All About Digital Photos - The Myth of DPI

Many people seem to get hung up on DPI (dots per inch) as a measure of quality of their digital photos. To set the record straight, DPI has NOTHING to do with digital image quality!.


Photo set to 1000 dpi	Photo set to 10 dpi

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 quality of the recording device (camera's optics and sensor, scanner's sensor).
2) The size (in pixels) of the digital image.
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 a measure of how a image is printed to a medium such as paper (or conversely, scanned from paper). Many software programs call DPI a measure of "resolution" which leads to more confusion since it is the resolution of the printed output, not anything to do with the "resolution" of the digital image. In fact the software programs that use this setting are really just doing so because they lack WYSIWYG (what you see is what you get) output. If you do use WYSIWYG software such as most word processing, desk top publishing or graphics programs such as Corel Draw, then DPI is really a meaningless figure - the quality of output will be determined by the pixel size of the original image, the paper dimensions you have chosen to print the image (i.e. 4" x 6") and the quality of the printer.

However, for programs that still kludge along with DPI (many older photo programs), the use of DPI is a method to determine the size of output on paper. In our example above, the photo set at 1000 dpi will print out from a non-WYSIWYG at 0.3 inches in width (300 pixels divided by 1000 dot per inch = 0.3 inches). Whereas the same photo set at 10 dpi will print out at 30 inches in width. The print previews of an 8.5" x 11" page below illustrate the effect:


Printed at 1000 dpi (photo is only 0.3 inches in width)	Printed at 10 dpi (only part of the photo shows since it is trying to print at 30 inches in width)

So - as can be seen DPI is really only used to set the size of the printed output from certain photo programs. If we wanted to print this photo (300 pixels in width) at say 5 inches in width using a program that used DPI to set the dimensions of the printed output, then we would set the DPI to 60 (300pixels/60dpi = 5 inches). If we took that same photo into say Word or Corel Draw at whatever DPI, we would simply size it to be 5 inches in width and the print result would be identical (no matter what the DPI of the photo is set to).

Photo software that comes with most recent digital cameras simply allows you to specify the output size you wish to use (i.e. 4" x 6", 5" x 7", etc.) and then automatically prints it at that size. Note that the internal DPI (which we now know is not relevant to digital image quality) of most digital camera photos is 72.

DPI vs PPI
Technically DPI means paper printer Dots Per Inch while PPI means digital Pixels Per Inch. But in many instances the term DPI is used when in fact PPI is what is really meant. Even in these articles there is crossover of terms, since many software programs continue to use the legacy term DPI to mean PPI. So, best to read the terms in context. When DPI is mentioned are they really talking about printer dots per inch or are they in fact talking about how many pixels per inch a digital photo is going to print out at? This too is where many print shops, graphic artists and magazines have it wrong when they are talking about required digital photo resolution, they mistakenly think the DPI setting in a photo has something to do with this, when in fact what they want is a certain PPI (see What Print Shops Really Want).

Some programs such as Adobe Photoshop are now calling the setting of what used to be shown as DPI as pixels/inch or pixels/cm. It's a bit of one step forward, two steps back since they still have it sitting in a section called "Resolution" leading people to mistakenly think that it has something to do with the digital resolution of the photo. Some camera manufacturers such as Canon have it right, they properly refer to digital resolution as the pixel dimensions of the photo.

To be clear, neither the DPI or PPI setting in a digital photo changes the digital quality of that photo. The resolution of a digital photo is its pixel dimensions.

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).

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.