All About Digital Photos
 
The Myth of DPI

Many people seem to get hung up on the DPI (dots per inch) or PPI (pixels per inch) setting within a digital photo as a measure of the quality of those photos. To set the record straight, the DPI/PPI setting within a digital photo has NOTHING to do with digital image quality! The resolution of a digital image is its pixels (usually expressed as megapixels).

The PPI of a paper print IS a measure of quality (of the paper print, not of the digital photo) - but it has nothing (in real world terms) to do with the DPI/PPI setting within the photo. Confused? Read on.

Photo at 1000 dpi Photo at 10 dpi
Photo set to 1000 dpi Photo set to 10 dpi
Can You Tell the Difference?

If you've read the section titled "What is a Digital Photo" you'll know that there are four main factors that determine image quality:
1) The size (in pixels) of the digital image.
2) The quality of the recording device (camera's optics and sensor, scanner's sensor).
3) The digital format it is stored in (lossless vs lossy compression).
4) The technical proficiency and the "eye" of the photographer.
Several other factors also come into play, but the above are the main factors that determine the overall quality of the original digital photo.

The size of a digital photo is measured in pixels (the smallest colour component in a photo). The two photos above are exactly the same size, 300 pixels in width by 225 in height, and both have been saved at 25% JPEG compression. Even though their DPI has been set to radically different values, the photos are identical in quality (the pixels don't change). So, what is DPI and why are so many people hung up about it?

DPI is short for Dots Per Inch and it actually refers to printer dots per inch. These days, when used in photo software, it means PPI, which are pixels per inch. DPI and PPI are two different things, yet often DPI is used when PPI is meant, so read its usage in context. PPI is a measure of how a image is printed to a medium such as paper. Many software programs call PPI a measure of "resolution" which leads to more confusion since it is the resolution of the printed output, not the "resolution" of the digital image (see Adobe Photoshop Image Size Dialog Box for an example of this).

If the pixels of an image do not change, then it has the same digital resolution no matter what number is parked in the DPI/PPI setting of the photo. One factor in the quality of printing to paper is many pixels per inch are delivered to the printer, a number between 200 and 300 is generally accepted to represent "photographic quality" at an arm's length viewing distance. But that quality, the PPI, is determined by how many pixels the image has - not the number parked in the DPI/PPI setting section.

Part of the confusion (and the myth) is generated by the fact that the PPI setting can (and is by some) used to determine the print size on virtual paper. It's often how graphic artists use photo programs, to virtually print size the photo. This has led to the erroneous view that this setting controls the resolution of the digital photo itself. But a digital photo's resolution are its pixels and if those don't change, the photo remains the same resolution.

The DPI/PPI number in the photo is simply a convenient at times conversion factor. Printing on all of today's home printers and most modern commercial printers involves remapping the photo, converting the pixels of the photo into printed dots. This isn't done on a one to one basis, one pixel does not equal one printer dot.

In illustration of digital resolution, with the two photos shown above, each 300 pixels wide, they will print to the same size paper at identical quality even though one has a setting of 10 dpi/ppi and the other to 1,000 dpi/ppi. If they were both printed to 6 inch paper, they would both be printing at 50 pixels per inch. The printer itself may be using 1,400 dots per inch, having remapped the pixels into its dots. With the printed output, you would not be able to tell which one was set at 10 dpi/ppi and which was set at 1,000 - since the printed photos would be identical.

So, when someone asks you for a ### dpi photo, they really mean a digital image of sufficient pixel dimensions to be able to meet their required pixels per inch requirement (usually 300 ppi) at the dimensions of the printed image. For a more extended discussion of this see What Print Shops Really Want. It doesn't matter what number is parked in the DPI/PPI setting of the photo - what counts is that there are enough pixels in the digital photo to meet their minimum PPI requirement.

Of note, camera manufacturers such as Canon have it right, they properly refer to digital resolution as the pixel dimensions of the photo.

DPI/PPI - Another Definition

For some, it is easier to think of the DPI/PPI of a digital photo simply as a conversion factor that some software uses to set the paper output dimensions. This is basically all it is - it is a 5 byte (very tiny) string in the header of a digital photo file. It's not actually part of the image, it does nothing within the photo file (it doesn't change the digital photo in any way), but some software will use it to set the paper output dimensions (in inches or centimetres).

Some software will also (unfortunately) use it to re-sample an image (see "The Horrible DPI Mistake" - below). My general advice is to never change the DPI unless some silly photo shop / graphics designer asks for it to be changed - then change it using my Change DPI instructions. When printing with a program such as Photoshop that uses the DPI to set paper output, print using the "Print with Preview" option, using the "Scale to fit media" selection to scale the photo to the print size you desire (without changing the DPI). If you must resize a photo, don't use DPI to do it, change the pixels (see the Change Size section).

