Colour management || Monitor calibrators || Colour Space || Photo Printing
What a photographer should know about PPI and how to calculate the size of prints
What does Resolution mean?
How resolution of a camera is calculated?
What does PPI mean?
Why is PPI important?
How to calculate the Print size?
The impact of viewing distance and print size on image quality
Aim: This is the third article in the series colour management and printing. The aim of this article is to make you familiar with Resolution, PPI and how to calculate the maximum size of print when you send an image for printing.
Audiences: This article is meant for professional photographers as well as photography enthusiasts with overall knowledge of photography and post processing.
Disclaimer: This article does not include the resolution of individual machinery and does not include TV screens.
In this article pixel resolution is discussed. Pixel stands for Picture element.
Pixel resolution is calculated by multiplying the number or pixels in one row by the number of pixels in one column.
In order to view an image with a good quality on a screen or print, the image should contain a certain pixel density. This is generally expressed in Pixels Per Inch or PPI. For viewing images on screen your image needs to be saved at 72 to 96 PPI. For printing, you should save your images at (240 to) 300 PPI.
You can calculate the maximum size of a print by dividing the number of pixels in one row and the number of pixels in one column of your image by 300 PPI. This gives you the dimension of your print in Inches.
Definition of Pixels Resolution
Pixels are the building blocks of a digital image. Each individual pixel in a final image contains information about the intensity and the colour of light that has been received by a pixel on the camera sensor. Each pixel can have only and only one colour/tone.
Pixels are important units in digital photography. The more pixels you have on a camera sensor the higher the pixel resolution of that camera is (not to be mistaken by quality!). It is very easy to calculate the pixels resolution of your camera sensor. Basically by multiplying the number of pixels in one row by the number of pixels in one column you can obtain the resolution of your camera sensor. For instance a Nikon D850 has a tremendous resolution of above 45 mega pixels. This is calculated as follows: there are 8256 Pixels in a row / and on the vertical direction 5504 pixels. By multiplying 8256 X 5504 we achieve 45441024 pixels, which we can round up to 45 mega pixels. This is the pixels resolution of this camera (camera sensor). Basically it tells us, an image made by a Nikon D850 contains 45441024 pixels and each pixel contains one colour / tone. All these pixels together, make up the image you are looking at. The more pixels an image has, the more details that image contains. However, note that pixels resolution does not necessary defines the quality of your image on output devices (monitors and prints). Simply said, if you print a very high resolution image on a low resolution printer, magic will not happen. Or when you watch a 4k video on a HD screen, you are actually watching a video at HD quality and not at 4K! In other words, pixel resolution will become meaningful only when considered together with the resolution of your output devices.
Furthermore, when you crop your images in the post processing (,which is a normal practice) you remove some pixels from your images -hence details-. Although this might be necessary from a compositional perspective, it reduces the size of the image you can print (discussed later on). In the Library module of Lightroom you can find the original and post-cropped dimensions of any image you are viewing (Figure 1).
Figure 1. In any photo editing software you can find the pixel dimensions of the image you are viewing. In this case, in the Library Module of Lightroom under Metadata section (in the right panel), you can see the original and cropped pixels dimensions of an image made by a Nikon D850.
At last, there is no standard pixel size. A full frame camera may have more and larger pixels. While a cropped sensor camera may have less and smaller pixels (think about the pixels on a tiny smartphone camera sensor). So, for viewing the images at an acceptable quality (regardless the size of the camera sensor or the size of pixels) on a screen or print we need to be familiar with other standards and terminologies that are discussed below.
