Chiroman8

I know nothing much about Dots & DRI vs Pixels and PPI's. I usually press that little button on the top of my camera & lo and behold I see an image and you know what ? I either like it or I don't like it. I guess this is why I've never won the Hasselblad Award yet but I'll keep on trying and you'll never know,Why? because I love & enjoy it ! But you people keep doing what you do because I also enjoy what you all do and I love it & enjoy that also. Thanks ever so much.
Maybe that is why my favorite photographer is Jay Maisel.

Strodav Loc: Houston, Tx

Those of us who have a background in the graphic arts industry, whether it be in offset lithography or digital printers (impact, toner or liquid) and have worked with both dots and pixels have a different, but deeper understanding than those who come from at the subject from more of a photography background. That may at least partially explain all the controversy around the topic.

Thanks to all those who made meaningful contributions.

burkphoto Loc: High Point, NC

The dpi header in an image file has nothing at all to do with modern digital photography. Long ago, it had MAJOR meaning for those of us who used Aldus — later Adobe — PageMaker. The "Place Image or Graphic" command in PageMaker used the image's dpi header to size the image when importing it — flowing it onto — a page layout. It absolutely had a role, because if you imported a 6MP (2000x3000 pixels) image at 72 dpi, it would be placed on a layout at a size of 2000/72 by 3000/72, or 27.77 by 41.67 inches!

So back in the day, desktop scanners would enter the scanner dpi into the file header, the assumption being that the scanner operator set the image dimensions in inches at 100%, at whatever dpi was desired for the layout. In other words, we did our math before scanning.

If we wanted an FPO (for positioning only) scan, we set, say, 6"x 8" at 72dpi. If we wanted a high resolution image for a film imagesetter to make a halftone, we set 6"x 8" at 300dpi. In both cases, the image would flow onto the page layout at exactly the same size.

If you printed the two pages to film on an imagesetter, though, one image would be sharply rendered with a 150 lpi halftone screen, while the FPO image would be quite full of jagged stair stepping effects using the same screen value. So [i]that is why there's a dpi header in image files, AND why it is accompanied by width and height fields, AND why it is labeled dpi and not PPI. The 'dpi' refers to the SCANNER SETTING, and was used in the graphic arts industry — not photography.



I agree with the appendix analogy. It was ONCE useful in a big way. (Recent medical research is identifying subtle reasons why we still have an appendix, and what it does...)



Well, the industry has no standard now. Most manufacturers used to default to 72dpi, because dpi is a required field in the EXIF definition. But my Panasonics put 240dpi in there, which when converted to PPI yield the maximum size image at minimum "photographic quality" resolution (according to Kodak's standards for 8x10s viewed at 12.8 inches). But still, it's meaningless unless you fire up PageMaker on a computer from the 1990s!



Again, monitors have dots. Some manufacturers call each sum of three dots a pixel. But they are physically-sized colored dots. What they call "pixel resolution" is just dot output resolution. A 1920x1080 "pixel" monitor has three times that many fixed-size dots on it, because it's projecting a tri-color image.



Once again, dpi is physical input resolution on a scanner, or physical output resolution on paper. The printer can spray ink at several different resolutions, one of which is 2880x1440 dpi. But do not confuse printed ink dots per inch with PPI used in the driver's creation of the signal driving the print head!

Some Epson drivers have an internal, advanced setting. It lets you choose the intermediate resolution between the original file coming TO the driver, and the raster image processor that converts the data to drive the head. That value converts file pixels to a grid of dot-like cells that are then represented by a lot more ink dots. The last printer I had with those settings had choices of 300dpi and 600dpi. So regardless of whether I sent a 72PPI image or a 300PPI image to the printer, the printer would first create from it, a raster image with either 300 cells per inch or 600 cells per inch — and THEN turn each cell into a portion of the 2880x1440 dpi output.

All of this is to say... It really does depend! You can use thousands of printer dots to represent one pixel from a camera. Imagine a billboard sized image... OR, you can use 4 pixels from a camera file to create one "pixel" (a set of three monitor dots) on a display, by setting the image size to 25% in Photoshop.

In all cases, pixels are just brightness values generated by digitizing camera sensel voltages or scanner sensors and drivers. Dots are the physical origin or manifestation of pixels. You don't have a dot unless you use a scanner or a sensor or a printer or a display. You don't have a pixel unless you have a file.

End, useless rant...

TriX Loc: Raleigh, NC

I would just add that as a bit of trivia, the 72 DPI/PPI field in EXIF data is there by default, and though widely quoted as some sort of standard now, there is no modern display that has a resolution of 72 PPI (or close to it). It originated with the early Apple Macintosh displays and Apple ImageWriter printer at least 30 years ago,

CHG_CANON Loc: the Windy City

Pixels map to Pixels.

Can it be stated any simpler or clearer?

Pixel-based images map to pixel-based display devices. There are no aspects of dots that impact this pixel to pixel mapping. Dynamic resizing occurs when the pixels of the image exceed the pixels of the display, but there's no way to discern the 1:1 match of pixels and an image dynamically resized as it's just a question of ad-hoc or the image is prior prepared to the target display size, the image becomes a 1:1 pixel match, either partially or fully displayed based on the pixel resolution of the display device.

burkphoto Loc: High Point, NC

Ahh, yes, I had one of those early Macs, an SE, both at work and at home. My Imagewriter I had a 1:1:1 dot for pixel for dot relationship with the monitor.

We thought it was ultra cool in 1986! It ran at about 8 MHz and had an 800K 3.5" Sony floppy drive and a 20MB hard drive and one megabyte of RAM. I upgraded both to 4MB RAM. We could do a lot back then with very little hardware...