Inch*pix/Inch = Inch*Pixel and not pixels –  Shaihi Aug 30 '10 at 8:30
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For printing, i usually scale photos to 300dpi. Since 10cm x 15cm are approximately 4in x 6in, that means scaling to 1200 times 1800 pixels.

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If you re-save at JPG quality 10 instead of 12, you'll get smaller files without sacrificing resolution. You may not want to do this as a general rule though.

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Depends on your audience. I have printed and sold a lot of 2MP Sony Mavica images printed on matte paper to 8x12 at 180 dpi. Some of the answer is in the paper you print on. Matte and semi matte paper have a tendency to wick the inks, they have less D-Min, but this helps mask the dots. There is also stitching software if you want greater resolution. I use a canon 7D now mostly at 10, sometimes 18mp, but my best selling images (all I sell are monochrome) were taken with a 5MP camera. My clients don't care about dots on the paper, just the feeling of the image.

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