DPI And Photos

What Print Shops Really Mean by DPI : Okay – your print shop, graphics designer, or magazine has asked for a digital photo at 300 DPI. What do they really mean by this?

What they are really asking for is a photo that will print at a certain paper dimension in inches at 300 pixels per inch (PPI). the term DPI is a holdover from when this setting in a digital photo would set the paper output quality (resolution) of a printed image (number of printer dots per inch). This is no longer the case, but people still confuse DPI with PPI.

Photo set to 1000 dpi Photo set to 10 dpi

Can you tell the difference ?? Answer : No you can’t, because there is no difference

Back to our print shop – if they are looking for a digital photo to print at 10 inches by 8 inches, at 300 PPI, then they are really looking for a digital image with a resolution of 3000 pixels by 2400 pixels (regardless of the DPI setting of that image).

What Print Shops Really Need:  The concept that 300 PPI = photographic quality is also a holdover from the quality of printing equipment a decade ago. Present day printers will output a good quality digital photo, with “photographic quality” at 200 PPI – so the requirements for a 10 inch by 8 inch paper photo become a good quality digital image with pixel dimensions of 2000 pixels by 1600 pixels.

A good quality digital photo is one:

  • 1- taken with a good quality digital camera (good optics and digital sensor)
  • 2- a photo that has not been enlarged either in post-processing or by in-camera digital zoom (never (ever) use digital zoom).
  • 3- a photo that has been properly shot (good lighting, no blur)
  • 4- a photo shot within the camera’s ideal ISO range (usually a low ISO such as ISO 100)
  • 5- a photo that has been stored in either a lossless format (i.e. TIF) or a very low compressed JPEG (highest camera JPEG quality setting).

Such a photo will reproduce on paper at photographic quality (assuming current printing technology) at 200 PPI.

A Note to Print Shops and Graphics Designers: I keep getting emails from poor folk who say that their print shop or graphics designer keep asking for digital phots at “number of” DPI (usually 300 DPI). Please STOP DOING THIS to your poor clients – ask for what you really need, pixels in your prefered format (i.e. low compressed JPG or a TIF). If your need is for a digital photo that can be printed at high quality at a width of 6 inches, and you think that 300 ppi is what is required to do this (based on your equipment), then you need a digital photo that is 1800 pixels in width (regardless of its DPI setting since it’s a meaningless figure).

So, ask for a digital photo that is at least 1800 pixels in width. Tell your client not to resize the photo. If the photo is less 1800 pixels, ask them to send it along anyway (so that you can test it) and that if it is larger than 1800 pixels it’s also okay (don’t have them resize a larger photo down to your “at least” pixel figure). If you are running older equipment/software that needs a particular DPI setting, then set it in the photo after you have received it.

Also, please don’t ask for a digital image of  0000 Kb in file size (I’ve had print shops do this). This is even more meaningless than the DPI figure since the file size depends not only on the format (JPEG, TIF) but the compression used with that format. If you are in a long term relationship with the client, educate the client to take photos at the highest quality setting their digital camera allows (and then ask for a copy of the original photo) or if they are scanning (where DPI does count in converting paper inches to digital pixels), specify the resulting pixel dimensions that you want.

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, photo printing programs or graphics programs such as Adobe Illustrator or 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.

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, Corel Draw or Adobe Illustrator 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.

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.

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 – 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.

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.

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).

If everyone simply worked with pixels we’d have a happier (and much less confused) digital photo world. ~ Courtesy of K. Watson

Still want to change the DPI Size in your photos, read this: How to change the DPI in your Photos.

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