Behind the Magic – Sizing: DPI, PPI & Pixels

There is so much confusion over DPI and sizing it’s untrue. I’m often asked for images for printing purposes in 300 DPI with no other information, this normally means that the person asking doesn’t actually know what they want other than the photo to be in high resolution. I will then reply asking what size will it be printed and at what PPI which can often lead to further confusion. This is a confusing subject so there is a TLDR (Too Long Didn’t Read!) answer at the bottom of the article!

Photoshop and many other programs refer to Resolution: Pixels/Inch or Pixels/Centimeter not to DPI

So what is DPI? It stands for Dots Per Inch which is a printing term about how many dots the printer can fit in per inch, the more dots the better the quality. DPI is often misused though in place of PPI, which isn’t payment protection insurance but in this scenario is Pixels Per Inch. When I say DPI is heavily misused this would be an understatement. I often see printing companies referring to DPI when they really mean PPI! Because of the misuse and it’s confusion many organisations will now refer to it as ‘Resolution’ and either pixels/inch or pixels/cm dependant on their unit of choice.

If you zoom right in to a digital image you can see the individual pixels that make up the image

PPI is how many pixels of an image are printed to an inch of paper. If you zoom right in to an image you will see it ‘pixelates’ into individual singularly coloured squares. Each of these is one pixel. The more pixels the larger an image can be printed without loosing quality. Different printing formats will use different PPIs but there are various industry standards. Acceptable PPIs for photographic prints range between 150 and a maximum of 300 PPI. 300 is the maximum output for printers.

Working out how many pixels a image must be to be printable at a set size is very simple. You simply multiply the desired PPI by the size so if you want to print an image at 6×4″ at a PPI of 300 the longside must be 6×300=1800 and the short side 4×300=1200. Below is a table showing that the pixel sizes need to be to meet either 150 or 300 PPI. If an image is larger that the size at 300PPI this is not an issue as the printer software will rescale the image essentially by ‘loosing’ some pixels in the same way that your computer would if you rescaled the image down to the required size. It is important though to remember if at any point you resize an image smaller any removed pixels are permanently removed, therefore it is always best to keep the largest possible file size.

The size of file a camera will produce will depend on it’s sensor. You will have undoubtedly have read your camera has a 20MP sensor or similar? This is simply the width times the length of the images it produces. For example the Canon 5DIII has a 22.1MP sensor as its largest file size is 5760×2840 which equals 22118400 or 22.1million pixels! Some cameras have larger outputs, for example Hasselblad have a 50MP camera and Canon have announced they have created a 120MP sensor but don’t expect it on a normal DSLR anytime soon! Now you may well be wondering (or not!) how huge prints are created as 5760 at 150PPI is ‘only’ 38″. The answer is images can be upscaled to a degree (adding pixels), and also if you look very closely at a larger image you will often see the individual dots (not pixels!) as printed at a lower PPI ratio but from the standard viewing distance the image doesn’t look pixelated.

There is a widely held belief that a 72PPI is required for web. This is misleading and these days actually wrong. In the very first days of Apple computers their monitors were produced with a screen resolution of 72PPI but this has long since changed. Monitor resolutions now vary widely brand to brand but for example a Apple Mac Retina 15″ displays (diagonal measurement) has a resolution of 220 PPI which means it will show 2880×1800 pixels. ‘Normal’ monitors will generally have a lower PPI and therefore display less pixels, 2048×1536 is a common screen size.

When saving your image for web use what is most important is the file size in MB as this is what affects the time the image takes to load on a website. A file’s size in MB is dependant on an awful lot of factors including colours, contrast and patterns therefore it is very hard to predict but you should aim at the most of around 1MB for an image intended to be displayed full screen or far smaller if only to be displayed at a small size. Lightroom allows you to input a maximum file size but I tend to adjust quality over the file size which leads to the same end result. Quality for print should ALWAYS be 100, but for web you can happily adjust to 80% ish without any perceivable change to the image. The quality reduces the amount of information in the file and therefore the file size, this won’t be easily visible on a monitor so is the perfect method to reduce file size for web use.

Ideal export settings for full screen web. 2048pixel longest edge, 300PPI (doesn’t actually affect image!) and 80% ish quality

Next important is the pixel dimensions. As mentioned above Mac 15″ Retina screens are 2880×1800 and other brands are around the 2048×1536 marker. For this reason the majority of my images I save for web if intending to view at or near full screen I export at 2048 long side size (short side will depend on the image ratio). The 2048 factor isn’t limited to monitors either it is also used by sites such as Facebook. I will discuss this further in a future article but if you upload an image to Facebook with a long edge of 2048 then Facebook’s servers won’t compress the image and it will therefore maintain it’s quality. If though you upload an image at any other size including larger (unless a multiple or divisor of 2048 i.e. 4096 or 1024) Facebook will compress the image which will reduce it’s quality.

Export settings for print. Quality setting at 100%, image size in pixels not changed and PPI to 300

So when saving your image should you set a PPI/Resolution, does it actually affect anything? Ultimately for the majority of instances it doesn’t matter what you put in the resolution box but it can make a designer’s life slightly easier, and when working with magazines on a regular basis it is sensible to keep them in your good books! The reason why, is that the PPI is put into the image’s Metadata so if the image is put into a document the program will read the PPI to determine its size. If set to 300 it is easily scaled down but some programs have difficulty scaling up if for example set to 72 or 150PPI. The designer can very quickly re-save the image to 300PPI in only a few seconds but if dealing with multiple images it takes up precious time. Therefore for ease always save your files at 300PPI, you can still print them at 150PPI or anywhere in between if needed. To make clear if you export the same image at 150PPI and 300PPI but the quality and pixel dimensions are the same then the file size will be the same. It is purely the image metadata which will be different.

For those who looked at went TLDR if saving a file for full monitor sized web use save it at 2048 on the long side quality 80% and 300PPI. If saving for print do not adjust the size in pixels, make sure quality is set to 100% and the resolution 300PPI.

In future if you are asked for an image at 300DPI ask for the intended print size as you can be sure they mean PPI. You know now that if they want to print at 6×4″ it needs to be 1800×1200 pixels minimum. Don’t worry about sending a larger file size. If for whatever reason your image is too small in pixel dimensions for example it was 900×600 it’s worth saying you can’t provide it at 6×4″ at 300PPI but you can at 150PPI which is industry standard for magazine printing as it will probably still be acceptable at the smaller size in most cases. If they don’t know the print size just send the largest file size you can.