DPI stands for Dots Per Inch which technically means printer dots per inch. Today it is a term often misused, usually to mean PPI, which stands for Pixels Per Inch. So when someone says they want a photo that is 300 dpi they really mean that they want 300 ppi.
PPI is simple arithmetic, it is the digital photo's pixels dimensions divided into the paper size to be printed. A digital photo itself has no PPI, the PPI only occurs when it is printed (more explanation later).
The resolution of a digital photo is its pixels, generally expressed as Megapixels or Mp - also simple arithmetic, the horizonal pixel dimension of a photo multiplied by its vertical pixel dimension. There is tons of detail on this website about what DPI, PPI and Megapixels all mean, this page will present the condensed version.
Okay, back to DPI/PPI. A digital photo is made up simply of pixels - that's all a digital photo, or any other type of bitmapped image, is. See the What is a Digital Photo page for all those details. To get a PPI number for any digital photo you need to know the intended print size. A request for a 300 ppi (or dpi) image is absolutely meaningless in itself - the request has to be accompanied by an intended print size. A meaningful request is for a digital image that will be 300 ppi when printed to 8" x 10" (or any other physical dimension). With that information you can now calculate the PPI of your digital image when printed to that size of paper.
In our specific example of 300 ppi for an 8" x 10" print, if you had a digital photo that was say 8 Mp in size it might have pixel dimensions of 3264 x 2448 pixels (those exact numbers will depend on the camera manufacturer). Divide those dimensions by your print size and you'll get 326.4 ppi for the long dimension and 306 ppi for the short dimension. They are different because the aspect ratio (length to width) is different (a common problem) - so the smaller number applies, your digital photo will print to 306 ppi on 8" x 10" paper (with a bit of cropping). It would meet the request for 300 ppi at 8" x 10" (more pixels are okay).
We could also look at it the other way - if the request is for a photo that will print to 300 ppi on 8" x 10" paper, we can multiply 300 ppi by those dimensions and we'll get 3000 x 2400 pixels or 7.2 Mp. So, to meet the request, you'll need a digital photo that is at least 7.2 Mp in digital size (again, more pixels are okay).
In fact the simplest and best request for a digital photo would be a high quality photo (see the What is a High Resolution Photo section below for a definition of high quality), at X Megapixels, in a specified file type. That's it.
DPI/PPI Setting in a Digital Photo
The original DPI term, as used with a digital photo, goes way back to the days when a digital image was printed to a printer using a 1 to 1 ratio (1 pixel = 1 printer dot). This has not been the case for several decades and today that legacy term is only directly applicable to scanning, where one scanning "dot" equals 1 pixel (and even then it's not correct since a scanner is really scanning PPI - but most scanning software still uses the term DPI). It's also not applicable to modern printers since they use a blended dot and they completely remap your image to convert it into print.
But you will still find a DPI or PPI setting within most photo software. This is where the confusion generally starts and I've detailed it ad nauseam on my Myth of DPI page. This setting in a digital photo is simply a conversion calculator, showing you what the printed size will be at any given PPI, or, given a specific printed size, what the PPI will be. It is a useful setting for graphic artists who are used to working on virtual paper and can be a useful setting to you simply to know what your print resolution might be for different paper sizes (rather than having to use a calculator). But, it has nothing to do with the digital resolution of the photo, those are its pixel dimensions.
This is also a very dangerous setting in most photo software since it can be used to re-size the photo which can lead to much grief and misery if not done properly. Please read the Change Size section of this website before you even consider touching the number in this section of your photo software.
What is a High Resolution Photo?
A "300 dpi photo" is sometimes referred to as a high resolution photo. Again this is a badly misused term, the resolution of a digital photo are its pixel dimensions and technically high resolution would refer to the resolving power of the pixels, the number of pixels mapping real world dimensions in the field of view of the photo. But a request for a high resolution photo generally means a high ppi (usually 300 or greater) when printed. The benchmark of 300 ppi being "high resolution" was made many years ago - it's not as true now as it was in the past (I would argue that 200 ppi is often sufficient for most printing). But for sake of argument we can use 300 ppi since that's what most people request.
However there are other things that go into perceived resolution, most importantly the quality of the pixels. Are you giving the printer 300 high quality pixels per inch or 300 low quality pixels per inch? That quality has four general components:
Item 1 is whatever camera you have (dedicated, smartphone, etc.). You can read reviews on camera sites to see what quality of photo it is capable of producing. Other factors come into play here such as the ISO setting, lens and apeture settings, and more - camera sites will usually detail this information.
- the quality of the recording device (your camera - its sensors and optics)
- the digital size of the image (its megapixels)
- the ability of the photographer (proper lighting, steady shot)
- the format (file type) the digital image is saved in
Item 2 is covered in an overabundance of detail on this website.
Item 3 is your skill at photography. The two most common mistakes are poor lighting (getting the right lighting on your subject) and an unsteady shot. If I was to give a single piece of photographic advice it would be to learn how to hold your camera steady. Lighting is critical but the advantage of good lighting can be blown by a bit of camera shake (camera/lens image stabilization helps but is no substitute for a steady shot).
Item 4 is one that many amateur photographers aren't really familiar with and the one in which the most mistakes are made - the file format your photo is stored in. The most common format is JPEG (or JPG). I'll talk a bit about that now.
JPEG is a "lossy" format, that it, in order to make the digital photo smaller is loses (throws out) image data. Your camera, at its highest JPEG quality setting, will be saving your image at about JPEG quality 96. JPEG quality goes from 0 (worst) to 100 (best) and even at 100 there is some compression (loss of data). Adobe has converted the JPEG 0 to 100 scale into 1 to 12, where 12 is equivalent to JPEG 100. You can see more about JPEG on the file types page.
A common error is to download your photos to your computer, do a little bit of editing with the photo software you have, and then re-save the image. Some software will default to saving at the quality it received the photo, but most has a default JPEG quality setting, often about JPEG quality 75. That will degrade the image. So, Rule 1, even before you learn how to properly save JPEG images, is to ALWAYS work on a copy of your image, never the original (keep your originals safe and untouched). A JPEG saved at lower quality cannot regain its original quality. If you take a JPEG that has been saved at quality 75 and re-save it as quality 100, it will still be at quality 75. Professionals tend to work with lossless image formats such as RAW (a photo format option found in higher end cameras) or TIF. But most amateurs will do just fine using JPEG as long as you learn how to properly deal with this format. A JPEG at Adobe quality 10 or JPEG 94 will be sufficient for most purposes including high quality printing.
Another error is to rely on built in software to email images. Most such software will reduce the image size by both re-sizing the image and reducing the JPEG quality in order to get the image down to a reasonably file size for emailing. That's great for posting on-line or sharing with friends, not so great if you're sending it to a magazine who have asked for high resolution image. With the latter you'll want to use a regular email program to attach your high quality JPEG so that it arrives unaltered. You can easily test your built in emailing software (ie. in a smartphone) by emailing the image to yourself and then comparing the file size of the emailed photo to the original. Are they identical?
So, bottom line, a high resolution photo is an image with high quality pixels, saved in either a non-lossy file format or a low compression (high quality) JPEG, that can supply the desired PPI (usually 300) for the intended print size.
Okay, you've got the gist of it - now go ahead an explore the rest of this website. Check out the details about digital image resolution, proper storage & backup, digital photo labelling, and filetypes. Happy reading!