Re: dots per inch?

HEMI-Powered wrote:
Scott W added
eugene wrote:

Someone has asked me to give them an image with 350 dots per
inch. Is this the same as 350 pixels per inch?

There will be no doubt another long debate about ppi vs. dpi,
but yes if they asked for 350 dots per inch it is a very good
bet that what they really meant was 350 pixels per inch.

There can only be one interpretation if asked for a digital file. It would only mean printer dot resolution if the request was for a paper print.

re: another long debate. I think you may be right here, Scott! As best I can tell, the term DPI originated in the printing industry for the earliest of what we now know as half-tone printing, which is the laying down of a pattern of "dots" in a geometrically regular pattern at an angle, often 45 degrees. In the days of B & W newspapers, the number and size of the "dots" laid down roughly to approximate the shades of gray in a continuous tone photo of the day allowed the black dots and white news print to fool the human eye into thinking they were looking at a real photo.

Interestingly, half-tone requires a huge resolution in digital files because it's based on the photographic process of developing through a screen with a grid of holes... with a film transparency overlaid. The brighter areas burn each dot down to a smaller dot, the dark areas make bigger black dots. And it's measured in lpi (lines per inch) just to make things more confusing... halftones can also be done with lines where the line simply gets fatter or thinner according to the exposure. Digital is not good at achieving this because digital can only jump by full pixels and cannot make gradual size increments like film.

Later, the same idea was applied to color photos.

Much, much later, when computer scanners were invented, the term DPI now had real meaning, in that it specified how many "dots" the user wanted the scanner to "sample" or scan per linear inch across the paper and down its length. Each "dot" resulted in a pixel in the final image. AFAIK, that definition of "DPI" is still valid.

I won't go into the debate about DPI for ink jet printers except to say that the number of "dots" laid down by the printer in order to "dither" to get the desired colors and density isn't at all the same sort of definition I used above. Yes, of course the printer is actually producing N DPI but it seems difficult for them that aren't mathematicians to understand what is really happening.

For us normal folk, who just want to get "good" prints from a given pixel resolution raster graphics image, I agree with you and most others in that "DPI" really means "PPI". I do not get any of my images printed by pro shops nor do I dabble in "printing" my images in publications that use today's version of half-tone printing, but I suppose there are places that are set up to calculate the finished print size using "DPI". One can obviously do the exact same thing using "PPI" but perhaps older print machines and/or older software may want DPI.

And with that, let the debate begin!

:-)


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Paul Furman Photography
http://edgehill.net
Bay Natives Nursery
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