Changing a JPG's PPI tag without opening it. Possible?

Hello

I have got a load of JPG files here, saved as maximum level 12 in Photoshop, ready to send as files for printing. Like an idiot I overlooked the fact that I haven't got the same ppi setting for every image, so now want to change them so that they're all the same.

Working on the assumption that the simple act of opening a JPG, then saving and closing without doing anything to it, will affect the image quality (correct me if I'm wrong there), I was wondering if it is possible to access the appropriate data tag and edit it without actually opening the JPG file in Photoshop.

If so, how might I go about doing this? Failing that I suppose I'll just have to re-do the whole lot, starting again with the original TIFF files.

Thanks very much,

Mike

Are you sure that your lab cares what PPI is set to? Most don't as it's a pointless setting.

It's not a lab but a magazine.

I have no idea about how they work, so I'd rather avoid the situation whereby they see that one of my images is set to 96ppi, then mistakenly re-sample it to 300ppi (or whatever ppi they want), then look at it at 100% and reject it as being crap, thinking that I've sent them a load of rubbish in the first place.

Mike

Well, I can give you a "down and dirty" approach, but use at your own risk and only with copies:

The "resolution" tag is in the tiff section of your file exif header. There are three associalted tags: xResolution, yResolution and ResolutionUnit. If you convert your jpeg copy to a text file and do a search for the word "resolution" you should see these three entries side by side and see the values associated with them. If you enter the value you want for the x and y resolution, save the file and change the format to jpeg, well, here's the question: do you feel lucky?

Like I said, try at your own risk.

You don't give specifics of how you are printing, but I wouldn't bother with changing the ppi unless your outside lab specifies that as a requirement. The vast majority of digital printers don't care about this number, although some printing of the offset type does use it and asks for it (which is why some stock agencies also ask for it, since they supply publishers).

Mike-DT6 wrote in post #10075822
It's not a lab but a magazine.

I have no idea about how they work, so I'd rather avoid the situation whereby they see that one of my images is set to 96ppi, then mistakenly re-sample it to 300ppi (or whatever ppi they want), then look at it at 100% and reject it as being crap, thinking that I've sent them a load of rubbish in the first place.

Mike

With the "answer" I posted above, I didn't take into account the fact that you are likely changing a two-digit ppi number to a three-digit one, and doing so could "break" your file (the file header also keeps track of pesky things like byte count).

Your safest bet might be to just redo the conversion from your original, although there are likely editors out there who will do this for you.

Thanks Tony, I'll delve into that possibility in a moment. I'm not quite sure what I'm doing there, but I'll have a look anyway!

EDIT, as you posted again whilst I was posting:

Okay, I think I'll re-do them from the start to avoid any problems. It wouldn't necessarily be from two to three digits as some are 240ppi, but I think I'll re-do it all to be on the safe side!

Mike