Image Size and Resolution, and Why We Must Understand It

Most beginners seem to skip lessons like image size and resolution as they do not find it as significant as the tricks and techniques of editing until they come to a point where they have to prepare it for specific usage. And no argument, there are many automated tools out there to take control of that process. The other reason it doesn’t bother them because the life cycle of their images ends in their computers and display devices. There are very few among those who claim themselves digital photographers are ultimately going to print their work.

And those who do not understand it is also very comfortable with their understanding because they know somebody will give them a recipe of Width, Height, and Resolution for the output operation. They are unaware of the damage they cause unconsciously to the quality of the image.

So, do we need to understand this? Yes, we are going to dedicate some time and effort to understand the image size and resolution because it is critical to maintaining the quality of an image. We should also remember that every edit we make we cause some degree of damage to the file. That damage is often necessary to make the image look good and every professional does it; they do it consciously.

Table of Content

What is a Pixel?

Pixel is the smallest unit of an image. To be more specific the individual grid of your camera sensor that records the photons. When the photons strike a sensor grid it records value in a direct proportion to the number of photons that strikes it. These numeric values associated with the pixels determine the tones and colors of the image. That’s all there is to it.

There is no need to know what those numbers represented in an image and it is great that we don’t have to. Or the image editing would be far more complex than what we can imagine.

We make the aesthetic decisions of adjusting tones and colors visually; not by numbers. However, you should have a computer monitor that you can trust for that matter.

Image Size and Resolution

There is no fixed image size and resolution of an image but pixel dimensions. Your captured images do not have any resolution. Resolution comes to exist when there is a fixed physical size of the image. Therefore, image size and resolution matter only for output devices.

Resolution is the density of pixel in an image measured in inch for a specific output device. The required resolution can be different for different output devices.

Let’s assume that you have two displays and both of them have the same pixel dimension of 3000 x 2000 px. Now you can view a 3000 x 2000 px image on a 3″ display─which forces the same number of pixels to fit into a smaller area. Or you can view the image on a large 27″ monitor─where the same number of pixels are redistributed to cover the larger area.

In the case of 3″ display there are a lot more pixels per inch than that of 27″ monitor. Consequentially the 3″ display has a higher resolution than 27″ monitor.

Width (px)	Height (px)	Resolution (ppi)	Width (in)	Height (in)
3000	2000	100	3000/100 = 30	2000/100 = 20
3000	2000	200	3000/200 = 15	2000/200 = 10
3000	2000	300	3000/300 = 10	2000/300 = 6.67

To determine the image size and resolution first you’ve to know where your image is going to be used. Generally, it will be used by three kinds of output devices, halftone output, continuous-tone output, and display output.

Halftone Output (LPI)

If you are preparing your images for printing press─for newspaper, books, or magazine─you are preparing it for Halftone Output. Halftone printing is not a continuous tone though it might start with a continuous tone image. It creates the illusion of continuous tone by overlaying dots of inks in patterns. These dots will become apparent if you see it through a magnifying glass or loop. It uses the CMYK inks. So the dots are made of Cyan, Magenta, Yellow, and Black.

Halftone printing technology uses a concept called Line Screen which is measured in Lines Per Inch (LIP). It is dependent on the paper and the technology being used to print the image. Lines Screen typically means the number of halftone lines printed in a linear inch.

The higher the lines screen the smaller the dots. The lower the lines screens the larger the dots.

So, what should be the resolution for halftone output? The general rule of thumb is two times the number of lines screen. And the only way to know how many lines per inch are there is to ask the printing press. After you get the number you can calculate your perfect resolution for that document which is two times the line screen of the printer. For example, if it is 100 lines per screen the desired image resolution is 100 x 2 or 200 (LPI).

Resolution for Halftone Output = Lines Screen x 2

Continuous-tone Output (DPI)

Inkjet printing simulates the effect of the continuous tone of a photographic image. This is why it is called Continuous Tone printing. Unlike halftone output, if you see the continuous-tone output through a magnifying glass and loop you won’t notice any dot patterns on it thought there are tiny microscopic dots of inks that simulate the effect of the continuous tone image. The desired resolution for continuous-tone output varies typically from 240 to 360 DPI (Dots Per Inch).

The digital camera often sets a default resolution to the images it captures. Since I am a Canon 6D user I know it sets the resolution to 240 by default. If do not make any changes to the default resolution and make a print it will be fine. The fineness it produces is far better than any computer monitor can show you.

But you can certainly try printing at 300 DPI and 360 DPI and compare all three types or resolution side by side. Then stick to the number that represents the image at its best. For most professional photographers and printing lab it is 300 DPI. If you send your images to the lab for printing it is good to ask them what resolution their output device is tuned in for.

Display Output (PPI)

Now the final and most common type of output device is the monitors and projectors. Unlikely, for display devices, there is no resolution at all. All that matters is the pixel dimension of the device. Because all the display technology has a fixed pixel dimension and what determines the actual resolution of that device, is the physical dimension measured in inch.

For example, you can have a monitor that has a pixel dimension of 1920 x 1080 px and you can also have a mobile device that has the same screen resolution. If you ask about the resolution of those devices the mobile device has far greater resolution than that of the monitor.

Display Device	Width (px)	Height (px)	Screen Size Diagonal Length (in)	Actual Resolution(ppi)
Computer Monitor	1920	1080	15.5	142
Mobile Screen	1920	1080	6	367

Since the mobile screen is physically smaller it has to compress all those pixels into a smaller area which increased the actual resolution. For the larger screen, the same amount of pixel is distributed across a larger area. Thus the actual resolution becomes low.

Now, to make things simple, consider you’re going to display your images on a screen that has a resolution 1920 x 1080 px. All you have to do is to set the image size to that exact number 1920 x 1080 in pixel. If you do so the display technology is then going to dedicate its every screen pixel to every image pixel. As a matter of fact, when you set the measurement unit in the Photoshop Image Size command to pixel, the function of resolution gets automatically disabled.

Conclusion

Next time you downsize or upsize an image for a specific output device keep these fundamentals aspects of a digital image in your mind. That’s the way you can ensure you’re getting the best of your image.