# When people ask "what's the DPI?"



## vv0nderboy (Oct 10, 2016)

First time participant here. Sorry if I'm in the wrong area.

I have a Canon EOS Rebel T3i that I have set up with a professional machine to capture old slides. Sometimes I get asked what's the DPI that I can "scan" the image at, yet since I don't really scan it this one stumps me. Is there a way I can associate the 18MP or the 5184x3456 resolution to a standard DPI for printing a 3x5 or similar print or what DPI it would be equivalent to?

What would be the best way to explain this to a customer without scaring them off that we don't "scan" the slides?

Maybe a better question, if I took a picture at the 18MP quality with the T3i, what DPI would it compare to if I had scanned it with a scanner?


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## Ysarex (Oct 10, 2016)

vv0nderboy said:


> First time participant here. Sorry if I'm in the wrong area.
> 
> I have a Canon EOS Rebel T3i that I have set up with a professional machine to capture old slides. Sometimes I get asked what's the DPI that I can "scan" the image at, yet since I don't really scan it this one stumps me. Is there a way I can associate the 18MP or the 5184x3456 resolution to a standard DPI for printing a 3x5 or similar print or what DPI it would be equivalent to?



No. But you do have a PPI figure since you know you're recording at 5184 x 3456. PPI is pixels per inch. Your digital images have a PPI value. They do not have a DPI value. PPI and DPI are often confused but they are in fact different and not interchangeable (doesn't stop the confused). Printers print Dots (DPI). You image is made up of Pixels (PPI).

If you display one of your images so that the horizontal side is 1 inch then the PPI is 5184. If you display that same image so that the horizontal side is 2 inches then the PPI is 2592. If you display that same image so that the horizontal side is 3 inches then the PPI is 1728 -- and so forth. Displayed with the horizontal side at 10 inches (8 x 10) the PPI is 518.

Joe



vv0nderboy said:


> What would be the best way to explain this to a customer without scaring them off that we don't "scan" the slides?


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## robbins.photo (Oct 10, 2016)

Danish Police Investigators.


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## KmH (Oct 10, 2016)

Print size is a function of image resolution (5184 x 3456 pixels) and print resolution (pixels per inch).
You can do some basic math:
*Image resolution / print size = print resolution*
5184 pixels / 5 inches = 1036.8 pixels per inch (PPI)
3456 pixels/ 3 inches = *1152 ppi
Notice the problem?*
A 3 x 5 print has a different aspect ratio(shape) than a photo that has pixel dimensions (image resolution) of 5184 x 3456 pixels.
5184 x 3456 is a 3:2 aspect ratio, not the 3:5 aspect ratio a 3 x 5 print has.

Using a print resolution of  1036.8 ppi you will wind up with a 5 x 3.33 inch print unless you crop enough off the long side of the photo to make it 1.67x the short side (5/3=1.67) so you get a 3 x 5 print.
If you use the print resolution of 1152 ppi you get a 4.5 x 3 inch print.

Using some basic algebra we  derive 1 more useful, but simple equations:
*Image resolution / print resolution = print size*
5184 px / 1036.8 ppi = 5 inches
*Print size x print resolution = image resolution (pixels)*
5 inches x 1036.8 ppi = 5184 pixels


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## TCampbell (Oct 11, 2016)

DPI and PPI seem to confuse a surprising number of people.  But what really surprises me is how the topic comes up... it seems like I'll usually hear someone ask this question because a "publisher" told a "photographer" that they need an image at, say... 300 dpi.

Immediately two things go through my head which is (1) why is a publisher even asking this question and (2) do they not know Photoshop well enough to realize that they, the publisher, are in control of this setting and they can use Photoshop to set any DPI value they want?

Both DPI and PPI only makes sense when you are trying to display the final product... either "print" the image in a magazine, newspaper (what's a newspaper?), etc. or even to print it for purposes of mounting and displaying it on a wall.  PPI assumes it'll be digitally displayed (on a screen).

Screens have resolution AND they have a physical size.  That means that there is a specific number of pixels per inch of screen size.

Camera images have resolution, but the images are stored in a file on a memory card.  The concept of a "physical" size doesn't make sense while it's stored.  You could print that image as a 4x6" ... or a 16x24" print.  You could display it on a phone or display it on a giant monitor.  All of these possible display formats have their own physical dimensions and the image can be displayed on any of them.  That means the image doesn't get a true DPI or PPI until you select a specific display output and decide what the true physical dimensions of the image will be.  

At that point the display has a maximum possible resolution.  

You don't have to use the maximum possible resolution... you could re-sample the image (change it's resolution) to fit a different size.

