edrobinsonjr Loc: Boise, Idaho

Sorry if this has been addressed before. Searched the site and found a few answers but still do not quite get it.

I am preparing images for an art competition for my wife. In the past the image requirements were expressed in pixels (height and width) and dots per inch. This time they have called for images to be 4x5 inches and 300 DPI. There seems to be a lot of confusion about this - myself included. I realize that the DPI unit is a print unit. My printer is specified at something like 1200 x 1200 and up to 4800 DPI in color. Is there a direct correlation between DPI and Pixels?

Can I assume that the 4x5 translates to 1200px X 1500px (300 x 4 and 300 x 5). Using Irfanview, resizing to 4x5 inches results in a badly distorted image.

I read where the long dimension should be used in resizing with the requirement that that the proportions remain the same.

Any help and enlightenment would be appreciated.

Thanks,
Ed

CaptainC Loc: Colorado, south of Denver

They are using PPI and DPI interchangeably. Wrong, but common.
If you are taking the whole frame (2:3) and just changing it to 4:5, then yes, you will distort the image. If you really want a 4:5 ratio, you will need to crop it TO the proper 4:5 ratio.

That 1200x1500ppi figure is correct for a 4" x 5" image at 300PPI.

OH, and there is NO correlation between pixels in the file and the dots a printer uses to put the image on paper. At least not one that you and I need to care about. There is an optimum PPI figure for specific printers (360 or 240 for Epson), but I have printed at 240, 300, and 360 and see no PRACTICAL difference. My lab likes 250, so there.

rgrenaderphoto Loc: Hollywood, CA

Adobe Lightroom allows you to export with specific file sizes, dpi, etc. See att screen shot from Lightroom CC

(Download)

edrobinsonjr Loc: Boise, Idaho

Thanks for the replies.

I shoot these images in my "home studio" i.e. spare bedroom with a black backdrop that is not really big enough so I have to crop the images first thing.

I'll work on it and get back.

Ed

burkphoto Loc: High Point, NC

That's absolutely correct.

In photography, pixels are always the individual picture elements in a file. They are DATA. Each pixel can be sized as needed by the output device, but HAS NO SIZE IN AND OF ITSELF. Sizing instructions MAY be included in the file header, but are (most) often ignored unless used by a page layout program when the image is imported.

Dots, on the other hand, are physical. They are either tiny ink dots on paper, or laser flashes onto silver halide paper, or something physical that can be measured in microns, picoliters of ink, etc. Dots are used to REPRESENT pixels. Many dots of different colors of ink can be used to represent just one pixel, for instance.

Where it gets a little tricky is with scanners, and the reasons why are historical. Scanners were introduced first into the PRINTING industry. They were used to dots, so they use the term loosely, to this day. Scanner drivers still refer to dpi, because they are talking about dividing up a physical dimension of the original into so many parts per inch. Before digital came along, those parts WERE dots, created by screens overlaid on graphic arts film in the back of a huge camera. So a scanner creates one pixel in a file for every "dot" (grid cell) it scans from the original. And in that sense, where a scanned dot becomes a pixel in a file, that's okay.

But it confuses the heck out of most people...

The important thing is to realize that you need a certain INPUT DENSITY of pixels represented on an output surface. In photography, the minimum is generally specified as 240 PPI (original, scanned, or in-camera captured pixels) spread over each inch of output. In the printing industry, they use 300 PPI as a standard practice, because editors like to be able to enlarge by 40-50% without going back to the client or other source for another copy of the original.

Whenever you resize, you need to MAINTAIN ASPECT RATIO to keep the same proportions. If you have a 5x10 original that is supposed to fit into a 4x5 space, you can crop it (BAD idea with artwork), or fit to 2.5x5 within the 4x5 space, or perform some compromise of crop AND fit. Your imaging software should have a "maintain aspect ratio" button or check box or other provision. However, Microsoft products are known for allowing distortion by default! (i.e.; Microslop Weird).

So if you make the long side 1500 pixels, and let the short side match the aspect ratio up to 1200 pixels, you'll give them what they want.

AZNikon Loc: Mesa, AZ

Well done! Thanks, Bob

edrobinsonjr Loc: Boise, Idaho

Thanks, Bob. Your last sentence makes it clear.

burkphoto Loc: High Point, NC

Labs like 250 because it's a nice round number that makes required or desired resolutions easy to calculate. Extinction resolution* at 12.8 inches from print surface (the diagonal of an 8x10 is 12.8 inches) is around 240 PPI. So rounding up made sense. At least, that's what the DP2 and scanner gurus at Eastman Kodak told me around 2002 when I was setting up their Bremson HR500s. Since a lot of pro labs still use DP2, the 250 PPI standard stuck. (*Extinction resolution is the input file resolution beyond which most people cannot perceive any more details in the output. If you spread 240 PPI coming from the camera or scanner across each inch of paper on an 8x10 inch print, using a much higher PRINTER DPI resolution for reproduction, you will maximize perceivable detail for over 98 percent of the population.)

In reality, once you get to 8x10, required resolution falls off! View a 16x20 print at 25.6 inches (it's diagonal, which is the standard or recommended viewing distance for any print), and it could be printed at anything over 120 PPI and still look good. I like to have at least 180 PPI when printing a 16x20 (and preferably more), though, because some people like to "pixel peep" (get closer).

marcomarks Loc: Ft. Myers, FL

They should be specifying inches and PPI, not DPI. Sad that somebody running a competition doesn't know that. 300PPI pretty much guarantees that a really nice print can always be made out of the file because printers vary in how much information they need to create the best print they can. Some printers print at 180DPI, some at 240DPI, and many commercial shops are higher than that. Most consumer printers don't print at 300DPI but you never know what printer is going to print that 300PPI file in the future.

You need to crop the file to 4X5 first, then save it as a 1200X1500PPI resolution (4X300PPI and 5X300PPI) to give them what they need. You or they can then print it on virtually anything at 4X5 inches and it will be wonderful. I personally would keep an 8X10 version at 2400X3000 as well so you can print a larger one to frame if she wins!

And true, there is no direct correlation between PPI and DPI. PPI is file resolution, DPI is dots of ink per inch. PPI can be afixed by you, but DPI will vary from printer to printer.