Friday 23rd January 2015
Printing images: what resolution do you need?
Many of our clients, and their printing companies, ask that our rendered images have a resolution of 300dpi without really knowing what this means in terms of digital images. If you’ve wondered what it actually means, read on. This post explains how ppi and dpi differ, why it matters, and why rendering an image at 225ppi is fine for most print jobs. It also explains why the total number of pixels in an image is the most important thing when it comes to determining the size at which you can print an image.
Pixels vs dots – two types of resolution
A lot of people say dpi (dots per inch) when they actually mean ppi (pixels per inch), and in our experience even print companies struggle when it comes to recommending the size and resolution of a digital image for print. Pixels are the building blocks of any digital image, whether it’s a render of a 3d model or a photo taken with a digital camera. The word pixel is a combination of pix (picture) and el (element). In digital imaging a pixel is a single point on a computer screen, tablet, smartphone or any other digital display. A digital image is formed of thousands or millions of pixels, each a different colour (typically red, blue or green) and brightness value.
Ppi (pixels per inch) is the industry standard term when referring to how digital images are displayed on a digital screen. It is often confused with dpi and many people (printing and visualisation companies included) and software applications use them interchangeably. But there is a big difference. Dpi (dots per inch) is typically used in reference to printing. More specifically, it is the number of dots of ink per inch the printer applies to the paper. But let’s ignore dpi for now, we’ll get back to it later in the article.
Resolution vs image size
Let’s start with digital photography. On the highest quality setting my Canon 60D digital camera records a digital image that is 5184 pixels in width and 3456 pixels in height. At 5184 x 3456 the photo has 17,915,904 pixels in total, which is close to 18 million, which is no coincidence because the specification of my camera is 18 mega pixels (where 1 mega pixel is 1 million pixels). It is these 18 million pixels that are important. Now everyone, (including the manufactures) call this number (18 mega pixels) the resolution, but it isn’t, it is in fact the image size measured in pixels. When I open the image in Photoshop it will automatically open at 300ppi, which will give me a printed image of 43.89cm by 29.26cm. It is key to understand that displaying the image at 300ppi is arbitrary. It is default in the industry that is based on the assumption it will give a photo quality image. We can print good quality images above and in particular below 300ppi.
Until we come to print a photo we do not need to worry about the resolution or physical size, we can simply work on the image in Photoshop and there will be absolutely no difference in the quality of the image if it is 72ppi, 150ppi or 300ppi, because the image will still have 18 million pixels. What is really important is the pixel count, the number of pixels in the image, because this is the true factor that will determine the size and quality of a printed image.
Translating digital pixels into printed dots
So now, let’s talk print resolution again… on a computer screen each pixel will be an actual colour. For each of these pixels, the printer will print a range of smaller dots to create the same colour as the pixel. This is because the computer screen can typically choose from 16.8 million colours, whereas a printer will have to create each colour by overlapping the dots of ink. My own printer is a Canon Pixma Pro 9000MKII (inkjet). It can print at a resolution of 4800 x 2400dpi. This means it can print 4800 dots of ink for every inch across the width of the paper and 2400 dots of ink for every inch down the length of the paper. So, when I give an image a resolution (ppi) I am simply telling my printer how many pixels need to correspond to what it prints per inch of paper. The printer will then convert this ppi to a dpi depending on its own capabilities and how many dots it needs to print to achieve the desired quality. If you print an image with a digital resolution of 300ppi image at 1200dpi, each pixel will be created as 16 dots (600 dots divided by 150 pixels will give 4 rows of 4 dots per pixel). This explains why printers have such a high dpi compared with the ppi of a computer screen and why there is no correlation between them. The difference between ppi and dpi does get more complicated if we start talking about other types of printer, but the important thing to remember is dpi refers to the number of dots laid by a printer.
How does ppi impact on the size I can print?
