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Image DPI? Does it realy matter

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  #1  
Old 11-11-12, 15:13
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omalanrf omalanrf is offline  
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Default Image DPI? Does it realy matter

I was always wondering when looking at the data of a photo I took, why it was always set to 240 and not 300 dpi, I know you can change this on photoshop but I came across this video on youtube.
Look at it and give me your input guys/gals

http://www.youtube.com/watch?v=33lWb...eature=related
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Last edited by omalanrf; 11-11-12 at 15:16.
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Old 12-11-12, 01:30
gordon g gordon g is offline  
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He is right talking about printing output as Dpi. I think a lot of photographers these days confuse this with Ppi (pixels per inch).
Dots per inch is an output measure - a way of looking at print quality. Most modern injet photo printers can output 1440-2880 dpi - way above the quality of magazines and newsprint, which is where the 300dpi standard came from. For large prints on fine art paper, I find 1440 perfectly adequate. Using 2880 adds little to the print quality at normal viewing distances, but considerably slows down the printing process. Higher dpi gives finer graduations of tone and detail because the printer is using smaller dots, but that doesnt alter the number of pixels in a digital image.
Pixels per inch is an expression of the resolution of a digital image. 300 ppi is a better resolution than most people's eyes can see - 240ppi is adequate for most images. But it does depend on the output device - computer screens are often set at around 72-90ppi, so they just cant show high numbers of pixels per inch. There is little point therefore saving images at 300ppi to be viewed on a screen.
Ppi does have an influence on the output size of an image. If you have a 3000 by 3000 pixel image at 300ppi it will be 10 inches square. If it is output at 100ppi it will be 30 inches square. What it doesnt do is change the number of pixels an image actually consists of.
Clear as mud I expect, but I hope that helps!

Last edited by gordon g; 12-11-12 at 09:41.
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Old 12-11-12, 08:22
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graham harcombe graham harcombe is offline  
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Excellent reply Gordon. Agree completely - and it's always been a confusing area for many.
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Old 12-11-12, 22:49
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This from 5 years ago.


We are too much taken by big numbers, and big bits of kit. Often we forget that just like computers other stuff has also come on in leaps and bounds in the last 30 years.

A little while ago I was looking to bring back into service some classic hifi speakers from the 1970s. I wanted to drive them from the computer but didn't want to have to run a long length of cable from the computer to some amp. I asked about and got well meaning solutions involving £100s or was told to just ditch the classic speakers and get wireless modern ones. I have a relation who is a sound engineer and his solution ran into £1000s.

Eventually some one gave me a perfect solution, I emailed back my relation and said I'd got a solution which would surprise him. He came round a week ago and I fired up the speakers. He nodded approvingly and said 'But where is the amp?" So I showed him and he spluttered 'but, but, but ...'
http://www.youtube.com/watch?v=1nNK8...ture=endscreen
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Old 13-11-12, 02:07
robski robski is offline
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I have spent many years on the technical side of the printing industry especially in imaging for Newspapers and Magazines.

Most of what the guy in the video is saying has a ring of truth but he does tend to fall into his own trap of not knowing exactly what his is talking about.

The most common error is folk confuse image (photo) resolution with printer (imager) resolution. Current imager technology scans the press printing plate directly with a laser.

Newspapers require fast imaging so the resolutions are typically 1016 dpi or 1200 dpi. Newsprint is a low quality paper and the print quality is determined by the smallest area of ink that will stay stuck to the paper as it whizzes through the press.

Glossy Magazines require quality and often use resolutions of up to 3600 dpi. In this case the paper and press used can print very much finer dots.

Newspapers will print photos using standard half-tone in the range of 85 - 100 lpi dependent of their press ( note lpi - lines per inch used but often you will also hear dpi used for the same term).

Glossy Magazine will print photo using standard halftone in the range of 120 - 185 lpi. Standard half-tone is where you see the picture made up from halftone dots. Each halftone dot is made up from a cluster of imager dots.

OK that's a bit of background info.

Lets get back to the question Does DPI really matter ?

Well the simple answer is it depends on what you are printing to as to what DPI should be used.

In the case of half-tone printing I am afraid strictly speaking the 2 times sampling theorem rule still apply to avoid quality issues such as moire patterning. so for 150 lpi you require 300 dpi or 85 lpi you require 170 dpi.

Halfway down the page on the link shows a brick wall with and without moire patterning.
http://en.wikipedia.org/wiki/Nyquist...mpling_theorem

There are other screening technologies such as stochastic which are said to required lower dpi requirements but rarely used in the real world.

Inkjet printers use various dithering techniques to produce a continuous tone effect so that no dots are seen. Good results are seen using 150 dpi.

The limit of the human vision system is said to be 7 line per mm which equates to 180 dpi or a half-tone of 180 lpi. So no point in going beyond that as the improvement will not be seen without a magnifying glass.
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