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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 17:04:29 -0500
The print size will be determined by whatever physical dimensions the 
printer is told to produce.  If told to make an 8x10" print that's what 
you'll get.  While not the only determinant, the print quality will be 
largely determined by the number of pixels available to fill that 8x10" 
space.  If the number of image pixels is large enough to provide 300 
pixels per inch on the print you'll get about as good as your eyes can 
handle.  If it's much more than that the additional pixels are wasted. 
Even if the printer can reproduce them you won't be able to see it 
without a magnifier.  That 300 number is based on the visual acuity of 
the human eye at 10" reading distance.  A 10" viewing distance is also 
assumed for depth of field tables and, for 35mm size images, typically 
assume an approximately 8X print magnification over the original source 
material.

The Luminous-Landscape article noted that 200 ppi was sufficient for 
good images.  I think that would work for many natural images and large 
images where you stand back for viewing.  But I don't think it would 
work well for architectural images where you have lots of straight 
lines.  The eye has a different sort of behavior when working on 
straight lines.

Chuck Norcutt

John Hudson wrote:
> Nathan ...
> 
> What you say I understand fully, but what I think is missing is the fact 
> that a print is a piece of paper covered in ink and that, other things being 
> equal, more ink within reason will make a better print.
> 
> The printer is driven by a digital image whose length and breadth is 
> measured in pixels. The print size is largely or wholly dictated by the 
> image's pixel resolution. As the pixels per inch increase the physical size 
> of the final print deacreases and vice versa.
> 
> Thus, an image with few pixels per inch being printed by a printer capable 
> of only a few dots per inch of ink delivery will result in a poor quality 
> print.
> 
> As the image's pixel resolution increases [more concentration of colour / 
> tone / contrast, etc information] coupled with a printer that can deliver 
> more dots per inch of ink must result in a better print because [1] there is 
> more image information for every inch of digital image, and [2] more ink per 
> inch is being put on the paper.
> 
> Thus, in my mind the key issue is optimizing the combination of [1] the 
> image's pixels per inch resolution and [2] the rate per inch at which the 
> printer can deliver dots of ink to the paper. If [1] and [2] are optimized 
> one gets an ideal print or so I think. If there is an imbalance between [1] 
> and [2] there is either too much image information [too many pixels per 
> inch] for the printer to cope with or not enough image information leaving 
> the printer to underperform.
> 
> Is there a formula or rule of thumb for optimizing the combination of pixel 
> resolution and the dot rate per inch at which the ink is put on the photo 
> paper.
> 
> John Hudson
> 
> ps: Both of these pieces are interesting reading
> 
> http://desktoppub.about.com/cs/intermediate/a/measure_dpi.htm
> 
> http://en.wikipedia.org/wiki/Dots_per_inch
> 
> 
> ----- Original Message ----- 
> From: "Nathan Wajsman" <>
> To: "Olympus Camera Discussion" <>
> Sent: Tuesday, January 13, 2009 12:04 PM
> Subject: Re: [OM] dots and pixels per inch
> 
> 
>> 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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>>>
>> -- 
>> _________________________________________________________________
>> Options: http://lists.thomasclausen.net/mailman/listinfo/olympus
>> Archives: http://lists.thomasclausen.net/mailman/private/olympus/
>> Themed Olympus Photo Exhibition: http://www.tope.nl/
>>
>>
> 
> 
-- 
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