Basically image in the camera doesn't really have a PPI or DPI _*until*_ you try to apply the image to some output.  HOWEVER... to add to the confusion, EXIF data will often include a PPI or DPI value even though strictly speaking it doesn't make sense because the image wasn't necessarily created for one and only one display medium and the real DPI or PPI will change _every time you change the display medium._


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## f64_or_bust (Nov 2, 2016)

TCampbell said:


> DPI and PPI seem to confuse a surprising number of people.  But what really surprises me is how the topic comes up... it seems like I'll usually hear someone ask this question because a "publisher" told a "photographer" that they need an image at, say... 300 dpi.
> 
> Immediately two things go through my head which is (1) why is a publisher even asking this question and (2) do they not know Photoshop well enough to realize that they, the publisher, are in control of this setting and they can use Photoshop to set any DPI value they want?
> 
> Both DPI and PPI only makes sense when you are trying to display the final product... either "print" the image in a magazine, newspaper (what's a newspaper?), etc. or even to print it for purposes of mounting and displaying it on a wall.  PPI assumes it'll be digitally displayed (on a screen).



A publisher may not make the DPI statement in the right context, but a printer would.  DPI will make sense in a finished printed piece.  They will typically request all images at 300dpi minimum, and mean it.  And no, they are not in control of this setting if they get a finished layout from Quark or InDesign, or a PDF from either.   They get what they get, and will not attempt to change anything in the document...its like a rule of the printing industry, "we only print what you send us."  BTW, that means we try to predict what the press will do to our greyscale too, and pre-compensate.  (see Dot Gain) 

Offset printing is very different from digital inkjet or laser printing, or fine art printing.  Because offset printing uses a halftone screen at 45 degrees, with a variable dot size, hitting that process with an image below a certain finished dpi creates issues.  The rule (with the printing company we use) is 300dpi, finished size.  We work in QuarkXpress, our printer uses a digital plate maker.  So, if we start with a 3000px wide image, but it has to fit in a 4" wide box on the page, we resize it down 300dpi.  There are two reasons, sort of  working against each other.   First is, at at the finished screen pitch the printer uses, 300dpi works and causes no issues.  Below that we can get moire patterns, and it's difficult to predict in the page-layout stage.  Going higher than 300 causes no problems for the printer, but it does cause problems for us.  Depending on Quark to handle all the resizing results in clumsy and huge Quark documents.  While the days of Quark not resizing photos well are over (resizing in Photoshop was the rule back then, Quark made a mess of things), we still pre-size in PS before import because the photo and files are just smaller and faster to handle.  There are also a few steps of pre-press photo processing to go through too, so that happens at that point as well.  

We sometimes get very low resolution .jpg files that we have to print.  They actually get resized up to end up at 300dpi too, again, avoiding interference patterns with the printer's screen.  The photo resolution is not improved, of course, but at least one problem is avoided.  And we do line-art at 1200dpi, bitmaps, which is absolutely necessary for crisp, smooth line art (no screen), but completely unnecessary for photos.


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## KmH (Nov 2, 2016)

DPI (dots per inch) refers to the _*output *resolution of a printer_ or image-setter, and PPI (pixels per inch) refers to the _*input* resolution of a photograph or image._ 
DPI, PPI, and LPI are not interchangeable values and today yet many use the term DPI when they are in fact discussing PPI or LPI.

But this is the age of the Internet where making sure one uses those term accurately seems to not be a concern.

Up to a point, inkjet printers having more dpi produce clearer and more detailed prints.
How many dots an inkjet printer head produces depends on the type of print head it is, but a printer does not necessarily have a single DPI value available, because the the print mode and driver settings can change the dpi value the printer uses.

It takes multiple dots to print a single pixel, which is why output device dpi values are always much larger than the print resolution (PPI) that with the image resolution (pixel dimensions) determines print size. Note to that while digital images use an additive RGB color model, printing uses the subtractive CMYK color model. In other words when you combine the maximum value of red, green, and blue light together you get white light. When you combine the maximum value of Cyan, Magenta, and Yellow you get Black. But inkjet printers combining just CYM don't make a very black, black so they have black ink to make the CMY black blacker.

As it is, human eyes cannot detect any appreciable difference in print quality at a print resolution greater than about 350 ppi, regardless the dpi the print device uses.

Next we have to consider that not all prints of photographs are made using an inkjet printer.
A common print type because it is inexpensive is the Chromogenic print that uses light sensitive paper. The paper has 3 layers of emulsion, a cyan layer, a magenta layer and a yellow layer.
Another type of printing is halftone priniting.
Halftone print resolution is in lines per inch values.
A 600 dpi laser printer typically has a halftone resolution of between 85 and 105 lpi.
Images on newspaper are typically printed at 85 lpi, while the coated paper used in many magazines is often printed at up to 185 lpi.


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