… without changing its dimensions? I scanned in a photo at 600 dpi but it saved at 72. I want to change it to 240 dpi so it prints at a higher resolution but when I try to change the resolution Photoshop automatically scales up the picture. I asked my teacher and he said no, but let’s just say I don’t entirely trust his judgement. Could someone else tell me if this is possible?
Uncheck ‘resample image’.
Ah, that’s it, thanks. Now it’s official: my teacher is useless.
And what you’re referring to is “ppi” (pixels per inch), not “dpi” (dots per inch). The former has to do with the resolution of the image itself; the latter has to do with the printed image.
I don’t know if your teacher is just clueless or not, but it a sense, he’s correct. You can increase the resolution of a photo just as GorillaMan said, but it’s not a good thing to do in most cases. You can’t create photo resolution out of thin air. A 72 dpi photo res’d up to 240 dpi will have no more data than the 72 dpi did in the first place and will likely look significantly worse than a ‘true’ 240 dpi photo.
And just to throw another nickle into the pot…
If your scanning an image in for printing, scan it in at the higher/highest resolution you can, and then resize the ‘print dimensions’ as required.
It probably ‘saved as 72’ either because you saved it as a JPG (or other format) that is self limiting to the lower ‘screen’ resolution.
Whenever possible, save to the native PSD (photoshop) format, then do your adjustment. If your saving it for the web, then you can reduce the overall resolution.
You simply cannot add data that does not exist to an image, no matter what the CSI techs would teach you.
Yep, I’m guessing your instructor meant you “shouldn’t” not that you “couldn’t”. Generally, it’s not a good idea to increase the resolution without decreasing the size, because, as stated, Photoshop can’t create what’s not there (the extra pixels needed). Sometimes it will try (I don’t know a lot about this because I try to stay away from letting Photoshop do stuff like this), but generally doesn’t do a great job. If you really want to do it right, you should re-scan the picture after figuring out why it scanned at 72ppi as opposed to what you thought you were scanning it at.
The OP was asking precisely how to avoid this. Photoshop defaults to resampling, i.e. creating extra pixels, whereas the OP wanted to only change the resolution setting. However, you’re right, that if you want a higher-quality printout (as opposed to just having a bigger one) it’s better to go back to the original and start again.
Ppi, not dpi. Right 
I scanned the photo at 600 dpi, but saved it as a png so it ended up at 72 again. I know changing the resolution doesn’t do anything to the image on-screen, I wanted to change it because he is insisting we have them at 240 ppi. That’s all.
IIRC, image formats such as PNG, JPG and GIF all leave you with 72 ppi resolution… it’s part of their algorithims… that and being designed for screen (as opposed to print) resolution, reducing to 72 ppi is part of the compression.
Let’s be perfectly clear here: a 1000 x 1000 pixel image has EXACTLY the same amount of image data whether it’s 72 ppi, 300 ppi, or 1000 ppi.
agreed… but if you attempt to increase the 1000 x 1000 from 72 to 300, you will not have sufficient data… going the other way, you are fine.
So, if you start with an image that is ‘already’ at 72 ppi (a jpg, for example) and attempt to make it 300 ppi, your gonna have bad results.
So, what I am saying (and where most people get confused) is that if you start with an image format that automatically reduces the ppi to 72, you’re not going to be able to ‘enlarge it’ by much, if at all.
You can of course, take a 300 ppi picture and save it as a jpg… but when you re-open the jpg, it will be at 72 ppi.
(I’m using jpg, png and gif interchangebly…)
Yes, it’s a tif now.
No, you can change it without changing the image at all except for a different number in the “resolution” box, I just couldn’t remember the button to make it go click. Like I said, it makes no difference whatsoever, my teacher just likes the number 240.
Not true of JPG. They can be saved in any resolution one desires.
I’m really not trying to be nit-picky here. Ok, yes I am. But not in a rude way.
I’m not sure we’re saying different things, but I do have a problem with your statement.
If you have a 1000 pixel x 1000 pixel image at 72 ppi and you change the resolution so that you then have a 1000 pixel X 1000 pixel image at 300 ppi, you will NOT affect the image quality. The only thing you will have affected is the image output size - you’ll have gone from a 13.8 inch X 13.8 inch image to a 3.3 inch X 3.3 inch image. But you’ll have the exact same amount of image data.
Now, if you have a 2 inch x 2 inch at 72 ppi and you attempt to keep the image 2x2 and increase the resolution to 300 ppi *then * you will cause interpolation to happen and Photoshop (or whatever) will have to create data basically out of thin air, thereby affecting the quality of your image.
That is exactly what I was trying to find a way to phrase after my hastily written post…
We are in agreement… you just have better words!
Returning to my original reply…we’re not talking about changing the absolute pixel width of an image. Changing the ‘dpi’ setting in Photoshop, once the resample option is deselected, means that the actual image remains completely unaffected. simster’s post (to take just an example) shows a misunderstanding of what’s going on here.
Okay, let’s go through this one more time.
If you have an image that’s one square inch in size:
at 72 ppi, the image has 5184 pixels
at 600 ppi, the image has 360,000 pixels
If you scanned a one square inch image at 600 ppi, then you changed it to 72 ppi, while keeping it one square inch you have thrown away 355,816 pixels worth of information, which you will never get back again, no matter what you do.
If you then change that one square inch image from 72 to 240 ppi, you will have more pixels again, but they will only be based on information available in the 5184 pixels thay you had left after your first conversion. You will be dividing those 5184 units into smaller pieces, but that won’t improve way they look to the eye when printed on an ink jet or on a commercial printing press.
The amazing (or not so amazing) thing is how many graphics professionals don’t understand this. But once you do understand it, it’s so simple.
Not if you don’t resample, as my first reply indicated. This is the dialogue box after disabling resampling - the pixel dimensions are fixed.