Pixels per inch (PPI) or the density of pixels in a unit of distance
Pixels are remaining important in all steps of photography. Because in order to have a good quality image on each device (screen / print), your image should have a required pixel density in a unit of distance (inch or cm). This is traditionally expressed in “Pixels Per Inch or PPI” (not to be mistaken or used instead of Dot Per Inch -DPI-, which I will discuss in the fourth part of this series). PPI tells us how many pixels should be in an inch (vertically and horizontally) of an image so the image could be viewed at a good quality on a screen or print, when viewed at a normal distance. To be clear, by normal distance, I mean the distance you have from your monitor, or the distance from which you view a printed photo on a wall in a gallery. This means when we observe the image we should not see the individual pixels. Instead we see a coherent image. Hence, how we perceive an image is also very much related to our distance to the image and at which scale the image is viewed (figure 3). In Photoshop -as well as any other photo editing software- you can assign a PPI to all images you are editing. You can do this by opening the photoshop preferences > Unit and Rulers, and then in the upper right section “New Document Preset Resolution” you can change the print or screen PPI resolution (Figure 2).
Figure 2. By hovering over “Photoshop” on the upper left corner of your photoshop screen, a dropdown list will appear. There you can choose for “Preferences”. From there you can adjust many settings including the PPI.
What is the required PPI when viewing your images on screen:
When viewing your images on screen you need only to have 72 to 96 PPI to view your image correctly. So, one suggestion would be to make an export preset for online images with a standard 72 or 96 PPI in order to make sure your images are viewed correctly. As technologies improve you may need to increase the PPI for screens in the future.
What is the required PPI when printing your images
Generally, it is accepted that for a good quality print (when viewed at a normal distance), your image should have 300 PPI. Because by increasing the PPI above 300, our eyes are simply not able to distinguish the differences in details. However, you can choose 240 PPI for less detailed images or when you want to print your images slightly larger. By assigning lower PPI to images you send for print, you increase the chance of seeing individual pixels on the print, when the image is viewed at a close distance.
PPI and the maximum size you can print your photos
The 300 PPI you assigned to your images for print, will immediately define what the maximum size of your print can be. For instance, let’s get back to the image made by a Nikon D850 with dimensions of 8256 x 5504 pixels. Divining 8256 ÷ 300 PPI = 27.5 Inches, and 5504 ÷ 300 PPI = 18.4 inches. Therefore, an original image (not cropped) made by a Nikon D850 can be printed in 27.5 by 18.4 inches at perfect quality. If you prefer the metric system (as I do), then multiply each number by 2.54: in other words; 27.5 x 2.54 = 70 cm, and 18.4 x 2.54 = 47 cm. The print size is independent of the brand or type of the printer you use as long as the printer can print images at your desired scale.
If you want to have larger prints, you can compromise the quality by reducing the PPI to 240, which slightly will increase the size of the print (e.g. 8256 ÷ 240 PPI = 34.4 inches by 5505 ÷ 240 PPI = 22.9 inches, which are equal to 87 cm by 58 cm) or to work with a camera with a higher pixel resolution. At last remember to adjust the size of your prints in case you have cropped your images (figure 1).
Size of the print and the viewing distance
The quality of the image you view, is not only a matter of pixels resolution and PPI, but also a matter of the print size and viewing distance. An image that is printed for a bilboard may have a very limited PPI, but since it is viewed from a large distance it looks quite nice. Similarly, a print of a photo made by a low resolution camera will look fantastic as long as it is printed at 300 PPI. The only issue is that the print size will be small. If you print it in a large format and view it from a close distance you will see the individual pixels. Remember that the number of pixels in a distinct image is fixed. By printing the image larger than the maximum allowance (at 240 up to 300 ppi) you will only enlarge the pixels and not increasing their numbers. Hence, if you print an image at a lower PPI, then you need to make sure that the image is viewed from a larger distance (Figure 3).
Figure 3. The image below is the featured image of this article, which is saved at 150 x 100 pixels. The upper image, the small one, should look fine. But when the same image enlarged, you will see the building blocks of the image. Now take a few meters distance from your screen and check whether you see this image better. This is a simple example of the importance of pixels, pixels density, the size of the image and the distance from which you are looking at an image.
I hope this was helpful, should you have any question or remark please do not hesitate to send me a message.
Triangle Photo Academy Team
This article was prepared by Armin Armani