Written by Carmen Jerrard
WHAT ARE PIXELS?
Pixels are the small squares that are placed next to each other, vertically and horizontally, that make up an image. If you open your images in Photoshop and zoom in (+) you will see that the image begins to break up into small squares. These are the pixels.
WHAT IS RESOLUTION?
Resolution is the term used to describe the number of pixels (small squares) used to display an image. The higher the resolution the more pixels are used to create the image. The more pixels we have in the image the crisper and clearer the image will be. When printing an image on an InkJet printer the ideal resolution is 300ppi (pixels per inch). To adjust the resolution (ppi) we need to resize the image in the Photoshop Image Size dialog box but it is VERY easy to loose quality in your image if you do this incorrectly so you will need to pay careful attention to the next steps in order to preserve the highest image quality possible. Note: when we are talking about image resolution we use pixels per inch (ppi) and when we are talking about printer resolution we use dots per inch (dpi). In the ‘old days’ there was only one term – dpi – as technology has advanced so much there has become a need to distinguish between ppi and dpi for numerous technical reasons.
IMAGE SIZE DIALOG BOX IN PHOTOSHOP
When you want to resize an image open it in Photoshop (drag the file into the Photoshop icon or open Photoshop and select File>Open and select your image). Then, at the top of your screen select Image > Image Size. You will see that a new dialog box opens that looks similar to Fig 1.2 below.
Here we can see all the details of the image.
In order to change the resolution WITHOUT loosing a huge amount of the original image quality, we need to be aware that the Photoshop image Size dialog box functions according to a size vs. resolution ratio. To keep the ratio balanced, what is taken from one side is ‘given’ to the other and visa versa. Thankfully Photoshop does all the calculations for us if we follow a few simple steps.
First un-tick the Resample Image block as in Fig 1.3.
Next change your ppi (pixel/inch) to 300ppi as in Fig 1.4 but DO NOT CHANGE THE WIDTH OR HEIGHT. You will see that the width and height of the image change automatically according to the ppi number you put in. This is Photoshop working behind the scenes to keep the size vs resolution ratio in tact so that you do not loose any image quality. Your image is now set to the maximum print size and quality.
Now click OK to apply these changes to your image. Please note that it is very important to make sure you have followed these steps correctly – if you have not followed them correctly and you save your image, you will loose the original image quality and there is then no way to recover the original size and ppi of the image. It is recommended that if you are new to resizing your images, that you practise with some duplicate files..
The lowest inkjet print resolution you can push your image to is 140ppi. This will allow you to print a much larger image. If you size your image to a resolution below 140ppi the image will begin to show visible pixilation when you print it.
NOTE: any images with text will need to be printed at 300ppi.
Once you have sized your image to the desired ppi, you may need to reduce the width and height according to the size you want your image to be printed at. To do this put a tick in the Resample Image block and in the Constrain Proportions block, now adjust your cm size – you will see that the ppi says the same. IMPORTANT: Remember that if you have set your ppi to 140 you cannot increase your image size – that is simply the maximum it can go. You can however make the image smaller because there is room in the equation to do so.
Your printing company will be more than happy to sit down with you and show you how resize if you are unclear about anything