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  #1  
12-28-2017, 06:44 PM
3d1l 3d1l is offline
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Hello,

My apologies if this is not the right forum. If it is not, I will appreciate if you can tell me where to go.

I'm scanning old 4"x6" color photos at 1200 DPI. In the computer the image look huge but when I go to print them the size remains the same 4"x6". For example when I check one of the photos with the program IrFanview it says that the pixel matrix is 4772x7069 pixels (33.73 Megapixels), but the size is 10.1 x 15.0 cm; 3.98 x 5.90 inches. I want to print the photo to fit in a 8.5" x 11" paper size. My printer prints at 600x600 DPI. All the software that I use is open source; Gimp, InkScape, IrFanView, etc.

Please advice.
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12-28-2017, 09:07 PM
sanlyn sanlyn is offline
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"DPI" stands for "dots per inch". Whether you have a 4x6 photo or an 11x17 photo, the number of pixels (dots) per inch of image can still be 1200 Dots per inch, but the total number of pixels in each image will be different. There will be a greater number of pixels in the large image than in the small image. if you multiply a width of 6 inches by 1200 DPI the number of pixels in the width will be 6 times 1200, or 7200 pixels. If you increase the width of the 1200 DPI image from 6 inches to 11 inches, the number of pixels in the width will be 11 times 1200, or 13200 pixels.

Meanwhile you are dealing with two different aspect ratios. A 4x6 photo has a width of 6 inches and a height of 4 inches, which is a width to height ratio of 1.5:1, and the geometric orientation of the 4x6 image will be landscape orientation (width is greater than height). An 8.5x11-inch paper is portrait orientation and has a width smaller than its height. You will have to orient the print paper to landscape, or 11x8.5 (WxH). An 11x8.5 image is a more narrow aspect ratio (1.294:1) than 4x6 (1.482:1). You will have to resize your 4x6 image to a 1.482:1 image that will fit the 11-inch width of your print paper.

If the width of your original image is 5.9 inches and you resize the image to a width of 11 inches, then the height of the resized 1.482:1 image will be 11 divided by 1.482, or a height of 7.422 inches. That is, for an 11x8.5 piece of paper to contain the full 1.482:1 image without losing image data from any dimension, the image will have to be 11x7.422 inches. Otherwise, the only way you can print a 1.482:1 image onto a 1.294:1 piece of paper is to fill the height to 8.5 inches and crop 1.6 inches off the image width.

This assumes that your printer can print the entire width and/or height of an 11x8.5 sheet of paper.
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12-28-2017, 11:13 PM
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kpmedia kpmedia is offline
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This is an easy problem to overcome.

When you scan at whatever DPI, the image remains the same physical size as the scan. So a 4x6 at 300dpi is the same physical dimensions of the image at 1200dpi. But what can change is the capture of the detail. And when you're scanning something small, in hopes of upsizing it large, that matters.

What you need to do in the image program is to both double the physical size, and half the DPI.

So a 4x6 at 1200dpi = 8x12 at 600dpi.

Since I use Photoshop, I can't show you a step-by-step. But it is likely almost identical (Image > Image Size, make changes).

I scan Polaroids at 1200dpi all the time, and then 4x the image size while changing DPI to 300dpi.

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12-29-2017, 03:30 AM
sanlyn sanlyn is offline
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To add some display problems, a 4x6 image at 1200 DPI will look huge on a PC monitor (which is usually configured at 96 or 120 DPI). A 1200 DPI image with a width of 7200 pixels won't display entirely on a standard 16:9 monitor that has only 1920 pixels in width.
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12-29-2017, 08:30 AM
3d1l 3d1l is offline
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Thank you all for the feedback.

I think I got it, in order to increase size I have to give up detail/resolution. If you are scanning an already printed photo, it doesn't matter if your scanner scans at 19,200 DPI, you won't get more detail out of it but you can print it larger without loosing the original detail provided by the photo. So if you have in mind to enlarge a photo is better to scan it using the maximum DPI provided by your scanner. If you only want to print it at the original size or view it on a monitor (whatever device it is), is better to scan at a DPI that allows you to process them faster.
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12-29-2017, 08:35 AM
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Quote:
Originally Posted by 3d1l View Post
I think I got it
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