JPGs DPI/Resolution & Print Quality Question.

ciaranhickey

Hi,

I was always aware that having a DPI close to 300dpi was best
for good print quality and that 72dpi was for web-based/on-screen
display.

However, JPGs produced by my 2mp Canon Ixus were 180dpi when
opened in PS and my 8mp Canon 350d seems to produce 72dpi
images at Largest Quality Setting (JPG Not RAW).

Can anyone explain the relation bewteen on-screen size (pixels x pixels)
and the print quality? Or why my JPGs are only 72dpi in this case?

I was under the assumption that it was Ok to reduce the resolution
to a lower DPI but not advisable to increase it as it would result in
splitting of pixels and lower image quality.

Any help on working with my 72dpi JPGs to get best print quality would
be great?

Thanks,
Ciarán

elven

I found this: http://www.photo.net/learn/resize/ which went a way towards explaining the whole DPI thing. Basically, as far as displaying on screen is concerned, it appears to be meaningless - it's all to do with the printing. But that's the bit i still have to get my head around... any plain english explanations are welcome!

ladgie353

An image is made up of a number of dots, its resolution (ex: 3456 x 2304 on a 350d).

When you print a picture, the image print resolution given in dpi (dots per inch) defines the size of the printed output.
Examples:
3456*2304 image @ 100dpi gives a printed size of 34 by 23 inches.
3456*2304 image @ 300dpi gives a printed size of 11 by 7 and a half inches.

If you increase the DPI setting (as long as you turn resampling off), all you do is reduce the print size of the image. If resampling is on, you increase the size of the image by adding details which doesn't exist.

Hope this helps.
Laurent

Alun

I think what the OP was referring to was the horizontal and vertical resolution that's embedded in the EXIF tag put there by the camera, and which is read and displayed by a lot of image manipulation software like PS or PSP. On my D70s, for example, it's set at 300dpi and on my Olympus P&S camera it's 314dpi or something weird like that, which doesn't have any bearing on anything at all. It's totally meaningless and should be ignored.

Dpi is possibly more relevant to scanned images, although even then the dpi at which an image was scanned is irrelevant unless you want to print it at exactly the same size as the original. What really matters is the image size in pixels ... if you want to print a 5"x5" image at a minimum of 300dpi, then you need an image of at least 1500x1500 pixels, and that's all the information you'll ever need. Whether the image was scanned from a 5"x5" original at 300dpi, or a 10"x10" original at 150dpi, or even a 30"x30" at 50dpi is completely irrelevant.

ciaranhickey

Thanks a lot to everyone for replying....

That's cleared things up for me.

ciaranhickey

Hello Again,
I'm back with another print related question, decided to add it to this thread instead of creating a new one.

I've been converting my photos (for print) to CMYK as I assumed all printers used this colour space but I noticed
the following on Conn's Cameras requirements for printing:

File formats: JPEG, TIFF and Windows bitmap.
Colour Space: RGB

Should I leave them in RGB? Or is it a possible mistake by Conns.

Has anyone any recommedations for print places beside Conns?

Thanks again!
Ciarán

antifuse

If they say RGB colour space, I'm guessing they know what they're talking about.

Heinrich

CMYK is used by commercial printers where four seperate plates are used to produce the coloured image (quadrichrome)

Cyan
Magenta
Yellow
K Black

Printing photos uses RGB

ciaranhickey

Thanks, so it's printer dependent really!
I just assumed all printers were CMYK.

Don't some home printers (eg. Epson inkjets) also take 4 ink cartridges CMY & K?
I guess they're trying to immitate the commercial plate based printers.

Thanks again

digitalbeginner

One of the reasons Conns would request RGB is that their software will handle the coversion to CMYK in a correct fashion. If you don't have your colour space correctly defined then you end up with a CMYK file that has lost valuable colour information.

Most inkjet printers are designed to accept RGB files for a similar reason, they handle the CMYK conversion in a proper fashion. In fact some inkjet printers make a bad job of printing CMYK photos. Also a lot of the low end photo manipuation softare out there cannot covert to CMKY anyway.

A minor reason would be that an RGB file is three quarters the size of an CMYK one, RGB has 3 channels each the same size and CMYK 4 channels. In other words a Grayscale image of a particular size might have 4 megabytes in its one channel, an RGB of the same size is 3 times that (12 megabytes) and a CMYK one 4 times multiplied (16 megabytes). Transmission of RGB files instead of CMYK helps people still locked in dial-up land. Particularly larger file sizes.