Take the 2-minute tour ×
Graphic Design Stack Exchange is a question and answer site for Graphic Design professionals, students, and enthusiasts. It's 100% free, no registration required.

This is a rather stupid question so I apologise in advance but I just wanted to be 100% sure! I have a set of photos at 300 ppi that are 800 pixels by 800 pixels and need to convert them in Photoshop to 72 ppi for web use (at the same size: 800px by 800px).

In Photoshop if I use the 'Resize Image' option and uncheck 'Resample Image' and then change the resolution to 72 obviously the pixel size will stay the same (but the physical dimensions in cm/inches will change). Presumably this will cause no quality loss at all compared to the original as all pixels will be retained there will just be less pixels per inch (as many as are required for web)?

I just wanted to make sure as one of my designers told me it would degrade the image quality but I don't think that's right?

The final image file used on the web will be a jpg but presumably in this same scenario TIFs would be the best source file type so as to not save to jpg twice (and suffer the associated image degradation)?

Thanks so much everyone!

Dave

share|improve this question
    
You may want to add more details to clear this up. If your image is already 800pixels by 800 pixels, you shouldn't be resizing the image. –  Eric Aug 12 '13 at 20:30
    
as user1803405's answer points out, the premise is slightly flawed...there's no need to change the ppi setting for web use since it's not relevant to web use. –  DA01 Aug 12 '13 at 21:31
1  
@dominic Are you sure? 800pixels by 800pixels in 300dpi print settings should still be basically the same as 800pixels by 800pixels in 72dpi. There are still the same amounts of dots, 640,000 to be exact. Now, an 8" by 8" document at 300dpi vs 8" by 8" at 72dpi, then the file sizes are different because we're talking about 5,760,000 dots vs 331,776 dots. If you're measuring the documents size in pixels, that's it, it doesn't get bigger or smaller. That's the fundamental unit, because pixels are the dots. DPI only comes into play when we're talking about how big to make those dots on paper. –  Eric Aug 13 '13 at 12:39
    
@Dominic no, that is incorrect. The ppi setting has nothing to do with the number of pixels in the image. It's merely stating how big said pixels will be when printed from PhotoShop. –  DA01 Aug 13 '13 at 16:43
    
My bad, you guys are spot on @user1803405 & DA01, I was thinking about print units and was on mobile so I couldn't check. –  user Aug 14 '13 at 12:43

4 Answers 4

up vote 4 down vote accepted

800 pixels by 800 pixels and need to convert them in Photoshop to 72 ppi for web use (at the same size: 800px by 800px).

There's no conversion to do. If they're 800 pixels by 800 pixels, that's it, they're the size you need. However, 300ppi at 800px by 800px is like a 2.3" by 2.3" picture, so are you sure the files are currently at 800pixels by 800pixels?


There's really no 72 dpi for web. This is an estimated resolution.

The web, and any computer in general, displays pixels with different monitors fitting different amounts of pixels into different amounts of space. 72dpi was an average to go by. If your monitor has a resolution (I really mean size in pixels, which isn't quite the same as resolution) of 1920 x 1050, then there are 1920 pixels across horizontally and 1050 vertically. So if your monitor's this size, and the display is 19" wider, then your screen displays about 100 pixels per inch.

Digital deviced just display the pixels. Physical size of those pixels will vary from device to device. DPI settings in files only affect the printed outcome.


To further explain this: Pixels are the dots, and DPI only comes into play when we're talking about how big to make those dots on paper, or measuring a doc in physical units of distance, like inches.

An 800pixel by 800pixel document at 300dpi settings still has the same amount of information in it as as an 800pixel by 800pixel document at 72dpi settings. The only difference is that when printed, the document at 300dpi will be printed as a square about 2.6" x 2.6". The 72dpi document will be about 11.1" x 11.1". The difference is, th pixels, which are the dots, are stuffed into a physical space according to their DPI.

Now, if we're working with documents that are set up in inches, as in an 8" by 8" doc, things change a bit. All bitmap images have pixels, or dots if you'd like to think of them that way. Every image that's not a vector has a height and width in pixels, and a user setting up and viewing their document in measurement of inches is just a feature for human beings to think of their documents in measuring units they're familiar with. This is dependent on having a DPI, whether it's one, or three hundred.

