Image Resolution Explained
June 17, 2009 | Design Basics
Running a design studio means that I receive frequent requests for images. Usually it is a client requesting a jpg image. It could be a jpg image of a photo that was used within a web site design or it could be a jpg file of a completed book cover design.
How are you going to use the image?
I nearly always find myself replying to the request by asking for more information. I am not trying to be annoying, I just need to know how an image is to be used so that I can provide the best image possible for your usage.
Let me briefly state that jpgs, gifs and tifs do not resize very well (the reason is content for another blog post, but just take my word). Therefore, I need to know the following:
- What size will the image will be? Is this going to be a thumbnail photo of the book cover design, or will it be a poster using the same photo that is on the home page of your web site?
- How is it being produced? Is this for a web site, for desktop/digital printing or for offset printing? Images for the web only need to be 72 dpi whereas offset printing requires 300 dpi or greater.
Resolution is based on total data: the size and density of information
An image’s final resolution is a combination of size and resolution. When you have an image there is a certain, finite amount of data that makes up the image. As you increase the resolution you will have to decrease the size. An image that is 2″ x 2″ at 72 dpi will be forced to shrink to .48″ x .48″ if you increase the resolution to 300 dpi. The size of the actual file (in kB) is the same, as there is no increase or decrease in the amount of data.
The image is 2" x 2" at 72 dpi. Note the file size is 60.8k
The image shrinks to .48" x .48" when the resolution increases to 300 dpi. Note the file size is the same, 60.8k
What this looks like
Why can’t you just increase the resolution to 300 and force the file size to increase? You know, somehow make it a high resolution file? Photoshop will let you do this, but there is no more data there to improve the image. You’ve just falsely increased the resolution. This will get ugly fast as your image looks digitized or mushy/blurry.
A corner of a web image that is 2" x 2" at 72 dpi
The resolution is increased to 300, but the quality has not improved
This is why you really can’t pull an image from a web site and try to resize it larger, and why you definitely cannot put it into a printed document that requires 300 dpi images. Therefore, when I provide a jpg image I need to know the final size and how it is being reproduced.
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