The Horrible DPI Mistake
Here's the scenario - a print shop/graphics designer/magazine asks a client for a photo at 300 dpi. They wish to print it out at 5" x 7". The client already has a beautiful digital photo with pixel dimensions of 2048 x 1536. The client notices that the photo editing software is showing that the photo is set to 72 dpi. So, following orders, the client types in 300 to reset the dpi to 300. In doing so the image is resampled and is enlarged over 4 times to pixel dimensions of 8533 x 6400. The client sends this enlarged 300 dpi photo. The print shop/graphics designer/magazine reject it (too grainy, too colour blotched). The client is crushed. The sad thing is that the client already had the perfect photo (2048 x 1536 @ 72 dpi) which would have been beautifully printed at 5" x 7" (at 292.6 PPI). The print shop/graphics designer/magazine didn't know what they really wanted - see What Print Shops Really Want and the client didn't know how to change the DPI without resizing the image to give the print shop what they mistakenly think they need - see how to change the DPI.

So - why should I care about DPI?
Well, if you are using an older photo program it may use DPI to set the the size of the printed output. With these programs you'll have to adjust the DPI in order to adjust the size of the printed output. This is becoming a thing of the past since most newer photo programs simply allow you to set a size output (i.e. 5" x 7") for the image, regardless of the DPI setting. Programs that use DPI to set the size of printed output are in fact using PPI, they aren't telling the printer how many dots per inch to print (DPI), rather they are sending the printer x number of pixels per inch (which the printer may well print at a much higher DPI).

Some programs such as Word Processors and Desk Top Publishing programs will use the DPI of an image to set the default size of the image. For example, a 1200 pixel wide image set to 200 dpi will load into such a program at a size of 6 inches. All modern programs allow you to easily visually resize such an image, but it can be convenient to set the DPI of images you plan to use in such programs to the DPI that will match the approximate paper size that you intend for that image. So it you have a 2100 pixel image that you want in a document at 3 inches in width, set the DPI to 700. Of note, always set the DPI without resampling the image. See the page titled Changing the DPI of a Digital Photo.

Scanning - DPI Does Count
Scanning is the process of converting paper to digital and in this process DPI is used to adjust the amount of detail of the scan. The DPI setting of the scanner relates to the final pixel size of the scanned image. If you put a 5" x 7" photo on the scanner and scan it at 300 dpi, the resulting digital image will be 1500 x 2100 pixels in size (5" x 300 = 1500 and 7" x 300 = 2100). In this case, DPI does relate to quality, since the higher the scanner DPI setting the more information is being collected. Keep in mind though the 200 ppi = photo quality concept, a minimum of 200 dpi should be used in scanning. Best results for paper photos are generally achieved within a range of 300 dpi (sufficient for most photos) to 600 dpi (if you want to enlarge the image). See the Scanning Page for a full discussion.

A Word of Warning
Some programs will resize a photo when the DPI is changed (see my example "The Horrible DPI Mistake"). Be very careful of this. To change the DPI without changing the pixel size of the photo you should click on the "maintain original size" (i.e. Corel Photopaint) or similar option that some programs offers, or click off "resample image" that other programs offer (i.e. Adobe Photoshop). See the page titled Changing the DPI of a Digital Photo.

DPI of Printers
As mentioned above that there is no direct connection between a pixel in a digital image and printer dot. When printing, the printer software takes the digital image and re-maps the digital pixels into printer dots using its own custom algorithms. At the same time it is converting the colour space of the image from RGB to CMYK (see colour models for explanation) - or even if the digital image is set as CMYK, into its own form of CMYK to match how it does colour.

In the printing process, the quality of output is dependant on the amount of digital data delivered into the conversion procession (the PPI), the quality of the printer software (the re-mapping algorithms the printer software uses), the physical attributes of the printer (types of printing heads, quality of ink, etc.) and the quality of paper.

The rated DPI of many home ink-jet printers is in the +1,000 range - the caveat with this number is that it represents a blended dot (overprinting a single "dot" with different colours to create the colour that is needed). There is no direct relation between a pixel in a digital image and the printer's "dot" - but both play a part in the quality of output. If you deliver 50 pixels per inch to a printer printing 1,200 dots per inch, you're not going to get a good print. The reverse holds true, if you deliver 1,200 pixels per inch to a printer doing 50 dots per inch, you're also not going to get a good print.

So, for good printing, it's not just the rated DPI of the printer that counts. It's how the printer does the conversion process and how it then delivers that to paper. So, when buying a printer, don't just read the specs, read the reviews that look at the quality of output. If you have a good printer and deliver enough pixels per inch from a good quality digital photo, you'll get a good "photographic quality" print (see home printing for more info.)

A Final Test
Congratulations if you've read all the words on this page (I tend to ramble at times). Did you have the "ah ha" moment when it comes to understanding DPI/PPI.

Test 1: this was the first photos on this page, both 300 pixels wide, one with a setting of 1000 dpi the other with a setting of 10 dpi. The test answer is that they are identical in digital resolution.

Test 2: which photo is higher resolution - a 2000 pixel wide photo set at 72 dpi or a 1000 pixel wide photo set at 300 dpi? Having read the material on this website you'll immediately know that the 2000 pixel photo at 72 dpi is higher resolution than the 1000 pixel photo at 300 dpi. If you don't agree/understand, then click here and start again :-)


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