Resolution and DPI

[GT98]

Ok I need some help here. I here the term DPI used alot when it comes to Digital Imaging. I know that Magazines and what not are normally printed out at 72 DPI and I think high rez is 300 DPI. How does DPI translate into say picture size coming out of a digital camera? Say if I shoot at 1600x1200 how many DPI is that?

[Tjalfe]

that depends on what size you print it
if you print it on a sheet of 16" by 12", then you will have 100DPI, if printed at 4" by 3" it would be 400DPI

[omegaRED]

There are 3 things that define the size of the image: pixels (say 1600x1200 px), DPI (Dots Per Inch), and the dimensions of the image (in cm, inches or whatever). These 3 things are connected.
DPI has to do with the physical size of the image. A picture of 1600 by 1200 pixels @ 72 DPI would yield an image with a physical size of 56x24 cm. The same image, but at 300 DPI, would result in an 13.5x10 cm physical image.

[VJ]

Most digital cameras put DPI information in the exif-header, but this as - as stated above - of no importance. The camera has a resolution of 1600x1200 or 3000x2000. This is the size of the digital image.

If you want to print the image, you'll have to determine the resolution at which you'll print (e.g. 600 DPI). The 3000x2000 image printed at 600 DPI would yield an image of 5x3.33 inch.

Many photo-editing software allow you to set the DPI of the image, but this is for printing purposes (also, some DTP software uses this DPI information to scale appropriatly).


Jörg

[TransformX]

Normal computer screens (beat me what's "normal" ) are 72dpi (photoshop's default is 72dpi).
Most printing machines use a 300dpi resolution (the big machines that make leaflets 'n stuff like that).

[UtwigMU]

There is also LPI

[TransformX]

All I know is that when I had to send stuff to the print shop, they told me their machines use 300dpi source material so I totally ignored pixels and worked using CM (centimeters) for size and DPI for resolution.

[VJ]

Originally posted by UtwigMU
There is also LPI

Yep, it means Line Per Inch.
It is mostly used in moire-reduction in scanners: you specify the DPI at which you want to scan it, and the LPI of the originial (e.g. if you scan from a magazine).


Jörg

[az]

In other words, don't worry about DPI. It's just the quotient of the number of the pixels through the physical size of the image (just as the name "Dots per Inch" implies).

AZ

[spadnos]

Originally posted by TransformX
Normal computer screens (beat me what's "normal" ) are 72dpi (photoshop's default is 72dpi).
Most printing machines use a 300dpi resolution (the big machines that make leaflets 'n stuff like that).

I think you may be off here.

The "standard" VGA screen has 96x96 PPI (or DPI) resolution (often approximated as 100x100). You can calculate the maximum PPI rom the dot pitch: a .22 dot pitch means 25.4/0.22 = 115.45PPI maximum. The actual PPI is visible image width(height) / # of screen pixels wide (high). Photoshop defaults to 72 dpi because that's what most print shops use for their screen resolution. (think silkscreen, not monitor)

DPI are important when deciding on output. A pro printing RIP (Raster Image Processor) will take input images in whatever resolution they start (you generally decide how big it will be in inches and calculate the DPI from the number of pixels, or you scale to a certain DPI value), and convert them to the number of pixels per inch (or lines) of the output.

PPI And DPI aren't necessarily the same thing on a printer either. That's part of the reason that a 300-dpi dye sublimation printer makes output that's better than a 1440 dpi inkjet. The dye-sub printer has 1 pixel per dot - each of its "dots" can be any color. (they also blend together, which makes images look better) On an inkjet, since it can only mix 3 or 6 colors in one spot, they need to combine multiple adjacent dots to make the full color gamut. (dithering) So a 1440 DPI inkjet, since they combine 8x8 (or larger) dot blocks to make individual pixels, is only a 180 ppi (max) printer.

So, when you scan or print, DPI are important. When you photograph or view on screen, they're not.

- Steve

[az]

72 dpi also was the Mac standard resolution, IIRC.

AZ