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Re: [OM] dots and pixels per inch

Subject: Re: [OM] dots and pixels per inch
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Tue, 13 Jan 2009 13:32:59 -0500
240-300 pixels per inch are common resolution values for a print since, 
unless you have very extraordinary vision, you will not be able to see 
the dot pattern at 300 pixels/dots per inch at a reading distance of 10" 
(25cm).  300 is commonly used for small prints which are viewed up close 
and 240 for much larger prints which are viewed from farther away.

You've gotten good feedback from others so far but no comment yet on why 
your own printer proclaims a capability of up to 2880 dots per inch. 
Why is that if you can't see dots finer than 300 per inch?

When talking about photo prints from your lab these are now typically 
exposed onto photo paper by CRTs or lasers.  A dot on paper in this 
process equates 1:1 with a pixel from the digital image.  The image will 
be chemically developed from dyes in or applied to the paper with the 
color and brightness dependent on exposure to colored light from the CRT 
or laser.  All very much like film.

Images that are truly printed by application of ink to paper are a horse 
of a different color. A digital pixel may have millions of color and 
brightness levels contained within a single "dot".  But printing presses 
normally have only  cyan, magenta, yellow and black inks to work with. 
Even high end ink jet printers are limited to a small set of inks, say 
6-9 colors including grays and blacks.  The only way for a printing 
press or ink jet printer to represent the vast array of color and 
brightness levels present in a digital pixel is to lay down a large 
array of even finer dots and or dots of different diameters in the space 
of a what equates to a single input pixel.  In the case of your printer 
claiming 2880 dots per inch it may mean something much closer to 288 
dots per inch with each of those dots made up of 10 sub-dots... or 
something like that.  Someone much better versed in the various printing 
processes could give a more detailed answer but my description should be 
close enough to get idea.

Chuck Norcutt


Nathan Wajsman wrote:
> John,
> 
> This is one of the most misunderstood aspects of digital photography,  
> and the amount of confusion out there is actually very simple. An  
> image, whether scanned or captured digitally, has a size, measured in  
> pixels. For example, 4000x3000 pixels (from a 12 MP camera, to use  
> round numbers). You can print such an image in various sizes, each of  
> which will have a different resolution. For example, if you print it  
> as 10x8 inches, the resultion will be 400 pixels per inch (measured on  
> the long side).
> 
> When your photo lab asks for images with at least 300 pixels per inch,  
> what they mean is that you should provide an image of sufficient pixel  
> dimensions to yield a resolution of 300 ppi. In the above example, if  
> you ask them to make a 10x8 inch print or smaller, you are fine. If  
> you ask them to make a print that is 2 feet on the long side, then  
> your resolution will be only 4000/24=167 ppi, and the print will not  
> have sufficient definition according to their standards.
> 
> So, in a nutshell, it is misleading of an image having a resolution of  
> x. It has pixel dimensions of x by z, and the resolution dependes  
> entirely on the size of the print you wish to make from it.
> 
> Nathan
> 
> Nathan Wajsman
> Alicante, Spain
> http://www.frozenlight.eu
> http://www.greatpix.eu
> http://www.nathanfoto.com
> 
> Books: http://www.blurb.com/bookstore/search?search=wajsman&x=0&y=0
> PICTURE OF THE WEEK: http://www.fotocycle.dk/paws
> Blog: http://www.fotocycle.dk/blog
> 
> 
> 
> On Jan 13, 2009, at 3:56 PM, John Hudson wrote:
> 
>> To quote www.NetLingo.com
>>
>> "Resolution
>> "A common computing term, it refers to several things. On a computer
>> monitor, it is the number of pixels (horizontally) and lines  
>> (vertically) on
>> the screen. For printers, resolution is a measurement expressed in  
>> dpi that
>> describes the sharpness of a printed image. With sound boards, it is  
>> the
>> number of bits used to encode sounds. And in business, it describes  
>> some
>> form of mutual agreement."
>>
>>
>>
>> My printer will work at the following print resolutions; draft,  
>> 360d[ots]
>> per inch through various steps to 2880 d[ots] per inch
>>
>> Whenever I get prints done out of the house the photo lab asks for  
>> images at
>> no less than 300 p[ixels] per inch.
>>
>> Is anyone able to provide a clear explanation of the relationship  
>> between
>> pixels per inch and dots per inch ?
>>
>> Does an increase in pixels per inch generate an increase in dots per  
>> inch or
>> are the two mutally exclusive ?
>>
>> If a printer's technology can only deliver a maximum or so many dots  
>> per
>> inch is there a formula to show that an image resolution beyond so  
>> many
>> pixels an inch is un-necessary ?
>>
>> If this question has already been flogged to death I apologise for  
>> asking
>> again.
>>
>> John Hudson
>>
>>
>>
>>
>>
>>
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>>
>> -- 
>> _________________________________________________________________
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>>
> 
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