As we’ve discussed, changing the digital resolution of an image does not alter the total number of pixels in the image (it will still have 18 million) but it does limit the size we can print if the image quality is to be acceptable. If you print an 18-million pixel image at a resolution of 300ppi then the printed image will be 43.89cm by 29.26cm. If we lower the resolution to 225ppi, we can print the photo at a larger size of 58.52cm x 39.01cm, which will still give a perfectly good quality print.
The resolution should therefore be determined by the intended use of the print, and in particular the distance from which it will be viewed.
- If the printed image will be viewed close up (e.g. a brochure) then we can stick to the industry standard of 300ppi, or to be honest we can go down to 225ppi without any noticeable loss of quality.
- If the image is to be viewed from a distance (e.g. a billboard) then we can go all the way down to 72ppi and often lower. At this resolution we can print the photo at 182.9cm x 121.9cm.
Digital photos vs rendered images
So far I have been talking about printing a digital photo, but there is a limit to the number of pixels that a digital camera can record. However, as a visualisation company we have no such limit on our digital images, because we can set the number of pixels to be whatever we want. What we need to know is what you want to use the image for and at what size so that we can work out how many pixels the image needs to be in total. For example, if you tell us you need an image printed full bleed (without a border) at A3, we know:
- the print size will be 42cm x 29.7cm
- it is likely to be viewed at close proximity
- therefore we will need to use a higher resolution, somewhere between 225ppi and 300dpi
So now that we know the print size and resolution (let’s assume 225ppi) we can work out the number of pixels we need to render the image at. The easiest way to do this is to use the ‘Image Size’ dialogue box in Photoshop, where we can enter each of the variables and see how they are each affected. If we enter the physical print size and the resolution you will see the number of pixels we need is 3720px x 2631px. So this is the size at which we will render the image (which is approximately 10 mega pixels). If you need the image for a billboard a few weeks later, the first thing we’ll ask is the size of the billboard and where it will be viewed from. If the billboard is 300cm x 212cm and it will be viewed from across the road, we can use a lower resolution. If we enter the pixel count (3720px x 2631px) and the physical print size (300cm x 212cm) into the Image Size dialogue box (you might need to untick the ‘Resample Image’ box for this to work) we can see it gives us a resolution of 31.49ppi. This might be a bit low but it will depend how far the billboard is from the viewer.
The great advantage as a visualisation company is that we can simply re-render your image at any image size you need. But, there are occasions when it is not possible or advisable to re-render the image. In such cases we might consider resampling the image, or more specifically upsampling, which means increasing the number of pixels in the image. In theory this will allow you to print the image at a larger size, but there is often a compromise with quality. This is because the software such as Photoshop will have to stretch the image and create new pixels in between the original pixels which can create undesirable artefacts. There are various resampling or interpolation techniques that can be used to improve this process and these can be useful when rescaling an image for printing at a large scale at a lower resolution. But in general we try and avoid any kind of resampling because there is no match for an image with its original pixels.
So, what does it all mean for your visualisations?
Deciding on the resolution of a printed image is not an exact science, and as mentioned it will be determined by the viewing distance. As the viewer moves further away the dots do not need to be so close together so the ppi can be lower. Most print or visualisation companies should be able to advise you on this, but if anyone ever asks for a large format image (e.g. 3m x 2m billboard) at 300ppi then you should question this because you simply don’t need that level of detail. When we render an image we can save it in any format, so deciding on which one depend on the final use of the image. For smaller progress images that we typically send to clients by email we will use the JPEG format because of its impressive balance between reducing the file size and minimising the loss of quality or detail in the image. For the final production images, that are intended for print, we will use an uncompressed or lossless format such as TIFF, so there is no loss of detail. Finally, if someone asks you for an image with a resolution over 300ppi or they ask you for an image where the resolution is in dpi then you should question what they actually need. In a future post I’ll explain how image size and ppi correlate when it comes to rendering images to use in presentations, websites and apps.