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Information on how to set up images for printing and how to maintain their quality, also gets in to how to know what images can be used on what size.
Often I get questions from people who have printed a file or have let it be printed, where the quality of the images is not as desired. Then it is often asked whether this is the printer and whether it could print such quality files, because it looked good on the monitor. I can be very short in this .... This is not due to the printer. In the era in which we live now, every current generation printer can deliver high-quality prints. Differences that are visible in the quality of various printers are mostly in sharpness of lines or small details and also in color gamuts. The latter I do not go into, color gamuts and color management is a very different story and maybe I will write an item on that later on. In terms of sharpness, a laser printer excels in sharp lines for example: text, graphs, illustration and lines. As for small details such as high-quality photos of detailed objects (animals, insects, flowers, etc.), it is best to use an inkjet (photo) printer. This does not detract from the fact that nowadays laser printers are getting better and better in photo printing and inkjet printers are getting better and better in sharp lines.
Enough about printers .... What causes disappointing prints is often due to the print file.
Files must have sufficient resolution to be printed so that they come out of the printer sharply and tightly. It is not 1-on-1 with what you see on your monitor, unless ... you also view the file in true format in the right resolution. When you view an image of 600x400 pixels on the monitor at about 15x10cm, it looks good. If you then print it at full A4 size, in many cases it will be of disappointing quality when it comes out of the printer. This is because this image does not have enough resolution to be printed in such a size. Since you have requested this size from the printer, the image will be stretched to that size while losing quality.
Screens work with pixels and these pixels are converted into dots in a printer. In general, most displays work with a pixel density of 96ppi, so it shows 96 pixels per inch screen. Based on the aforementioned example, a 600x400pixel image on a monitor is approximately 15x10cm where it is sized appropiately and will look good. For printing, however, it is better to use a higher resolution, and if an image has to be printed larger, it must also contain more pixels. To calculate image resolution (quality) for printing, it is good to do a pixel density calculation in dpi. Dpi stands for dots per inch, this dpi is basically (to keep it easy) proportional to ppi (pixels per inch). A file is of sufficient resolution (number of pixels) if it can achieve a dpi of between 100 and 300dpi.
- 100dpi is often enough for text and simple illustrations,
- 140-200 dpi for drawings and other quality images,
- 300dpi is mainly used for extremely high detail and high quality photos and is a standard for printing.
An image may always contain more pixels than required, but preferably never less than 100 dpi. Usually 140dpi is a good starting point for general use. This means that if you have an image of 140 pixels long, it can be printed on 1 inch length, 1 inch is 2.54 cm.
Calculating the maximum size and resolution
To calculate the maximum printable size without loss of quality use the following formula:
Pixels: desired dpi x 2.54 = the number of cm that the file can be printed as a maximum
Imagine you have found an image of 400 pixels long, you want to use it at 140 dpi, it can be printed max:
400: 140 x 2.54 = 7.25cm long
You still need to apply the image to a file with a maximum size of 7.25cm. When you make this smaller, the dpi for that image is automatically increased in many cases, when making an image smaller than the calculated maximum size is no problem.
Depending on the software you use, you can often set a PPI or DPI for the relevant document in advance. I will not go further into how this would be done and if this is possible, but often through a google search you can find how this should be done. If you can only set a document format in the form of the number of pixels (in some drawing programs) you can calculate it yourself with the formula below. The above calculation can also be done the other way around.
Suppose: You know how many centimeters an item should be, but want to calculate how many pixels the file should be. Then you use the following formula:
Number of cm : 2.54 x desired dpi = number of pixels
Suppose you need a logo of 12mm on the long side at 100dpi minimum (more is allowed), then you need at least: 1.2 : 2.54 x 100 = 47 pixels Or, in the case of setting up an A4 document in pixels, use: For the longest side: 29.7 : 2.54 x dpi = ......... (at 140dpi this would be 1637 pixels) For the short side: 21 : 2.54 x dpi = ......... (at 140dpi this would be 1157 pixels)
It is very important that:
- Before you start a document, you set the program to the desired dpi.
- The program is set to the correct format media (A4 format or 297x210mm).
- The final document is saved or printed without loss or compression .
- The images used (being applied to the correct resolution and format) are of good quality.
Suppose you find an image of 1500 pixels and it is not sharp or clear and you will then print it at 5 cm, then the image is usually not nicer. It gets much worse if you make an image larger than it actually is, so for example a 400 pixel image in a file of 140dpi on 10cm.
Do not confuse dpi settings for a document with the quality/resolution of a printer or the setting in the printer driver.
It is rather confusing that printers are indicated with 1440 dpi or, for example, newer photo printers up to 6000 dpi. This is the dots density of the printhead, or how many drops the printhead can lay down per inch. This value therefore indicates print quality of the printer, and not what the image quality of the supplied file should be. As soon as you send a file to the printer, it will calculate of how many drops it can put down per pixel in the file. If your printer (like many epson's) shows the print quality in dpi, it only determines how much ink it will use/try to lay down where possible. When using a 300dpi file you can simply print it with a print quality of 1440dpi to achieve the highest print quality. You do not need to make files of 1440dpi to use the indicated print quality in the driver, which would make for a photo with a filesize of 1gb.
Not related to means of this article, but don't let "them" mislead you:
Okay, this has nothing to do with image quality of print files, but with the aforementioned print quality of a printer. There are often great talk about very high numbers when it comes to print quality of printers. For example, with laser printers, there are sometimes advertisements talking about 3600 dpi printers. Unfortunately, this is mostly marketing because it involves digital operations / additions. Laser printers at the moment of this writing (2017), have a native print resolution of 600dpi unless you spend thousands of euros for a professional graphic machine which can contain more native resolution. Inkjet printers, on the other hand, often have a native resolution of approximately 1200-1440 dpi depending on the brand and printhead configuration. All extra dpi that is thrown around in advertisements are mostly artificial improvements and tricks to achieve better quality. The printer often prints over a spot several times, therefore consumes more ink and this often results in a slightly better quality print. However, often the quality difference between the native printer resolution and this artificial resolution is negligible.