What are these jargon terms in photography? How do they affect your image and how can they be used?
Firstly let’s just lay some cards on the table;
DPI (Dots Per Inch) and PPI (Pixels Per Inch); these are measurements of resolution which have different uses. They are not the same as Miles and Kilometres which measure the same stretch of road they are more like volts and amps. One confusing thing about PPI and DPI is that they both count dots, though don’t think of pixels as dots think of them as units. These units make up the digital form of your image. Dots on the other hand are things that you print.
Once you understand their differences it’s important to understand how the pixels of the digital image and the dots of the printed image are connected. The resolution of one does not equal the same as the other which is where confusion can begin. It is also becomes more confusing when the determining factor of these resolutions is a third one.
If you were to open your favourite photograph and then zoom in at 100% you will see lots of squares of colour, each square of colour is a pixel.
If you were then to zoom out by 50% then you are looking at the same image but a down sampling the image. (When down sampling an image the computer groups adjacent pixels into one and then calculates the colour from the pixels it is made of.) It means that the computer is recreating the image with fewer pixels. This is why when editing an image it is always best to zoom in at 100% to check the area edited.
The digital image has its own digital resolution (PPI) which will never match the resolution of your printer (DPI). PPI and DPI won’t match because your computer and your printer produce images differently.
On a computer every pixel in an image is a set size and each one has the possibility of being a different colours and tones. In a printer the Ink has one colour and one tone, so the printer has to trick the human eye into seeing different tones and colours. The printer tricks us by printing different size dots.
In a perfect world all printers would print using different size dots but most printers can only print a dot at one size. To print an image the printer must print the image at a lower resolution thereby grouping dots and printing them in clumps to trick the eye into seeing a larger dot.
These larger fake dots as we could call them, are known as a Halftone screen. This is the third resolution between your digital image and your printed image. When you prepare your image it is this resolution that you need to deal with. The higher dot resolution of your printer means a higher halftone screen resolution and everything is interrelated.
How does this affect my image?
For printing this has a direct effect on the minimum PPI needed to print with good colour fidelity and clarity. The rough calculation says that your PPI needs to be roughly twice the halftone screen value of the physical size of the final output image. Look at the table below to see which resolution you will need.
|Output Device||Device Resolution||Expected Halftone Screen for that Device||Minimum Resolution Required for Device|
|Laser Printer||300 DPI||60 LPI (lines per inch)||120 PPI|
|600 DPI||85 LPI (lines per inch)||170 PPI|
|Image Setter||1200 DPI||120 LPI (lines per inch)||240 PPI|
|2400 DPI||150 LPI (lines per inch)||300 PPI|
|Inkjet and Dye-sublimation printers don’t print in halftone dots. For a desktop inkjet printer I would assume a halftone screen of 100 LPI, so you would need a minimum resolution of 200 PPI. For high quality proofers, assume a halftone screen of 150LPI, you will need an Image resolution of 300 PPI|
When scanning, it is easy to prepare your images for the correct resolution. Selecting the area you want scan first then entering the size that you want to print the image at and also setting the appropriate resolution for your printer.
There is a potential problem here, what if you don’t know how big you want to print the image or you want to print at different sizes. Does that mean you would need to scan and edit the image multiple times just to have the image at the correct resolution?
No, but you need to think big, not small.
If you were to scan a smaller image than you wanted to print you would need to upsample the image the oppersite to down sampling that I mentioned earliar ( down sampling merges pixels to create an average pixel for a smaller image than the orignal.) Up sampling works by resizing the image and instead of merging pixels and stretches them over a larger area.
The best idea is to scan the image bigger than needed. Because down sampling is better for an image than trying to stretch it and make it bigger.
Fixed resolution images
Most images now are fixed resolution images. These are images from Image libraries or from your Digital Camera. These pictures have a maximum size at which they can be printed without compromising their visual quality. You can calculate the maximum print size using a calculation involving your printers’ halftone screen and the images pixel resolution.
The easiest way is to find out the maximum output size is to use Photoshop’s Image Size command (In Photoshop >Image>Image size).
To use the feature uncheck Re-sample Image option, and enter the minimum PPI resolution you have for your printer, remembering the rule of thumb that it needs to be twice the size of your printers halftone screen. Photoshop will then show you the maximum size your image can be printed to.
This has been a lot of information and at first it can seem a little daunting, but with some simple ideas clear in your head it hopefully will all fit into place.
- Both DPI and PPI deal with dots and are a form of resolution but PPI is the resolution for your image on your screen and camera; DPI is for your printed image.
- Your Printer has a Halftone Screen (LPI) which affects the minimum resolution an image can be.
- Your Image resolution (PPI) should be at least twice the resolution of our printer (LPI).
- Don’t up sample your image, if you want your image for multiple uses prepare the original image at the largest size you will need.
- Try not to mix elements of different resolutions in the same image.
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