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understanding DPI
joshhuntnm
Registered Users Posts: 1,924 Major grins
First, I think I understand essentially what DPI is. Here is what I don't understand. I took a picture Canon 60D in RAW full resolution. Edited in Lightroom, then sent to PS for further edits. Hit save for web and device, created a full resolution .jpg.
The graphic guy emails back and says he needs this photo at 300 dpi. I go back into PS and check that setting. Sure enough, in PS it says 240 DPI. I changed that to 300, hit save for web and device again; we will see if he is happy.
here is what I don't understand. It seems DPI is simply a function of the resolution of the camera and how big you are printing. In this case, the original image was 3456 X 5184. I thought this meant you could print at 11.52 by 17.28 print at 300 dpi. (3456/300 and 5184/300). If you wanted bigger, you would have to print at lower dpi, given the original megapixals of the camera.
when I changed the dpi setting in PS, it moved that 3456 X 5184 number up, but is that really accomplishing anything?
The graphic guy emails back and says he needs this photo at 300 dpi. I go back into PS and check that setting. Sure enough, in PS it says 240 DPI. I changed that to 300, hit save for web and device again; we will see if he is happy.
here is what I don't understand. It seems DPI is simply a function of the resolution of the camera and how big you are printing. In this case, the original image was 3456 X 5184. I thought this meant you could print at 11.52 by 17.28 print at 300 dpi. (3456/300 and 5184/300). If you wanted bigger, you would have to print at lower dpi, given the original megapixals of the camera.
when I changed the dpi setting in PS, it moved that 3456 X 5184 number up, but is that really accomplishing anything?
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The other possibility is to resample (add or subtract) pixels. You could in theory need output from a 2000x3000 pixel file that is 3x6 at 1000dpi. Then you would have to add pixels, resample. No free lunch here. Its not the same as having a capture device that produced a true 3000x6000 pixel file in the first place but sometimes you do need to sample up and often sample down.
Author "Color Management for Photographers"
http://www.digitaldog.net/
that is what I am thinking. I think the graphic guy I am working with is confusing dimensions with the mg of the file size. It is 90% jpg compressed. it is an 18 megapixal camera, but this jpg compressed file is only 4, which sounds about right to me as the subject has a plane white background, which, as I understand it, jpg compression is able to compress a lot.
Las Cruces Photographer / Las Cruces Wedding Photographer
Other site
So assuming the resolution tag is 240 and he wants 300, all he has to do is go into Photoshop’s Image Size dialog, turn OFF Resample and set the field for 300 instead of 240. Photoshop will not add or subtract pixels, it will simply update the math used to set a “size” upon output assuming 300 vs. 240 with the pixel dimensions of the document.
Author "Color Management for Photographers"
http://www.digitaldog.net/
Moderator of the Cameras and Accessories forums
but converting from 240 dpi to 300 dpi doesn't really improve the quality of the output, does it? I mean, isn't this just stretching the picture?
I think my guy is confusing mg size with dimensions
Las Cruces Photographer / Las Cruces Wedding Photographer
Other site
If you cause the graphics person more work then you risk not getting the best from them.
"dpi" is a somewhat arbitrary term in photographic usage but in the publishing world it can mean something important. 240dpi submissions, or imagine a myriad other possibilities like 180dpi or 72dpi, to a publisher who has some document automation set for 300dpi can lead to a significant problem depending upon the volume of work involved.
If the graphics person is asking for 300dpi specifically there may be a very good reason for the request. If they are asking for a 1.5" x 1.5" @ 300dpi that means they already have a frame set up and ready for your image to drop in at exactly those settings. If you supply a different dpi then they may need to scale your image with the potential for an error or at least extra manual work for them.
I agree that it should not be this way; printer/publishers really should only be interested in dimensional pixels. The fact remains that many publishers do still have workflows that rely to some extent upon dpi/ppi, like it or not. I always found it best to just send them what they wanted since it was pretty trivial for me to do it and they would, in turn, tend to pay attention to the more important things about using my images and cooperate to my benefit.
This may help to explain part of the problem:
http://www.rideau-info.com/photos/printshop.html
http://blogs.mathworks.com/steve/2006/03/03/help-my-publisher-wants-a-300-dpi-tiff/
Moderator of the Cameras and Accessories forums
The term you need to use is pixels, or pixels per inch. A digital file is captured in camera with a certain number of pixels on each side of the image, and the user can then decide whether they are expressed in pixels per inch, or pixels per centimeter, or just plain pixels. There is no need to discuss pixels per inch, unless you are determining whether a digital file has enough data to print an image of a specific size at a specific level of quality.
