The meaning of the term, "resolution" in Photoshop

[Tom Mann]

There are two very different and equally valid meanings to the term, "resolution", as applied to digital images:

#1) "Resolution" is the dimensions of the image in pixels.

#2) "Resolution" is the number of pixels per inch (on a display) or dots per inch (when printed).

The amount of confusion caused by having two meanings for the same term is unbelievable.

Resolution #1 tells you the amount of information contained in an image, no matter whether it is on a tiny screen or spread out over a poster-sized print. #1 can't be increased without asking your software to guess at the new information (RGB values of pixels) that didn't formerly exist. This is what happens when you check the "resample" box in the "image size" command in PS.

Increasing the number of pixels is commonly called, "up-rez'ing" and decreasing the number of pixels, "down-rez'ing".

Resolution #2 simply tells you how far apart the pixels are going to be when you display them, or how far apart the dots are going to be when you print them. Changing Resolution #2 (ppi / dpi) without changing Resolution #1 (pixel dimensions) means that the final image is going to change its real-world, physical size, but no new information (new pixels) is created. This is what happens if you don't check the "resample" box in the "image size"command of PS.

The minimum acceptable value of Resolution #2 (ie, ppi/dpi) depends on how closely you are going to examine the image. Specifically, you can get away with much lower dpi/ppi for a billboard (always viewed fro more than 100 feet away) compared to a small print in a gallery where everyone's nose is going to be a couple of inches away from the print.

The pixel dimensions, ppi and size are related by a simple formula: #pixels = (#pixels per inch) * (# of inches). Photoshop conveniently takes care of these conversions in it's "image size" dialog box, constraining you to manipulate numbers only in the correct fields depending on whether or not you have the "resample" option checked.

HTH,

Tom M

PS - This is an excerpt from an old thread. This question arose yet again, so, here it is for reference.

 

[rachidshop]

than so much brother for your info

 

[MrToM]

I appreciate this is an old thread and will probably be closed after this but the term "Resolution" has been a particular bug-bear with me for many years....so here is my take.

Digital Images are constructed of 'pixels'. (Picture elements)
Digital Image dimensions are defined in pixels.
There is no definition of how 'big' a pixel may be, (in terms of linear measurement), so a single pixel could be the size of pin head or the size of a continent.

Due to this, digital images have no native 'size' in terms of linear measurement......only number of pixels.

With that in mind:
The term "Resolution" is defined as a ratio between two elements......A:B, mph, dpi, mpg, 47% etc.
If we take 300dpi with 'A' to be the number of pixels and 'B' to be the linear measurement it becomes obvious that a digital image with no linear measurement (see above) cannot have a "Resolution"...the ratio fails A:?

Resolution therefore can only be used with tangible, physical objects, such as a PC monitor or a printed image, anything you can physically measure with a ruler. Both of these CAN have a resolution as each 'pixel' has been defined a real physical linear measurement.....the spec for your LCD monitor may even tell you the pixel 'pitch.....basically its size (give or take) and a dot of 'ink' is well, the size of a dot of ink, but it IS measurable.

The dpi setting for an image is only information saved with all the other meta-data for that image, like its name, what camera was used, its dimensions in px, the time and date, the focal length, the aperture, etc.....it does not change the image in any way, shape or form. This is the info you see in 'Bridge', under the 'metadata' tab.

A printer will read this information and use it to calculate the image size when printing. Photoshop uses it in a similar fashion except before the printing stage rather than AT the printing stage, giving you feedback as to the potential size of the printed image and if it will fit the paper supplied.

It was mentioned somewhere that the "Greater the resolution larger will be the size of file"....simply not true, (read negligible). The dpi of an image is just a 'note' attached to the image, like writing on the blank space of a 'Polaroid'...its just info.

Agreed you can have 'high resolution' images when printed on canvas or a billboard but while an image is still a bunch of binary digits it has no resolution at all.

Regards.
MrTom.

 

[ALB68]

Mr Tom,
That is some interesting logic. I personally would like to see the thread stay open for some more discussion. As Tom Mann has stated, it is huge source of confusion to many people and would serve to enlighten many. Thanks for posting.

 

[Tom Mann]

Hi Tom -

It would be a lot simpler if the word, "resolution", was only used in the sense you described in your post (which seems to be more or less the same as my definition #2 in the 1st post in this thread), ie, essentially, how large (in units of inches or cm) each pixel in an image is. This definition tends to be more commonly used to describe the output end of things (eg, prints and images on monitors), but can equally well be used to describe input devices (eg, "a CCD sensor with 1000 pixels per cm").

Unfortunately, by far, the more common usage of the term, "resolution", is that it is the number of different "things" that one can see in an image (ie, my definition #1). This is effectively the total number of pixels in an image (ie, megapixels) or the pixel dimensions of the image. This is the common meaning of the term by laymen, as well as definition used by camera manufacturers when they characterize their latest and greatest product as being a high resolution, 99 zillion megapixel DSLR, LOL. This information content definition is also the what is meant when you read in the newspaper that a spy satellite or the Hubble telescope is "ultra high resolution".

There is no getting around the fact that there are two different, equally common uses of the term, and if one tries to convince yourself or others that they should ignore one or the other definition, you'll be fighting a battle that you can't win. IMHO, the best approach is to use the context to determine which definition is being used at any particular moment, realizing, of course that the writer may not even be aware of the other use of the term.

I try to avoid such confusion in my own writing by referring to one as "dpi" or "ppi", and the other by "pixel dimensions", especially, the first time I use the term resolution in an article, in class, or in a web post.

Tom M

PS - re your statement, "...A printer will read this information and use it to calculate the image size when printing...", for the vast majority of prints made in the world (eg, by printing kiosks in stores, when ordering prints at a camera store or on-line at places like Mpix or Snapfish), your statement simply is not true. Rather, the customer submits a file with some pixel dimensions (probably unknown to them), and specifies the size (in inches) of the print that they want. The vendor's software then calculates the resulting ppi, and if it is too low, automatically up-rez'es your image so that the customer don't see individual pixels, just a bit of a blur. The customer/user never has to deal with concepts like ppi, and if some ppi metadata happens to be stored in your file, it is simply ignored.

In fact, in many cases, even if you are sending your file to a local printer directly attached to your computer, the same also holds. The driver software for many consumer grade printers simply asks you to fill in what size print you want, and it does the rest. This is in contrast to more professional grade programs (like PS) and pro or semi-pro printers that work the other way around and make use of the ppi metadata as you suggested in your post.