  • An 8" by 8" document in 300dpi is equal to a 2400pixel by 2400pixel document, because f or every inch, there are 300 pixels in it.

  • An 8" by 8" document in 72dpi is equal to a 576pixel by 576pixel document, because there are 72 pixels in every inch.

If you were to set up an 8" by 8" document and draw on it, add rasterized text, etc, and the document was set up in inches, at 72dpi, then you converted it to 300dpi, now we're adding pixels by scaling the document up from 576pixels by 576pixels to 2400pixels by 2400pixels, and any non-vector images will lose some quality (on your screen mostly, but this is a whole other discussion), or at least not truly be the quality of a document originally designed at 300dpi.

But, if you're starting out by setting up the document in pixel measurements, then these wont change. At this point, the only affect changing the DPI settings will have is on the physical size of it when it's printed.

share|improve this answer

You will see no quality loss on the web. In fact, it won't make any difference at all since the browser will read the pixel dims and ignore your ppi settings.

If you print that same file, the quality will be lower because it has effectively been zoomed. Same source pixels, different device ppi.

Definitely keep the source tiff. As you noted, resaving a jpeg progressively loses data.

share|improve this answer

No resizing should be necessary, and here's why.

DPI and PPI mean Dots Per Inch and Points/Pixels Per Inch respectively. This means that they are a resolution measurement with relation to Inches, intended for print media.

On a screen, pixels are mapped to pixels, so if possible, your 800 pixels will fill 800 screen pixels, whereas printed to paper at 300 dpi, your 800 pixels will be packed into inches, 300 per inch. You can see the pixels : DPI : Inches relationship by playing with the numbers in PS Image Size.

When I design for screen media, I do design at the standard 72 DPI, but I could also design at 300 DPI if I know I'll be printing some part of it later on. They will look the same on screen, but the 72 DPI will print natively smaller on paper.

share|improve this answer
    
"72 DPI will print natively smaller on paper" = actually, the converse is true. Given two images with the same pixel dimensions, the one with the LOWER ppi setting will print LARGER (when printed from software that looks at the PPI setting...namely PhotoShop) –  DA01 Aug 13 '13 at 16:45

It seems to me that the OP knows the relationship between pixels and physical extent, but the question seems to be whether the Photoshop image size logic even if resample image is off for some reason changes pixelvalues as it is changing the pretty simple math which doesn't relly concern itself with the image as such.

Maybe the question could be rephrased from another angle by shedding light on how to compare two images pixel-by-pixel. I guess nobody with access to the source code is here and able to tell us what really happens, but as the described workflow (change resolution) is pretty simple, the question could quickly be measured by simply doing the transformation and compare.

My guess would be:

  1. Use the original image at the bottom of a stack.
  2. Duplicate the "resized" layer on top of it. Set the blending mode to "Difference"
  3. Create a Brightness/Contrast adjustment layer, with both sliders to the max

Any practical difference should now be visible. (At least I suppose so? Anyone with a different view?)

Coming from a web background, I'm to too sure that passing resolution data is so redundant after all, especially considering the move to high resoultion (e.g. Apple's Retina) displays. But this is another question altogether.

share|improve this answer
    
When changing PPI settings (and not resampling), you are only changing meta data about the image--not the actual image data. As for the web, it's not redundant, but irrelevant...at least for now. At the moment, no web browser concerns itself with the PPI meta data. Even retina screens still depend on the concept of pixel dimensions (needing 2x the pixels in each dimension) –  DA01 Jan 26 at 3:39
    
Yeah, it's irrelevant for now, but this might change, as the Retina and other displays demonstrates. Still, this is off topic (sorry for bringing it in), as the question is what we can do to prove to the OP's graphic designer that merely changing the DPI doesn't change the image? Or, with an honest mindset stand corrected because "Well, it still happens because of XYZ, even if it really shouldn't". I love to be proven wrong if it leads me to deeper knowledge, and the only way is to inquire, investigate, experiment and observe keenly. The general "shoulds" it seems most agree to. –  picture performer Jan 27 at 8:19

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.