Dpi and pixels

[Justinr]

No doubt this is an elementary question but how can I predict the maximum size of a print at 300dpi from a jpeg of a given size in pixels?

I have come across a requirement from a buyer who wants to print to 29"x23" at 300dpi and so scaled up part of an image to match those dimensions the result looks perfectly acceptable. Yet the original frame size is 4256 x 2832 pixels which if I dived by 300 should only give a print of 14" x 9.5" at 300dpi.

What am I missing?

To be honest I have been completely lazy up until now and gone by rule of thumb of 1-2mb for A4 and 100 - 150kb to fill a screen, depending on complexity of subject.

[Mark D Segal]

Your calculation assumes no "scaling-up". The "scaling-up" you mention is actually a resampling of the image, or put otherwise, increasing the number of pixels per inch with algorithms in the software that essentially invent information intelligently. There are limits to how far you can take this before image quality begins to visibly suffer, but if the extent to which you have done it looks fine than it is fine.

[Slobodan Blagojevic]

What you are missing, my friend, is that you need to uprez the image to match the client's requirements. In other words, you need to deliver a file 8700px x 6900px. See? (Jigsaw) puzzle solved

[Mark D Segal]

Correct, and in case this is not what you have done knowingly, you uprez the photo in Photoshop's image size dialog by checking the resample image box and entering the PPI and linear dimensions you need.

[Justinr]

Correct, and in case this is not what you have done knowingly, you uprez the photo in Photoshop's image size dialog by checking the resample image box and entering the PPI and linear dimensions you need.

Inspired by the advice of you two kind gentlemen I think I now have it all worked out.

Many thanks.

[Mark D Segal]

Super - you are welcome.

[eliedinur]

To be honest I have been completely lazy up until now and gone by rule of thumb of 1-2mb for A4 and 100 - 150kb to fill a screen, depending on complexity of subject.

Megabytes and kilobytes have nothing to do with it. A 2400x3600 pixels image for an A4 @ 300 ppi could be a 1 MB compressed jpg or a 50 MB 16 bit tif.

[Justinr]

Megabytes and kilobytes have nothing to do with it. A 2400x3600 pixels image for an A4 @ 300 ppi could be a 1 MB compressed jpg or a 50 MB 16 bit tif.

Which is why I said rule of thumb.

[eliedinur]

Which is why I said rule of thumb.

Invented by somebody with hands like a duck's feet. Donald lives!

[Justinr]

Invented by somebody with hands like a duck's feet. Donald lives!

But it works for me. Jpeg file size is an indication of how much info is contained within it, usually 1-2mb minimum will have enough for a quite acceptable A4 print while 100 - 150kb is usully more than sufficient to look good on most, if not all, monitors and any bigger is just too slow to download.  Of course it depends upon the complexity of the subject but if there is not a fair degree of similarly coloured sky or background involved then I'll simply send a larger file. No problems to report as yet and whatever quality you send to a magazine the printers never fail to butcher it, but then I'm not working for the top glossies.