DPI Secrets Revealed

72 dpi
300 dpi
3,000 dpi
Kristine browses the menu at 72 dpi - 23.7k   Kristine browses the menu at 300 dpi - 23.7k   Kristine browses the menu at 3,000 dpi - 23.7k

DPI is TOTALLY confusing!!!

It just about drove me around the bend in the early days of digital darkrooms, since there was no web at the time. I kept looking for a relationship of dots-per-inch (dpi) in processing, dpi in scanning and dpi in printing. Information was extremely scarce, and much of it SEEMED contradictory.

It WAS contradictory, since I was looking for a relationship that simply did not exist. Each exists as an entity unto to itself and none really bears much relationship to the other - they just happen to use the same convenient term. In some cases, a more precise term would be pixels per inch, but in practice reference to dots per inch and pixels per inch have been pretty much merged.

Monitors

Computer screens have resolutions between 72 dpi and 105 dpi at common settings but dpi has no relevance at all in practice - monitors have no understanding of dpi at all. All contemporary image processing applications let you zoom in and out enlarging or diminishing the image without altering it.

In sizing pictures for the web, only the pixel dimensions are relevant. Thus a common 640 x 480 image on my web site will look exactly the same size on your screen, whether I set the dpi at 72 dpi, 300 dpi or 300,000 dpi. Furthermore a 640 x 480 image that I compress to 48k will be 48k no matter if it is 72 dpi, 300 dpi or 300,000 dpi. Browsers ignore dpi TOTALLY. See images above.

Scanners

Scanners specifications speak of the scanner's optical resolution in dpi. A film scanner may be as high as 4,000 dpi. This means that scanning a 1" x 1-1/2" 35mm frame, one will get a file roughly 4,000 pixels by 6,000 pixels if one scans at full resolution. In this case, dpi describes the relationship between the size of the original material and the resolution of the scanner - NOT THE FILE IT PRODUCES - except in terms of absolute pixels. The file it produced can be any dpi you want - 72 dpi, 300 dpi or 300,000 dpi or whatever and it won't mean a thing at this point.

However, most scanning software allows one to scan to a print size and now dpi does become meaningful and resolution in pixels does not. My specific printer wants to see a file with the dpi set to around 240 dpi. So if I were wanting to make a 5" x 7" print of an image I am about to scan, I can set the dimensions to 5x7 and the dpi to 240. Then I can draw a rectangle on the preview to select the area I want to scan and the scanner will give me a file that will print exactly that size. It does the calculations and will set the pixel dimensions to suit. When I open it in Photoshop and select print preview, it will show the image as being 5x7. I can then put 5x7 paper in the printer, select a paper size of 5x7 in the driver, select borderless and click on print. A perfect borderless 5x7 emerges from the printer.

Printers.

Now there is also the resolution that the printer prints. Again, this has absolutely NOTHING in the way of a relationship to what dpi the scanner does, or the dpi set in the image processing program. It is the number of ink-splats per inch in effect. Thus if I wanted a quick draft of an image while finding the best combination of settings, I could set my printer resolution at its lowest - 360 x 360 dpi - and it would quickly print. I would use the recommended FILE resolution of 240 dpi! Again no relation - the printer wants to see a 240 dpi file coming in, but the driver and firmware in the printer will convert it to 360 dpi! Now if I want to print a final high quality print, I will change the PRINTER resolution to 1440 x 720 - but still print from my 240 dpi file!

So you scan either at the maximum resolution of the scanner - if you simply want to archive an image with the maximum detail the scanner can gather. Or you can scan with a print in mind, so you will need to tell the scanner the resolution and size of the image and let it set the actual pixel resolution.

Once in the image processing program, dpi is only meaningful if you are making prints. If you are processing for a slide-show, web-site or e-mail, dpi is of no relevance at all.

Digital Cameras

If you are dealing with a digital image from a camera for a slide-show, web-site or e-mail, dpi is of no relevance at all.

My camera's maximum pixel resolution is 2560 x 1920 and the size of the picture and the file would be exactly the same if it was saved at 72 dpi, 300 dpi or 300,000 dpi. Until an image is scaled or cropped for printing, the dpi is just a place holder in the file format.

There have been messages in forums where a new camera owner discovered that his camera saved at 72 dpi while his friend's camera saved at 300 dpi and was irate at the company for foisting off a low resolution piece of junk on him, and wondering if he should return it. It makes no difference at all until the image is scaled or cropped for printing - as above - until then, the number is just a placeholder of no significance at all.

If you are making a print from a digital image from a camera, then you will need to crop or scale the image to the size of the print, using the dpi that the printer wants - in my case 240 dpi or thereabouts.

Just how important is this stuff, anyway?

Notice I said "or thereabouts"? A printer will accept a file at any resolution without complaint. If I were to make a panoramic print at the maximum size the printer will handle - 13" x 44" at 240 dpi - the file would be 94.2MB. I know that a print that size would not be viewed closely, so I could probably drop the resolution to half that - 120 dpi. Now I only have a file 23.5MB, which is easier to store and requires much less RAM to process. Since the human eye is limited in its resolution, the print would still look sharp and richly detailed from a normal viewing distance.

What about coming from the other direction? Can you gain added sharpness and detail by exceeding the 240 dpi recommendation? Well, I actually tested this. I took a five megapixel image (2560 x 1920 pixels) and reduced it to 5" x 7" at 240 dpi (1200 x 1680) and printed it on one side of a sheet of high-quality letter-sized photo paper. Then I took the same image and reduced it to the same size at 360 (1800 x 2520) dpi and printed it on the other half of the sheet of paper. I showed it to a number of people, and some said they could see a difference, but no one would say that one was "better" than the other.

©2003 Larry N. Bolch
December 14, 2003