how can I change resolution from 72 ppi to 144 ppi?

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Level 1

104 points

how can I change resolution from 72 ppi to 144 ppi?

Adobe's CS allows me to change the resolution of an image from 72 ppi to 144 ppi (Image | Image Size). Does iPhoto have a similar capability? Terence Delvin has previously posted how to resize an image to a specified dimension (iPhoto 8 | Installing | Resize), but I also need to change the resolution. iPhoto is so wonderful it's a shame to have to export a file and open it in CS if this is all I wish to do. Thanks, in advance.

MacBook, iBookG4, Mac OS X (10.5.6), iLife9, CS4, Canon Pixma Pro9000, Sony DSC-H9, Canon XTi and Canon SX10IS

Posted on Apr 1, 2009 3:24 PM

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Answer 1

PT

Level 7

20,894 points

Apr 1, 2009 4:23 PM in response to BPWMorro

OK I have to ask why? Because many people do not really understand what they are changing or the reason the think they are doing it for is wrong/flawed.
The ppi setting is a print setting to determine the size print you will get from your printer.
iPhoto allows you to print different sized prints, therefore it does in effect change the ppi setting behind the scenes as you choose print sizes. But it won't change the ppi setting in the photo's exif data permanently.
But if you send the photo to another application for printing, if that application also offers ways to change the print size, then you are effectively changing the ppi setting again.
Contrary to popular belief, the PPI setting of a photos has NOTHING to do with how it displays on a computer monitor.
Patrick

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Answer 2

BPWMorro Author

Level 1

104 points

Apr 1, 2009 4:37 PM in response to PT

Patrick,

the answer to "why?" is "because that's what I'm told to do by others who think they know."

Seriously, it is a photo competition where digital images are displayed through a computer/digital projector setup. The organization's requirements (not mine) are 1024 pixels on the longest side and maximum 144 ppi. Adobe resizes like this in both the CS and also Elements and when the file is then saved it keeps the 144 ppi and its size decreases (for example, from 3.22MB to only 751KB). I just would like to make my life simpler and use iPhoto to accomplish the resize.

Bert

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Answer 3

PT

Level 7

20,894 points

Apr 1, 2009 5:39 PM in response to BPWMorro

BPWMorro wrote:
Seriously, it is a photo competition where digital images are displayed through a computer/digital projector setup. The organization's requirements (not mine) are 1024 pixels on the longest side and maximum 144 ppi. Adobe resizes like this in both the CS and also Elements and when the file is then saved it keeps the 144 ppi and its size decreases (for example, from 3.22MB to only 751KB). I just would like to make my life simpler and use iPhoto to accomplish the resize.

OK I see. I am still confused as to their requirement of 144 ppi as the 1024 on the longest size will resize down for you.
I don't think iPhoto can change the ppi but you can resize down. Pick the photo, go to File -> Export... and then under size pick custom. On the pull down if you pick "dimension" then type in 1024 in the box under that, it will automatically output a new version of your photo with 1024 as the max size and you get a nice smaller file.
However, the PPI setting remains the same since this is a print setting and when displayed on a monitor or digital projector, it will display at exactly 1024 by whatever the other side is, assuming the program displaying it displays at 100%.
Let me poke around a bit more and see if I can come up with something.
Perhaps a simple exif editor (rather than having to open it in Elements or CS) that you can quickly change the PPI setting of the exported image. I'll have to look and see what is available.
Patrick

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Answer 4

LarryHN

Level 10

106,142 points

Apr 1, 2009 6:43 PM in response to BPWMorro

DIP is Dots Per Inch - digital files do not have inches, only dots - there is a DPI field in the (I think) EXIF data (or maybe it is in the ITPC data) but it really does not mean anything

DPI comes from displaying or printing an image - the dpi is the pixel dimension divided by the display size - for example a 3000x2400 pixel image printed at 10" x 8" is printed at 300 dpi

Read +The myth of DPI+ for a longer discussion of DPI -
http://www.rideau-info.com/photos/mythdpi.html

LN

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Answer 5

PT

Level 7

20,894 points

Apr 2, 2009 7:49 AM in response to BPWMorro

Bert,
I couldn't find anything in iPhoto that allows you to somehow get to the tag and change the ppi number. And a quick look around the internet for small apps didn't turn up anything that can do it. There are small apps to modify other exif data in your photos but they tend to focus on things like date, time, comments, etc. I guess people normally do not have a need to change just the PPI tag.
So my next suggestion is you may want to contact the organization and see if you can get clarification as to the reasons for their requirement. Is it for smaller file sizes, to limit photos to maximum pixel sizes, etc. You might explain that you can easily resize the photos down to 1024 on the longest size but the ppi setting would remain the same but that it shouldn't matter on a computer display or projector. If they say their main concern is file size and photos that fit nicely on the projector, then your iPhoto resized pictures should fit the bill. Maybe you can simply submit one resized in iPhoto with whatever PPI the file is and see if they accept it.
Also, for future use, you might check your camera menus. Some newer cameras (especially dSLRs) will allow you to set the ppi setting to whatever number you prefer and apply it to all photos you take. So you could set it to 144 and you are all set. This setting has no effect on how the camera takes the photos. It simply changed the number written into that particular field of the EXIF data.
Regards,
Patrick

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Answer 6

Apr 2, 2009 8:28 AM in response to BPWMorro

I have heard that Preview can do this, too. iPhoto cannot. As others have pointed out, it is really just an EXIF tag that has as much to do with the image as the time and date it was shot.

You could probably right a script that can use exiftool to do it, too.

Message was edited by: Keith Barkley

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Answer 7

BPWMorro Author

Level 1

104 points

Apr 3, 2009 11:49 AM in response to PT

Patrick (and LarryHN, also),
Thanks for all your help--as for another app, I will stay with Adobe since iPhoto doesn't do the trick.

Otherwise, I think it is time that I study the manual to the projector since "those who know" tell me it is a 4 year old requirement of the projector. I'm more prone to think it's a file size issue with an old notebook computer that has a lousy video card....the specs on the projector seem to be very high-end and I don't think its the problem. (Also, I'm going to just try sticking in an iPhoto image reduced to 1024 but ignoring the 144ppi and see if it works!)

LarryHN's suggestion of the Rideau link
http://www.rideau-info.com/photos/mythdpi.html
has also been very helpful; if you have time to browse it you might find it of interest.

Thanks to you both.

Bert

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Answer 8

BPWMorro Author

Level 1

104 points

Apr 1, 2009 5:48 PM in response to PT

Patrick,
will appreciate your assistance.
Bert

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Answer 9

Apr 3, 2009 12:15 PM in response to BPWMorro

If it shows up on the screen it will show up on the projector.

It could be that the software they use to view the photos uses dpi to scale the image somehow.

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