There are two important things in ALL image editing software concerning the sizing of your image – Resolution and dimensions. This is not specific to any specific program, it is applicable to all of them.
Firstly these factors depend on how you wish your image to be used. Will it be for the web and only ever exist on screen or will it be printed out?
Normally I always start by making my documents for print. This is because it requires a greater resolution than screen and with images you can always make them smaller but you can’t always make them bigger. So lets sort out resolution.
Resolution in this context refers to how much information can be stored within a set area. So the more information you store, the more detail in your image, in our case its how many pixels can be stored in an inch (2.54cm). On a typical computer screen it can display up to 96 pixels per linear inch, commonly in practice resolutions for screens are still set at 72 pixels per inch. So for every inch on your computer screen there are 72 pixels.
With printing, its slightly more complex, you are limited by how many dots/ droplets the printer can produce in an inch. Typically most printers can now handle 1200 dots per inch (Most Epson, Canon + HP inkjet printers), but this amount of data will create a very large image file, so best practice is to set a print resolution of 300ppi as generally you won’t notice anything more than this on your average paper. Professionally your image resolution will depend on the type of paper your printing to, photographic paper can handle the fine ink droplets of upto 1200ppi but your bog standard photocopy paper would be a big mess if you printed out something at this resolution – this comes down to the papers weight and how dense it is.
So to recap, 72ppi for screen based work, 300ppi generally for print based work. Always start with print based settings as you can always scale down – its hard to go the other way. Why? because if you have only 72 of something you have to stretch it to be 300, that means a lot of data gets added to make up the difference. Whereas reducing means losing data in the process which you wouldn’t notice on screen.
Now we have a basic understanding of resolution, we can move on to the size and dimensions of your image and you’ll see that on screen resolution has a big impact.
Because it is easier to comprehend lets start with print sizes, Whilst we measure resolution in pixels or dots per inch, with dimensions its actually easier to measure in metric mm/cm. Typically in print it used to be measured in points and picas but I prefer millimetres.
So when you create a new image in Photoshop it will ask you for a width and a height – you can change the measurement from pixels to various settings. For now I would suggest always doing this in millimetres (mm), this will be the size that the image prints out at. Typical paper settings in the ISO A series are:
A6: 105 x 148.5mm
A5: 148.5 x 210mm
A4: 210 x 297mm
A3: 297 x 420mm
A2: 420 x 594mm
A1: 594 x 840mm
Any paper that you buy will always have the measurement on it and as a rule of thumb if you want to do borderless photos I would make your document 6mm wider and taller than your paper. Doing this is called ‘bleed’ what this means is you can print to the edge of your paper and you won’t risk showing a nasty white edge even if the paper loads in crooked. A lot of inkjet printers (not just the more expensive photo models) now have modes that let you print to the edge but this will only be for certain types of paper – normally photographic media.
So to recap, if you’re printing your image make sure the resolution is set to at least 300ppi and your width and height dimensions are set to your paper size, with 6mm added on if you want borderless pictures, or the dimensions are set to the physical size of how big you want your image printed.
When you create your image this will look huge on screen. This is down to resolution, your computer screen can’t display the 300ppi resolution so your screen will show you your image based on the pixel size of the image.
Thats right, on screen your image is measured in pixels – not any physical measurement! So please do not let people catch you measuring your image on screen with a ruler – I have more often that not seen this from our marketing department when they get confused with resolution.
So here’s how it works (briefly)…
You should be able to figure this out that for every inch you have 72 ppi so a 1 inch square at 72ppi will be 72 pixels wide and tall. If that same inch has a resolution of 300ppi then it will be 300ppi wide and tall. That Inch will print exactly the same size but on screen it will have much bigger dimensions. This comes down to your screen size, for instance most common screens are set at a size of 1024 x 768 pixels so my 300ppi inch square is going to take up a a third of my screen area when viewed at 100%. Now if my screen size is 1280 x 1024 that same square at 100% will take up around a quarter of my screen area and so on. This is whats called a relative measurement and this is why you can’t physically measure what’s on your screen.
So when you’re making an image for the internet, you stick it at 72ppi and then you’ll know exactly how big it will be on everyone’s screens – also this keeps the file size down remember? The higher the resolution, the more detail and the bigger the file. You can see from the image above the difference (hopefully) between the same image at 100% with the same print size just at different resolution, the left is 300ppi, the right is 72ppi.
So to recap:
Unless you are specifically working for screen/TV/web ALWAYS start your document with physical print measurements (cm/mm) and a resolution of at least 300ppi. This way you will rarely be disappointed by fuzzy/ blocky images when you print them out. You can always scale down later for web usage if you need to.
Briefly lets discuss colour modes. When creating your document you will see different ‘Color’ modes. Basically leave it at 8bits per channel and leave it as RGB – you can always convert your document to black and white later on. Remember like resolutions, colour contains more data than grayscale so you can lose it and convert, but you can’t convert grayscale and get back your colours because this data is lost and won’t exist.
In a printed world we would use the CMYK color mode as this best represents the pigment/ ink setup of a printer Most printers have just the 4 inks Cyan, Magenta, Yellow and Key (Black) but computer colour profiles are generally good enough now for it to be able to handle RGB and you’ll get just as good a result. To handle RGB, printers now come with extra ink colours to help produce more vibrant results. It’s also worth noting that the RGB colour mode contains much more colour information than CMYK, so it’s best to start in RGB, especially since this is the colour mode your digital camera will use.
Use RGB, start at 300 dpi/ppi and work in physical sizes for your dimensions and this should sort you out.
So now hopefully you understand this a bit better (I hope). To create a new image in Photoshop, go to the File menu at the top left and select ‘New’ and go from there, or to alter an existing Images size go to the Image menu at the top and then select ‘Image Size’.