DPI Question

[mabas9395]

I thought I understood this, but apparently not as well as I thought.

I have been playing with a trial version of Lightroom (which I give a big thumbs up, btw ) and I noticed something when I export my photos. It requires you to define your resolution in pixles per inch. Why does it do that?

When I open an untouched photo in Photoshop CS, it gives my dimensions as 3456x2304 but it also says 72dpi and the ruler bar on top goes out about 48 inches (3456/72=48). But when I export a file out of Lightroom and I set the resolution dpi to 300, the file still says 3456x2304 but now it says 300dpi and when I look at the Photoshop rulerbar, it now only goes out to about 11.52 inches. But both pictures look the same.

My question boils down to this: since my pixel area is the same for both fotos, do I really need to worry about what dpi photoshop/lightroom says when it comes time for me to print? Won't that be decided by the resolution of my printer? If I am right about this, then what is the point of having lightroom state a dpi?

 

[photo_chick]

The dpi is only really important to output size. Basically it tells the printer how many dots/pixels per inch the photo is so the printer knows what physical dimensions the image is. The resolution needs to be set in your image.

A 400x600 pixle image will look the same at 300 dpi on the computer as it does at 72 dpi, but when printed they are very different. On screen the pixel dimensions are what matters. In print the DPI matters.The 72 dpi will print larger as the dots are physically more spaced out in the print. It will also be a lower quality print because of the lower resolution. The 300 dpi will print much smaller since the dots are closer together and be a better quality due to the higher resolution.

Did I make any sense with that to you, or did I totally miss what you were asking?

Feel free to tell me I'm wrong guys. I know what I am trying to say is right, but I am once again not sure I am conveying my thoughts well!

 

[seashoreCM]

Given that you must specify DPI, can you choose anything you want or must you choose a round number like 300 or 450?

Given pixels for example 3456x2304 and pixels per inch (dots per inch) for example 300, you can always compute final picture dimensions in inches (11.52"x7.68" or 11-1/2"x7-2/3") exactly although in a roundabout way.

If you export the same picture using two different DPI's, are the export files the same size?

What happens if you print the two files? Do you get the picture dimensions you computed in the above roundabout way?

What happens if you export the same picture using two different DPI's and re-import those pictures one at a time? Are they still the same?

Disney hints: http://members.aol.com/ajaynejr/disney.htm

 

[Groucho]

I think the only reason Photoshop might emphasize DPI more is that they're targets pros whose pictures will end up in printed form rather than most of us whose photos never leave the digital domain.

I believe you can put it any DPI you want. It certainly won't affect the image itself when it's on your PC, that quality is only determined by resolution, color depth, compression level (if using a lossy format like jpg), etc.

Furthermore, usually programs that let you print photos let you choose the size of the print yourself, no matter what the DPI is set for... though they will let you print from the original DPI.

I am only concerned with DPI with it's something that has to be printed at a specific size - like if I scanned in a document that I want to print at the same size as the original, or if printing out multiple-page paper art (the kind you cut, fold, and glue together, like the ones from The Disney Experience) where the multiple pages need to match in order for the pieces to be the same - which is often not a problem as those are commonly in PDF format.

For photos, I could not possibly care less what the DPI is set for. I get the pic the size I want and if I want it printed, I tell it to print at the size I want it to print, DPI (of the picture, not the printer) be darned.

 

[photo_chick]

I guess I am kind of meticulous about setting the DPI since DH worked in print for so long and he is the one who taught me about how to handle digital files.

I will say that printing at home I have gotten good prints from as low as 200 DPI, and passable prints down to 150 dpi. But I think a lot of what I can feel I can get away with is because of my printer. I have noticed that when I send them out I can't get away with quite as much of a departure from the 300 dpi "print resolution".