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When I open an image then I can see it's size details from Image > Image Size..., it shows me:

Photoshop Image Size

The image has 600px = 8.33in width with resolution 72dpi. Now when I open this image in a browser it is exactly 600 css px wide. But it is not 8.33 inches wide. When I measure with actual scale on screen it comes out to be around 6 inches both when opened in browser and in Photoshop. Even if I set the width to 6 inches in Photoshop it is lower than 6 inches when measured with actual scale. Another thing when I change the resolution in Photoshop it keeps the width same in inches but changes the width only in pixels.

My questions are:

  1. Are pixels in Photoshop logical or physical?

  2. Is resolution in Photoshop physical pixels / physical width in inches or logical pixels / physical width in inches?

  3. Why do inches in Photoshop not match with real life inches of a scale?

  4. Also when I right click on the image on my computer and check it's size from properties it shows exactly same pixel size and resolution as shown by Photoshop and web browser. So does my computer show size and resolution of the image in physical or logical pixels?

  5. When I capture images from my Andriod mobile then is the size shown physical pixels or logical pixels? And is the resolution of the image related to logical pixels per inch or physical pixels per inch?

  6. If I have an image X opened in Photoshop. I half it's resolution and save it as Y. Then I take original X again and decrease it's width to half keeping original resolution intact and save it as Z. Which has better visual quality Y or Z?

P.S: Please note that real life 1 inch is not 96 css pixels as I have understood from my another post https://stackoverflow.com/q/40480617/3429430

Edit: Thanx to everyone for helping me. I have been watching TastyTuts beginner photshop tutorial. In chapter 3 Raster Image principles he has explained very well which I find satisfying explanation.

And image has four attributes:

  1. Image Size
  2. Image data size
  3. Dimension
  4. Resolution

Image Size is the physical size of an image, that is the size when the image will be printed. In photshop under Document Size it shows width 8.333 inches. When the image will be printed it will be exactly 8.333 inches wide.

Dimension is the width/height of an image in logical pixels. In photshop it is under Pixel Dimesion section. When we capture an image from camera the resolution mwntioned is in logical pixels. Physical Pixels are no way related to a raster image -- be it photshop, Computer or camera. The reason is that different digital devices has different dpr. An image must look similar on all the digital devices, e.g. on a ratina display or simple CRT monitor.

Resolution is the number of square blocks(pixels) per inch on a print media. Two images of same dimension, say 600px, one with reolution 60px/inch second with 120px/inch. Both will look exactly same on digital screen. But when both will be printed second image will be 5inches wide and first image will be 10inches wide. Second image is small but sharper than first. What happens is, the printer for image1 will divide one inch width on paper in 60 equal parts. Each part will get color of 1 logical pixel of the image. On the other hand for image2 the printer will divide 1 inch of paper in 120 equal parts and fill each part with one logical pixel of image2.

For print media we should care about Image size(Document size in photshop). Consider two images both having image-size 8.33 inches but first with lesser photoshop-resolution and second with higher photoshop resolution. First will have greater dimension(pixel-dimension in photshop) than the second. So for digital media we should care about Dimension.

Image data size is the size in bytes on a computer. It is directly related to the number of logical pixels an images has. E.g. if all pixel's color is in rgb format then 1 pixel at least requires 3 bytes.

With this information I thing my 7 questions are not too broad and each can be answered in a few lines as:

  1. Are pixels in Photoshop logical or physical?
    Answer: Pixels in photshop are logical pixels.

  2. Is resolution in Photoshop physical pixels / physical width in inches or logical pixels / physical width in inches? Answer: logical pixels / (physical width in inches of image on print media).

  3. Why do inches in Photoshop not match with real life inches of a scale?
    Answer: They do if the image is printed. Inches will not match on a digital screen becasue digital screens do not have consistent logical-pixels per inch of screen value. E.g. we can simply show either 1367px on our screen or 1980px on same screen by changing resolution from operating system options.

  4. Also when I right click on the image on my computer and check it's size from properties it shows exactly same pixel size....
    Answer: It is not how computer shows that size. the image in itself has the meta information of about how much logical pixels it spans on any digital media and how should it look when printed(resolution). so both photshop and computer shows the same information.

  5. When I capture images from my Andriod mobile then is the size shown physical pixels or logical pixels....
    Answer: With any device camera the image size shown in pixels are logical pixels. The resolution mostly is something like 800*600 -- This is again image size in logical pizels. The resolution shown in px/inch is logical-pixels per inch of print media.

  6. If I have an image X opened in Photoshop. I half it's resolution....
    Answer:If X was 100px wide then Y will be 50px wide on digital screen. If X was 1 inch wide on paper the Y will also be 1 inch wide on paper but with lesser quality(sharpness). Z will be half of X on both digital screen and print media. On print Z will be as sharp as X. Z is sharper in quality than Y on print.

