Are pixels in Photoshop logical or physical?

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

Answer

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.

Attribution
Source : Link , Question Author : user , Answer Author : Cai

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