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Question

9.11.23_PPI & Dimensions are interchangable?

Forum|Forum|2 years ago
September 11, 2023
5 replies
617 views

Hey all after some research ive come to the conclusion of the title based on the factor that

Resolution = ppi x dimensions

So that means dimensions and ppi are mathematically interchangeable right?

If I work at 20x20 300ppi

It converts to the same exact quality as

10x10 600ppi

Nothing is lost in conversion/ no difference

when going to print or even when working right?

Windows

This topic has been closed for replies.

c.pfaffenbichler

Community Expert

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.

Conrad_C

Community Expert

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

and

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.

D Fosse

Community Expert

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.

Conrad_C

Community Expert

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)

or

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

and

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.

D Fosse

Community Expert

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.

G

Gene27167201cqusAuthor

Known Participant

Right!
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

NB, colourmanagement

Community Expert

@Gene27167201cqus you kinda right, the number of pixels is the vital thing - but as @jane-e explained the PPI is important for print as it in combination with pixel number defines printed image size

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

jane-e

Community Expert

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.

Jane

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