Edit finshes here

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  • 1
    Voting to close as you're asking 6 rather different questions here. It's a bit broad.
    – DA01
    Dec 1, 2016 at 22:34
  • But to answer the title of the question: If you're working in pixels, just pay attention to pixels. Forget about the physical measurements.
    – DA01
    Dec 1, 2016 at 22:35
  • I have included a kind of self answer. I am not sure it is technically correct so I did not post it as an answer.
    – user
    Dec 2, 2016 at 7:05

6 Answers 6

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A pixel is a pixel is a pixel.

Resolution (as in PPI/DPI) is meaningless* in a digital context, the only time it is meaningful is when you are printing (or otherwise transferring to a physical medium) your image.
A 100 × 100 pixel image saved at 72PPI will show on your screen exactly the same as the same as a 100 × 100 pixel image saved at 1PPI or 1000PPI.

72PPI has become the de facto standard for screens, so stick to that, but in reality you can ignore the number—it's irrelevant.

Physical dimensions are equally irrelevant in a digital context, Photoshop or your web browser (or any other image viewer that I know of) don't care what the physical resolution of your screen is and work on pixels alone—if you're output is a screen, forget about physical dimension (inches, cms etc).

* Mostly. Some applications are resolution aware, so importing your Photoshop document in to Illustrator for instance, will take resolution in to account. But that is mostly a convenience for sizing—it doesn't actually affect the image content.

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  • Thats because illustrator views everything from print perspective. Even when you explicitly tell it not to. It just simply can not, not do that.
    – joojaa
    Dec 1, 2016 at 9:52
  • "72PPI has become the defacto standard for screen" How come? phones, laptops and desktop screens all come in different DPI, 100-300 is super common
    – Kos
    Dec 1, 2016 at 15:08
  • @Kos of course, screens haven't actually been 72PPI for decades and (as I said) it's really irrelevant, the whole screen = 72PPI thing is nonsense...
    – Cai
    Dec 1, 2016 at 16:07
  • This is... unfortunately less true than it used to be. Someone had the great idea that on high-resolution screens, browsers should automatically apply some zooming to prevent things from getting too tiny. So the default zoom of a webpage might be 125%, not 100%, because the resolution is high, and that will affect how the image is displayed by the browser. Newer versions of Windows even have a system setting relating to that scaling. Not all programs pay it any mind, but some do, like those aforementioned browsers.
    – KRyan
    Dec 1, 2016 at 20:56
  • 2
    @KRyan that is very true and it is a lot more complicated than it used to be yes, but all of that is still completely irrelevant to any PPI or physical dimensions attributed to an image (in e.g. Photoshop)
    – Cai
    Dec 1, 2016 at 21:12
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It's only for print/manufacturing conversions. The operating system does not, most of the time, know what kind of screen is attached to the computer. So it does not even try to guess. Because it can not know it can not scale to physical size. But even in cases where it does, it does not even try because that would be even worse.

DPI/PPI/LPI and physical units are only meaningful in a manufacturing context such as printing, laser cutting, vinyl cutting, embroidery etc. It does not have many uses in a digital context. Which causes people some mental trouble.

Are pixels in photoshop logical or physical?

Neither, pixels are pixels. They are just samples, they have no dimension - no size. Since showing pixels in a digital display at any other size than one pixel is one pixel. So that is how the digital domain works. Your image is simply as big as your display device's pixel dimension happens to be. So different sizes for different monitors.

What you see is just 2 different cultures of thinking of the problem. Neither side is more right or wrong.

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  • If I have two images 600 photshop px wide one with reolution 72px/inch another with 36px/inches both have same size in mega bytes. Which means photshop doesn't change physical px in an image.
    – user
    Dec 1, 2016 at 9:21
  • @user obviously so. Pixel amounts did not change. the reslution is just a metadata tag.
    – joojaa
    Dec 1, 2016 at 9:22
  • Would Photoshop behave similarly on retina display computers? E.g. consider a 2dpr laptop. Would changing resolution in Photoshop keeping width in px intact not change size in mega bytes?
    – user
    Dec 1, 2016 at 9:24
  • user yes. The pixel doubling is just a trick for apple to make sure old web pages would not suffer from the significant screen resolution change. It was a very bad decision. Affects nothing really.
    – joojaa
    Dec 1, 2016 at 9:29
  • can you confirm this by actually resizing and saving image in retina laptop e.g. apple mac book?
    – user
    Dec 1, 2016 at 9:32
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Both.

Because there are diferent types of pixels.

Pixel is (could refer) just a chunk of information (Type A). One raster image is just an array of bits and bytes of information. They need to be reinterpreted as something.

In some 3D programs for example, they can represent not only color, but probably height, displacement, angle of a surface. On a 3D printer they could mean volume.

On the other hand you have small collections of squares on a screen (Type B), this is a different type of pixels.

