72dpi - resolution - resampling - dpi - ppi

[Mad Aussie]

Ok..and I just read this..does this make any sense?

"If you need to change the embedded DPI for some other reason, this can be done within photoshop and most other image editing programs. In Photoshop, the DPI can be changed by: Image>Image Size>Resolution (pixels/inch). Make sure to uncheck the "resample image" box at the bottom, otherwise the number of megapixels in your image will change (using interpolation).:"

Yes Kat ... that is the correct way to resize the image. DEFINITELY make sure 'Resample Image' is not checked.
It's fine (and reccomended usually) to have 'Resample Image' checked if you are just resizing for uploading to the net.

Lots of nice technical advice in here.

In short ... your monitor at best will show you about 72 - 96 ppi (dpi) so changing it 300 doesn't look any better on the screen.

The printer asks for 300 dpi to save them work when they print. Like someone said ... it's just a code that tells the printing device what quality to print at.

Many photo labs have a default at 300 dpi so even if you forget and send them a file set at 72 dpi you'll get an image printed at 300 dpi anyhow. I (actually LeAngel) tried this recently and it worked.

[Marko]

Excellent info in this thread.

This part of kat's question is still foggy:

I've also notice that I have some images pop up at 240 dpi and others 300 dpi. And I haven't changed the settings on my camera for the last year (nor the computer - but then again, we do have two and I use both).

I've noticed that do and I know how to fix it but why does that happen? Is it a portrait/landscape thing?

Ps. - I added terms to the thread title so guests/members can find it in many ways.

[tirediron]

Excellent info in this thread.

This part of kat's question is still foggy:
I've noticed that do and I know how to fix it but why does that happen? Is it a portrait/landscape thing?

Ps. - I added terms to the thread title so guests/members can find it in many ways.

I suspect that this may be the result of using smaller-size .jpg images.

[Marko]

I don't think so T.I., I only shoot RAW and this happens to me too.

[tirediron]

Okay, I'm confused. A RAW image displayed in the same software will sometimes appear as one DPI and at other times, another? What application, and what, if anything else is different?

[edbayani11]

it may come out 250 or 300 dpi in different softwares but the file size in megabites does'nt change. what changes is the length and width measurement if the dpi changes.

[tirediron]

it may come out 250 or 300 dpi in different softwares but the file size in megabites does'nt change. what changes is the length and width measurement if the dpi changes.

Correct; different applications may have different default display sizes set, but I made the assumption that most people would be using the same application...

[edbayani11]

if you want to resample in photoshop simply because you have no choice, ans you want to increase your measurement (length and width} do it in small increments until you reach the size that you want.
this is from experience when i was still doing billboards that measure 40 x 60 feet. i start the image at around 45mb and when approved for printing in large format printers, i slowly increase the measurements slowly in small increments until i reach 40 x 60feet. the file size sometimes go up to 350 to 470mb. this takes time but i noticed there is little degredation compared to resizing it in one shot.
the resolution for this size is about 20 dpi only but since this will be viewed from a distance, it looks smooth and sharp.
i know you won.t be doing this kind of extreme resampling

[ridicwilco]

actually, dpi does matter. here's an example why: if you scan an object on a flat bed scanner at 300 dpi then you can print that image with a higher level of detail at 20x30in than if you were to scan that same object at 72 dpi, convert it to 300dpi and print it out as 20x30in.

when you change the dpi of an image the file size return is larger. that's because you're adding more information.

you have to think about it in terms of information vs detail

more detail means more information but more information doesn't necessarily mean more detail. yes more detail, but more of the SAME details.

say you change a 72dpi image to 300dpi. you're literally multiplying the same information to make for a larger image vs an actual 300dpi image which has a more diverse information matrix.

[Alex Wilson]

Some good info here, and I thought I'd chime in:

- Most of the time DPI and PPI are used interchangeably when talking about the pixel resolution -- the exception being when talking about the resolution of a printer. For printers, the DPI output is talking about how small the dots are it uses to make the image. In that case, it is not the same thing, since it take many dots over a larger area to render a single pixel of the source image. If you've got a 1440 DPI printer, you don't want to print your images at 1440 PPI on it.

- On a scanner, the PPI is very important since what you're scanning is of a fixed size. Being able to double the PPI means a higher resolution image.

- For an image file, PPI means absolutely nothing unless paired with the image dimensions. A 300 PPI 8x10in file contains exactly the same information as a 75 PPI 32x40in file.

So why do you need to know or care about PPI for an image?

Photoshop and other image applications will use it for printing. If you've set the PPI (or the setting has carried through from your raw file) then that's the size it will print at -- unless you override the print setting to scale it to fit to the paper, etc.

It also makes it easier when talking about printing formats and such. For the print shop I take all my stuff to, it's much easier for them to say to provide the files at 300 PPI rather than list the pixel dimensions of every possible image size.