Resizing images: 72ppi to 300ppi

[stylish_maker6609]

I use Photoshop frequently, but I need a refresher on resizing images. Many cameras take photos at 72ppi and the size in inches is huge. 

I need to provide images to someone with a  "miminum 300dpi", and it occured to me I'm just winging it when I resize images. I recognize ppi and dpi aren't the same, but they seem to get treated interchangeably.

The screenshot I included is a decent quality image, 36.667" x 27.472" at 72ppi. Is this image equivilant to a 4" x3" 300ppi image? How should I resize it?

I didn't have much luck finding a decent tutorial, maybe I'm just salty beause AI search results make everything difficult these days, ha ha. Can anyone point me to a good resource or give me the quick and dirty rundown?  

[davescm · Accepted solution]

Hi,

You are correct to use ppi (pixels per inch). dpi (dots per inch) is used in printing and describes the dots that are printed on paper, in a pattern, to make up those image pixels. Stick to ppi.

72 ppi could be set by the camera, but it is also the default value used by Photoshop when no ppi value is contained in the image metadata (data stored alongside the image).

So to answer your question, go to menu Image > Image Size uncheck Resample and set the Resolution to whatever value you require. With resample unchecked, the image itself is not changed - it will have the same number of pixels and not a single pixel will be changed. Only the ppi value, stored with the image, will be changed which means that the rulers and reported image size in real world units, i.e inches or centimetres, will change based on the new resolution value.

Dave

[Conrad_C · Accepted solution]

The screenshot I included is a decent quality image, 36.667" x 27.472" at 72ppi. Is this image equivilant to a 4" x3" 300ppi image? How should I resize it?

By @stylish_maker6609

It’s actually equivalent to 8.8 by 6.3 inches at 300 ppi.

The math works like this:

36.667 inches * 72 ppi = 2640 pixels wide

2640 px wide / 300 ppi = 8.8 inches wide

The demo below shows that Image Size does that math for you. Note that because we don’t want to change the total number of pixels in the image for this, I turn off Resample like davescm suggested, so that the image stays at 2640 x 1978 px both before and after changing the ppi Resolution:

You said you want 4" x 3" @ 300 ppi. If you set your image to 4 inches wide, then:

2640 px / 4 inches wide = 660 ppi.

But you want 300 ppi, so to maintain 4 inches wide while cutting the resolution from 660 to 300 ppi, some pixels will have to be thrown out. This is when you enable the Resample option, as shown in the demo below:

As you can see, after dropping it form 660 ppi to 300 ppi at 4 inches wide, the image had to go from 2640 px wide to 1200 px wide (4 inches * 300 ppi = 1200 pixels).

[stylish_maker6609]

Thank you both, incredibly helpful!

Any chance you can give me tips for routinely resizing large images for various uses, assuming I do need to resample in this case?

This screenshot is a nice high quality image, but unwieldy for use in InDesign, especially for email sized PDFs. Say I want to keep the original, plus create a print quality version and a screen quality version - three images total. What would your "standard" parameters be? 

 

[davescm · Accepted solution]

Take the 'original' and change the resolution to 300 ppi without resampling. Save that as your master in PSD format. That gives you something easier to work with in InDesign.

For InDesign embed a link to that master and let InDesign handle resizing on the page. When you export a PDF you can choose to downsize any images over a certain ppi value in InDesign's export settings. If you feel the need to downsize first then use Resample and enter a smaller physical size. Save that as a downsized version and embed a link to that version. The disadvantage is that if you change the master you have to remember to change the downsized copy. Personally, I link to the master and keep the size down when exporting to PDF.

For screen use ppi is irrelevant, it is used to calculate a physical size when the image is printed. All that matters on screen is pixels. Check the required pixel size for the platform on which you intend to post, be that social media or a website, then use Image > Image Size with resample checked and enter the required pixel size. Then use Export or Export Save for Web to export a copy at the required size. Note that Save for Web will not include a ppi value (as it is irrelevant for screen use and will be ignored by browsers) and if you open it in Photoshop, Photoshop will add a default value of 72ppi.

Dave

[stylish_maker6609]

Thank you, I feel smarter 

[Conrad_C]

Say I want to keep the original, plus create a print quality version and a screen quality version - three images total. What would your "standard" parameters be? 

By @stylish_maker6609

It’s difficult to boil it down to “print quality” and “screen quality” today.

The way I think about it is, I have one original, and from that I derive a resized copy for whatever the next job needs, print or screen.

For print or screen, job requirements can vary dramatically because the potential media have diversified so much. For example, client A needs a photo as a two-page print spread in a magazine of 8.5 x 11-inch pages, so they need something 17 inches wide (2 * 8.5 inches wide) x 11 inches tall at 300 ppi. If we account for a 1/4-inch bleed, I need to send them:

17.5 inches wide * 300 ppi (which is 5250 pixels wide).

Client B wants the same photo as a full-screen background for a corporate presentation shown from a 4K projector. 4K is 3840 pixels wide, so I resize a copy of my original to that.

Then Client C asks for the photo to be printed on a page at 4 x 3 inches at 300 ppi (your example).

Then Client D wants the photo for their website, and the space allocated for it on the web page is 800px wide. 

Let’s summarize what happened here:

Client	Medium	Resolution	Width in pixels	Purpose
A	Print	300 ppi	5250	Full bleed magazine spread
B	Screen	Not used	3840	Full frame of high definition 4K video
C	Print	300 ppi	1200	Part of a print page layout with other elements
D	Screen	Not used	800	Part of a web page layout with other elements

First observation: The image sizes don’t divide neatly between print and screen. Client B needs more pixels for their screen-based version of the image than client C needs for their print-based version! This has been happening ever since high definition screen resolutions started approaching or in some cases exceeding print resolutions (some phone screens are over 500 dpi). 

Second observation: There is no standard print or display size, there are too many potential variations depending on the job and medium. Even just within print, a small magazine picture might need fewer pixels at a higher ppi (300 ppi) while an image for a 20-foot-wide billboard might need more pixels at a lower ppi (100 ppi or less, because billboards are viewed from so far away). So even if you saved a “print” version of the image it might not have the right numbers for many print uses.

That’s why the main thing I focus on keeping is a full resolution, full quality original from which a copy can be adapted for any delivery requirements. When a client comes along, I get the specs (Dimensions? PPI? Bits per channel? RGB/CMYK? Color space?), save a copy to those specs, and send that off.