But this type of pixels normally, when a program inteprets the Type A pixel, they translate to 1 Type B pixel. Normally if the bitmap is not re-scaled, for example zooming in.

The ppi information (it is ppi, not dpi) has nothing to do with this conversion from Type A to Type B. N O T H I N G.

They are used to be printed on a... I was going to say phisical medium, not electronic, but I will rephrase... printed on a printed medium. with fixed dimensions.

On some electronic mediums there is no way to tell what is the phisical dimension of an image (or its pixel), for example on a projected image.

But in all the screens there is some specific ammount of pixels on that screen with phisical dimensions. It is a type of resolution now called pixel density. But you do not define it inside the file.

Now the systems are more aware of this new kind of resolution, pixel density, and they are starting to declare it. But for now they only have one unit to handle them, and it is the 2x or 3x density.

Some answers:

2) In Photoshop you prepare the ppi data on the image to be printed. and makes the calculations on what ammount of pixels you will have if you want some printed resolution.

If your screen-operating system is not calibrated with its internal rulers, they just assume one default value, for example the 96 ppi.

This is a configuration of the operating system, which almost no one uses:

enter image description here

If you do modify it or calculate your ppi or pixel density by hand you can adjust your view to be phisically acurate.

a) Measure the full width of your screen in inches (not diagonal) for example 20in.

b) Read the declared resolution of your monitor, for example 1920.

c) Divide 1920/20 = 96.

But if your screen is smaller, or the resolution is diferent, this numbers will change.

And CSS pixels is just an aproximation assumming most of the screens have more or less that pixel density.

Take in account that smartphones with high density screens, declare a low resolution and makes later the internal 2x or 3x conversion for the images.

I am adding something else. Do not bring into the mixture other concept that has nothing to do with a file pixel information. File weight. The Mb. (Mega byte) That is simply how much disk space a file has.

The "meta" information on a image file for example 72 ppi declared on your photoshop jpg file is taken by the software to shoot the imformation to a printer. Yes, unless is overwridden by anithing else, like a "fit to page" instruction on the printer software.

The ppi information is a linear one, so with 72 ppi you will have 72x72 "square" blocks (sorry @joojaa).

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  • I would say neither, but there are no different kinds of pixels. Pixels are pixels.
    – joojaa
    Dec 1, 2016 at 9:22
  • 1
    Yes they are... No they are not... are they?
    – Rafael
    Dec 1, 2016 at 9:24
  • It will probably in the future a diferentiation between the "information matrix" pixel and the "screen element" pixel, but right now we need to live with both shaked and stired.
    – Rafael
    Dec 1, 2016 at 9:25
  • Pixels are just samples that are most definitely not square. read A pixel is not a little square, A pixel is not a little square, A pixel is not a little square.
    – joojaa
    Dec 1, 2016 at 9:26
  • I already did for my other controversial answer. That is my concept of information matrix. But yes, there are other type of pixels, that although not square, they react (and are reinterpreted) as such.
    – Rafael
    Dec 1, 2016 at 9:27
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It works as a calculator:

When you want to print a business card for example, normally sized 3.5 x 2 inch, you can enter that first.

You can then choose how precise / pixely you want the end-result to be.

The difference in DPI when zoomed in

The higher DPI, the more detail you can see. Maximum DPI varies per printer.

Image used: https://en.wikipedia.org/wiki/Vector_graphics#/media/File:Vector-based_example.svg

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  • Thanx for the example.
    – user
    Dec 2, 2016 at 6:13
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The problem with the premise for your question is that you assume your monitor is 72dpi, it isn't. In fact most monitors are not 72dpi, they're usually higher resolution, or even much higher resolution.

600px at 72dpi is 8.33inch, you cannot however bring out a ruler and measure on your own monitor as the monitor isn't 72dpi. The physical dimensions are going to be different. Digital is completely uninterested in dpi, it only cares about pixel dimensions. The monitor it is viewed on has a set dpi which doesn't change ever, the OS then scales the image to an appropriate size depending on the setting.

72dpi is pure bollocks, don't ever actively change to it. The only dpi worth specifically setting is 300dpi for print work. Other than that just use whatever dpi came out of the camera/scanner. Since InDesign and Acrobat will resize images for you when exporting to PDF you don't really need to bother with setting 300dpi either. Care about dimensions, not dpi (in Photoshop).

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For those who want a short answer.

A: Only dots on a printer (or from another output device that makes real, physical picture elements) are "physical" pixels. Anything else is a representation -- a virtual "spot" of color. The size of a virtual pixel is not fixed because it's not a physical entity. Yet.

The real world cannot be stretched (OK, ignoring Silly Putty™). And the virtual world of Photoshop cannot be made real (physical) without outputting the virtual pixels as actual ones; only then does a pixel have a dimension.

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