PPI or Pixels per inch is an important part of file analysis if you are planning on printing a project. Let’s look at the differences between Actual PPI and Effective PPI
Actual PPI is the resolution of an image at 100% in Photoshop. Effective PPI is the resolution of an image that has been scaled in InDesign.
As a Graphic Designer for 20+ years I still get images sent to me that have really low resolution. When I ask for a hi res version, I sometimes get, just go back into Photoshop and change it. Unfortunately printing doesn’t work like that. Read on to find out the difference and why it matters.
Actual PPI is the amount of pixels or points per inch of an image being viewed at 100% in Photoshop.
What does that really mean? Well, virtually no one prints anything direct from Photoshop. All images usually make their way to InDesign where they are placed in a document.
The Actual PPI of a file can be as low as 72ppi, but you must increase it when you place it in another program in order for it to be sharp when you print it.
The higher the PPI at 100% in Photoshop the better your image will appear, but it also depends what you are going to do with the file, and what size your final output will be.
Effective PPI is what effects how your file will ultimately print. It all is dependent on scaling, either up or down, within InDesign.
If, for example, you have a large image in Photoshop, say 10” x 15” at 72ppi. When you place this image in InDesign, on a document size of 10” x 15” your ppi will remain at 72ppi.
However, if you scale this image down to 50% then your ppi will become 50% higher and now have an Effective ppi of 144ppi. The same applies if your scale it upwards. Your effective ppi will now be half or 36ppi.
Effective PPI is ultimately what makes the difference, but you do have to have a good solid Actual PPI in Photoshop to begin with.
How does ppi affect your print document?
The general rule is that if you are sending a file to a print shop it needs to have a ppi of 300. Slightly less may still work depending on what you are printing but printers get a little angry when the resolution is too low.
So if we have pics or images that are too low, how can we make them correct? All is not lost if we have a low resolution image.
If we initially open our file in Photoshop, view at 100%. Go to Image and Scroll down to Image Size this will tell us our Actual ppi
Make a note of the size of the image and compare it to the output size in InDesign.
You cannot change the resolution of a file in Photoshop to make it better, but you can change the Image size.
It seems a little complex but it’s not really. Let me explain with some diagrams.
By increasing your original image size in Photoshop and then scaling down in InDesign you can change your Effective PPI to work in a print format.
So what if your Photoshop file is 300ppi and you scale it to 50%, you will then have an effective ppi of 600ppi. But realistically your file will not print at 600pp because when you export it, InDesign will reduce everything back to 300ppi wherever it can.
Otherwise you file size would be enormous and of no benefit to you print wise.
We basically can’t have Effective ppi without looking at Actual ppi. Although they are completely different they do work in conjunction with each other. The rule you need to remember is if you want sharp print images ppi must be within 260 – 300ppi range.
As a last point to remember, we need to think of what size your print output is going to be. If you are printing large scale format, these have quite a spread out ppi, but this is only noticeable if you are up close to the artwork, however, they are not designed to be looked at close up, and so when you are on the other side of a convention hall or on the highway, they look perfectly fine from a distance.
If you are generating images for the web then 150ppi is perfectly fine.
When scaling either up or down try and use rounded numbers to easily get to effective 300ppi.
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