When printers demand a file in 300 ppi, they are merely stating that they do not either want or need to alter the file to meet their printing needs. Like Ziggy said, they just want to drop the file into a preplanned format for their output. Specifying 300 ppi, at a specific size, means that the image does not need resizing or altering for their printing. ( Or maybe they do not understand how to alter image size in Photoshop, or are too busy to do that ....)
I wrote about pixels, ppi and dpi here - http://pathfinder.smugmug.com/Other/Resolution-Resizing-and-Dots/2246604_CgPjC
Moderator of the Technique Forum and Finishing School on Dgrin
This was exactly what I was thinking when my guy asked me to send a jpg in 300 dpi. A jpg can be in what ever dpi the printer prints it at
Las Cruces Photographer / Las Cruces Wedding Photographer
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Theoretically, you can print a 1 single pixel file at 1440 dots per inch ( dots per inch being how many dots of ink there are per inch on paper) - but it would just look like a single tiny dot on paper.
Like Ziggy said, winning an argument with the printer and correcting his usage of dpi, may be counter productive. You some times have to know when to fold'em, and give him the file he wants and needs, ( and not the file he actually asked for .) After all, you have NO CONTROL over the dots per inch of the file he will print for you - that is a function of the printer hardware and software, and has nothing to do with your digital file. But you can provide a digital file at 300 ppi, and at the image size he needs at 300ppi. The printer will then be happy ( none the wiser ) , and you will be happy too.
Moderator of the Technique Forum and Finishing School on Dgrin
Going back to what arodney said...it will stretch the photo if Resample Image is turned on, it will not stretch the photo - only change the size of the existing pixels - if Resample Image is turned off.
I think the printer meant to say 300 pixels per inch...at the desired inch dimensions of the print. Both pieces of information. But too many printers leave out the second part. Like not mentioning resampling, it is forgotten that not mentioning dimensions can change the nature of the image completely.
If the printer had asked for ppi plus dimensions, it removes the ambiguity of whether resampling was meant to be on or off. By just saying "300 ppi" leaves open the possibility that the customer will change the ppi in Photoshop to 300 without resampling, and call it a day, even though without resampling, what dimensions that 300ppi are matched up to depends on how many megapixels the original image had. Could be too little, could be too much. But "Give me an 8x10 at 300 ppi" is very clear about what has to happen to the image: If exactly 300 ppi is required, and changing dpi alone does not match up with 300 ppi, then resampling must be applied too.
Las Cruces Photographer / Las Cruces Wedding Photographer
Other site
I received a Canon Powershot SX-30 IS (14 MP) for Mother's Day because I wanted a high quality p&s camera to use when I don't feel like lugging around my dSLR and lenses. The pixel dimensions are in the 4000 x 3000 -ish range, but the dpi is 180. I have an older 5 MP Kodak and those images come off the camera at 230 dpi. I was feeling mightily disappointed with image quality on my new camera because I expected so much better from Canon until I read this post!
Thanks again!
Sherry
Yes, our print requirements have us set the photos to 300 dpi, but that's not something the contributor needs to be concerned about (every file we get in gets run through Photoshop anyhow to convert it to CMYK and embed our print color profile, so we change the dpi during that process if we need to). If someone thinks "the magazine needs an image at 300 dpi so I'm going to scan this slide at 300 dpi" the result will be useless (unless we only run it 1.5 inches across). We tell people "We don't care what dpi you use -- just give us a file that's 3000 pixels along the long dimension." That's the most foolproof way to give us an image that will be usable 99% of the time.
The Railroad Photographer
www.railroadphotographer.com
Moderator of the Technique Forum and Finishing School on Dgrin
ppi and dpi are not related any way when speaking of an image file. :bash
dpi is a function of ONLY the printer and has no meaning with regard to an image file. :bash
Image file size (MB) has no bearing on ppi. :bash
Pathfinder and other have explained extremely well. :bash
RAW, .jpg, tiff, etc are file formats and have no bearing on ppi. :bash
Total pixel count vertically and horizontally divided by ppi will provide the image size in inches. :bash Altering ether the total pixels or the ppi will provide different results. :bash
Ok...............I will chill now......need a nice quite place, bottle of Scotch, a bag of ice, a valum or two..........................
Sam
DPI is used to describe how printers put dots of ink on paper, and has nothing to do with an image file. To talk about dpi in an image file, is nonsensical.
Images are composed of a finite number of pixels, horizontally and vertically. When you know how many pixels you have, you can compute a pixels per inch needed for a specific image size by at a specific pixel density -
An inkjet printer typically puts 5-10 or more dots per pixel ( from the digital file ) on paper.
Moderator of the Technique Forum and Finishing School on Dgrin