When your image will be on the web or in another digital format, all that matters is the number of pixels for the width and height.
When your image will be printed, you need to be concerned with pixels per inch (ppi). 300 ppi is standard for professional printing. If your image is 20px x 20px and you print at 300 pixels per inch, the image will be teeny tiny.
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your example image at 20 pixels wide, with image size set to 300ppi would be 1/15 inch across printed.
I hope this helps
neil barstow, colourmanagement net - adobe forum volunteer - co-author: 'getting colour right'
google me "neil barstow colourmanagement" for lots of free articles on colour management
This is really simple:
The image is just pixels. Pixels have no size. The size is defined by the ppi number. Pixels per inch. Read that literally, it means exactly what it says: how many pixels to an inch.
So the pixels never change...which means even if we print for 300ppi
all that does is stuff the amount of available picels you have into that
but that doesnt efect the final pexel --- x ---- px file itself
You said the equation is
Resolution = ppi x dimensions
but it’s actually
Number of pixels along one dimension = resolution (ppi) * physical dimensions (inches/centimeters)
Resolution (ppi) = Number of pixels along one dimension / physical dimensions (inches/centimeters)
(Your equation seems to use the definition of “resolution” for video and computer displays, which means pixel dimensions, as in “a resolution of 1920 x 1080 pixels.” But in Photoshop and when talking about print, resolution means pixel density, as in “300 ppi,” so in Photoshop/print terms, resolution is pixel density/ppi.)
In your example,
20 inches * 300 ppi = 6000 pixels
10 inches * 600 ppi = 6000 pixels
and the reason that their quality is the same is that they both equal 6000 pixels.
Now, where some people get confused is if the image is both resized and resampled. Resampling does not use that equation because it interpolates or merges pixels to reach the new combination of dimensions and ppi. Doing that can compromise image quality.
Resolution = ppi x dimensions
That equation seems nonsensical.
What do you mean by »dimensions«? Height times width would describe an area.
Resolution gives the number of pixels per unit of measurement of length.
ppi is a unit in which Resolution is given.
Depending on which sources they used for their research and how they combined the info from them, their equation can make sense under the definition of “resolution” used for screen media (web, video, apps), where it means “pixel dimensions” and not pixel density. Using that definition:
Resolution (one dimension in pixels) = ppi (pixel density) * physical dimension (in inches/cm)
so their example works out to:
6000 pixels = 300 ppi * 20 inches
6000 pixels = 600 ppi * 10 inches
which is OK.
But, because they discussed it within the context of printing, to avoid the confusion we had, they should use the print definition of resolution (pixel density) instead, which is how I reframed the formula.
Those two meanings of the term "resolution" might seem different - but actually they are the same, just different notation, as it were.
In both cases it means "number of sample points per unit of measurement". It's just that the unit of measurement can be either a fixed area in full, as on screen, or it can be a small section of a potentially infinite area, as in print. In the latter case you need to define the size of the section since it's not given. That's the inch in ppi.
You could define monitor resolution as ppi, and you could define print resolution as total number of pixels W x H in the image. But it's not very practical.