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When working on a project, its obviously important to use the right resolution. My question is, how important is it to use the right one?
For example, why can't you always use a high PPI for all your projects and just scale down when you only need a web version of your file.
Or, why can't you design an A3 poster and then scale it down to A4 if needs be?
Other than the file size are there any other factors to consider when choosing the resolution for your file?
When working on a project, its obviously important to use the right resolution
Well, it's important to work at the correct size, although arguably not that important*. Work in vectors for anything and everything that isn't specifically an image, then resolution doesn't matter (vectors have no resolution).
It's important to use the right resolution for assets. So your placed photographs or background images or whatever certainly should be at an acceptable resolution, but a whole project in Photoshop (or equivalent) shouldn't really happen (unless you're working for a screen output, where resolution isn't as much of an issue).
Ok, so maybe I'm being pedantic but I thought it should be said.
why can't you always use a high PPI for all your project and just scale down when you only need a web version of your file.
You can.
why can't you design a A3 poster and then scale it down to A4 if needs be?
Again, you can!
I often design posters that are used in multiple sizes. I can design at A3 and have the exact same file used to print A4's, A1's, A0's, even A6 flyers.
As Alex said (and is very much correct in saying so), that isn't really the ideal thing to do but sometimes you only have a budget for a single poster (and working within constraints is just part of being a designer). And as I said, working with the correct tools (i.e. vectors) means it's only the linked assets within those posters that need to be of a certain resolution.
* The beauty of A sizes. I can work at any A size I like, and output to any A size I like; and resolution probably isn't as much of an issue here as you'd think — you can generally get away with printing an A0 poster at a much lower resolution than an A6 flyer.
In theory, starting off with very high resolution assets and scaling them down for output is just fine. However, as you note, file size is a big concern here.
"Why can't you start at A3 and scale down" is a a somewhat different question. You might well use the same image assets within both an A3 and an A4 poster - scaling the image down for output for the A4 version - but you would likely want to re-set text and possibly even re-work the design. This is because nice, big 16pt type at A3 shrinks to harder-on-the-eye 8pt at A4.
That said, you can design a poster to work at A3, A4 (and, at a push larger and smaller sizes), it's just unlikely it will work as effectively at each size as something purposefully created around that form factor.
There are probably some fundamental problems with a "correct" methodology here:
why can't you always use a high PPI
This is implying two things. You are working in a raster program for all the project.
Lets analyze some common steps when designing something:
If you simply use a vector based program and insert a photo of 24Mpx, you have inside a Photo of 24Mpx. If that is the photo you have, you use the photo you have. The resolution will be "elastically" changing if you make it bigger or smaller, but will be fine for a magazine cover or a street poster.
If you need to retouch your image to add effects in Photo-whatever program, you use your 24Mpx image and work on your still 24Mpx image. Unless you crop it guess what size of image you still have.
If you know there is a chance you need to "push" a bit more your 24Mpx original image you could yes increase the size and add some new details on the image at larger scale.
I have not talked about any PPI at all yet.
So, lets talk about physical size.
Here is a question where I understand the user expects too much for a tiny image, most likely for web.
Autodesk Graphic: saving vector file to 200x300 pixels
You can not simply design a poster and expect that would work for a banner on a webpage.
Other than the file size are there any other factors to consider when choosing the resolution for your file?
This implies the resolution needed for print on not only the paper size, but also the paper type or finish.
This implies if an image will be 1x or 2x for a retina display.
But a final note is, if you are downsampling things it is more important the final size than the resolution.
If we do a completely different take on your question than the other good answers. Then it boils down to this:
Simply it helps. But design is always confronted with a unknown number of unknowns. You simply can not know what future holds for you. The client may come back with a totally different requirement next day, month or year. The printer may have switched to a different process that has different requirements and so on.
While you can always compensate for this by slightly overestimating any requirement, like resolution, this is not without a cost. Sometimes this cost isn't that big and may be worth taking, but sometimes it's just extra overhead. In the specific case of over resolution the problems are:
So you see it clearly is a trade-of, you gain some you lose some. Also it's arguably not so bad to be below certain thresholds commonly cited as good quality as long as your main items are good enough (like text which is, or should be, vectors anyway).
But no its not, always a winning strategy. Try not to optimize things for the sake of optimization, but for a real need.
Its important, but really not that complicated. Start from the below basic principles:
