DPI - Unravelling A Mystery

[Dumien]

I don't really understand how the dpi thing works in post processing. I don't really have a clue. So, could someone tell me how it works when you want, say, to save something with 1080dpi and what happens if you save it with, say, 72dpi? How do you how many dpi does a picture have whan you first open it? (I basically use only DPP for post processing).


If you need any other kind of information please tell me.


Thanks for you time and patience.


Andy

[Colin]

I believe it's dots per inch, as opposed to pixels per inch. If i'm right, it only matters if you print from applications that use the information to print. I print from DPP, and I just size the image in the printing menu. I may be wrong, but it seems to work well.

[Keith B]

I believe DPI is term inherited from the print industry, but it refers to pixels in this case. With 1080p the p stands for progressive and it it referring to the height of the image but isn't dot per inch, it is total.


In Photoshop you can go into the Image menu choose image size turn off resample type in the width you are planning on printing and the resolution will change to what the actual dpi of the image it will try to print.


You could also do the math. If my image sensor is 5616 pixels wide and I want to print that at 7x5 I end up with apx 802 dpi which is severe over kill for a 5x7 image.


Most ink jet printers can only handle about 200-240 dpi so anything over that the rip will throw out. I would down sample in Photoshop, so you can see the effect of the reduction before printing. As always I'd suggest printing both, one down sampled and one left as is and compare the results.


Some higher end printers with more memory built into them may handle more. I'm not sure though.

[Citronsyra]

How come that you can save an image with a fixed resolution and still change the DPI? Is it to aim for the correct end result or what? This is how it looks like in Aperture. Is it just as a reminder so that the image can be printed later on with correct DPI?


Thanks in advance!


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[Keith B]

My guess here is that Aperture doesn't do anything destructive to the original. So I guess it would do the averages and some cropping. How it chooses the cropping is beyond me. I guess it would just center it.

[RocklandPaul]

There is actually a pretty good, accurate, and comprehensive explanation in Wikipedia under "Pixel":


http://en.wikipedia.org/wiki/Pixel


The whole article is worth reading, and re-reading.


Here is a relevant sub-section copied from that Wikipedia "Pixel" entry:





<span>A pixel is generallythought of as the smallest single component of a digital image. The definition is highly context-sensitive. For example, there can be "printed pixels" in a page, or pixels carried by electronic signals, or represented by digital values, or pixels on a display device, or pixels in a digital camera (photosensor elements). This list is not exhaustive, and depending on context, there are several terms that are synonymous in particular contexts, such as pel, sample, byte, bit, dot, spot, etc. The term "pixels" can be used in the abstract, or as a unit of measure, in particular when using pixels as a measure of resolution, such as: 2400 pixels per inch, 640 pixels per line, or spaced 10 pixels apart.


The measures[url="http://en.wikipedia.org/wiki/Dots_per_inch][i]dots per inch[/i][/url][i](dpi) and[/i][url="http://en.wikipedia.org/wiki/Pixels_per_inch][i]pixels per inch[/i][/url][i](ppi) are sometimes used interchangeably, but have distinct meanings, especially for printer devices, where dpi is a measure of the printer's density of dot (e.g. ink droplet) placement.[/i]<sup class="reference" id="cite_ref-6"][url="http://en.wikipedia.org/wiki/Pixel#cite_note-6]<span>[i][[/i][i]7[/i]<span>[i]][/i][/url]</sup>[i]For example, a high-quality photographic image may be printed with 600 ppi on a 1200 dpi inkjet printer.[/i]<sup class="reference" id="cite_ref-7"][url="http://en.wikipedia.org/wiki/Pixel#cite_note-7]<span>[i][[/i][i]8[/i]<span>[i]][/i][/url]</sup>[i]Even higher dpi numbers, such as the 4800 dpi quoted by printer manufacturers since 2002, do not mean much in terms of achievable resolution.[/i]<sup class="reference" id="cite_ref-8"]<span>[9<span>]</sup>


The more pixels used to represent an image, the closer the result can resemble the original. The number of pixels in an image is sometimes called theresolution, though resolution has a more specific definition. Pixel counts can be expressed as a single number, as in a "three-megapixel"digital camera, which has a nominal three million pixels, or as a pair of numbers, as in a "640 by 480 display", which has 640 pixels from side to side and 480 from top to bottom (as in aVGAdisplay), and therefore has a total number of 640 &times; 480 = 307,200 pixels or 0.3 megapixels.


The pixels, or color samples, that form a digitized image (such as aJPEGfile used on a web page) may or may not be in one-to-onecorrespondencewith screen pixels, depending on how a computer displays an image.


I think that one place where a lot of people get confused is that they forget (or don't realize) that neither "Dots Per Inch" nor "Pixels Per Inch" says anything specifically about the physical size of that "Dot" or "Pixel". All it is saying is how many of these (whatever size) dots or pixels there are in one inch. Many people leap to the conclusion that if a printed image is at "600 DPI", that each Dot must therefore be 1/600 inch in diameter. Not necessarily true!


There's a lot more to this discussion, but start by reading the Wikipedia entry.


-Rockland Paul

[Dumien]

Thank you guys, these were all good answers. They helped me a lot =)

[Keith B]

Photoshop renders pixels the same regardless.


Pixels from my 5D Mk2 were .004" at 240dpi


Pixels from my 40D were .004" at 240dpi


If I set the image size to 300dpi in Photoshop (resample off) both become .003"s.


72 dpi = .014"


So pixel size is determined when you give it the arbitrary value in Image editor. But ultimately determined by the size you print it.


Rules of thumb are 304.8dpi is the standard for (magazine-print ad) printing, set in Europe some time ago. It is actually a perfect 12 pixels/mm.


200dpi is the cutoff of where the human eye can begin to depict the pixels instead of smooth image.


Canon Raws are 240dpi by default (happy medium?). Jpegs 72dpi.


In practical terms, when you print an image without interpolation the pixel will scale down to what it needs to be. If you scale it up it will scale up and once it goes beyond the 200dpi point the pixels will start to become apparent.


So I can print an image from my 5D Mk2 at 28 inches wide before I detect the pixels (200dpi).


If you scale it down, once you get below 75% or 17.55" it will start to throw out pixels (down sample), reducing image quality.





In summation, in the camera pixels can be different sizes but once in an image editor pixels become a standard size.DPI determines pixel size.


You can have too many pixels. Scaling down can actually reduce image quality.





I hope this helps.

[Citronsyra]

Interesting it is I must say!





But Keith: How did you came up with the fact that the maximum print of an image is 28 inches wide for your 5D MkII?

[Keith B]

at 28 inches wide my images will be at the 200dpi point and you will start to see blockiness after that. I can obviously go as large as I want but the pixels will start to become apparent.


Although once an image gets so large, one has to be a good distance back from it to view the whole image, then the eye is more forgiving.


Like billboards, that you see on the roadside, are usually printed at 100dpi and they will have no problem interpolating those images either because the eye isn't going to detect all the fine details from 100 feet away.


Photoshop does a really good job of resampling, If your image starts out sharp you can easily double the size with out loosing much detail or at least be able to see it with the naked eye at a reasonable viewing distance.