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What is the difference between image size, Pixels, resolution and dpi?

Posted on 2013-11-04
Medium Priority
Last Modified: 2013-12-06
Question is in the title. Kindly explain.
Question by:sheana11

Author Comment

ID: 39621021
Also, can you convert an image to 72 dpi and 350 x 350 pixels in Photoshop?
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Expert Comment

by:Sigurdur Armannsson
ID: 39621143
In Photoshop select the Crop tool
In the Control Panel type: 350 and 350

Crop the picture the way you want and it will  be in the proportion of 350x350.

Go into the Image Size (Alt + Command/Ctrl + I )

In the Pixel size choose 350 and 350 if it is not already so.
LVL 54

Expert Comment

by:Scott Fell, EE MVE
ID: 39621223
The size refers  to the physical print size of the image, the resolution is the vertical and horizontal size in pixels and the dpi refers to how many dots fill up the space.

If you scan in an 8 X 10 at 300 dpi you end up with a resolution of  2400 X 3000 and that image will be over 20meg.  If you scan the same image that remains at 8 X 10 but this time change your settings to 72 dpi, the resolution will be 576 X 720 and the file size will be just over a meg.

Is there something specific you are trying to figure out?

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Expert Comment

by:Scott Fell, EE MVE
ID: 39621236
> 72 dpi and 350 x 350 pixels in Photoshop?
Yes you can assuming your image was already square.  If you are using photoshop and wanting to create an image for the web, you will want to use the "Save For Web" option.  Click File > Save For Web & Devices.   You can play with changing the quality of the image to get the lowest file size and still keeping an acceptable image.  This will drastically reduce the file size.  You can also use online services like http://www.smushit.com that will remove data bits reducing the file size without harming the screen output.

Author Comment

ID: 39629261
Padas, how did you calculate the resolution for your example image? Also, I am trying to understand the difference between size, pixels, dots per inch, and resolution. I also need to know where you can adjust these options.
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Accepted Solution

Scott Fell,  EE MVE earned 2000 total points
ID: 39630018
>How did you calculate the resolution for your example image?

8 inches by 10 inches was my sample.  If you have 300 dpi (dots per inch) then 8 X 300 = 2400 and 10 X 300 =3000 so  an 8" by 10" @ 300dpi yields a resolution of  2400 X 3000 (pixels).

>trying to understand the difference between size, pixels, dots per inch, and resolution

Let's say you have a square that is 100 pixels by 100 pixels.  This has a resolution of 100 X 100.  The resolution is the size.    The dots per inch refers to how detailed the image is.  Think of an impressionist painting.  If you can make your painting by only touching the canvas 72 times vs 300 in the same area, which painting is going to have more detail?  

>where you can adjust these options.
In your scan software or photo imaging software such as photoshop or gimp.

Author Closing Comment

ID: 39632518
Thank you, Padas, for your detailed explanation and help. Your answer is the reason I'm a member of Experts-Exchange! Thanks again.
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Expert Comment

by:Scott Fell, EE MVE
ID: 39632590
Thank you for the kind words.
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Expert Comment

ID: 39670967
Saw this question mentioned in the EE newsletter.  Even though an answer has already been accepted I thought I could add some relevant detail.

There is some variation is how and when some of the terms are used.
DPI is generally referred to in association with a print.
PPI is generally referred to in association with on-screen display.
Physically a pixel is the smallest dot that can be shown on a monitor, but it can also refer to the dots of which an image is composed.  Most monitors have a PPI of about 96 (in the past Apple monitors had a PPI of 72, but this isn't the case anymore), so in Photoshop I would always adjust it to use 96.  If using Windows, you can adjust the PPI (in Win7 it is referred to as "adjust custom text size (DPI)"), using presets or comparing an on-screen ruler to a physical one and adjusting them to match.  If you had Photoshop use this PPI, then an object measuring 5 inches and displayed at 100% resolution/zoom would be exactly 5 inches as you're looking at it.  Without changing that setting you will likely see some slight variation.  Windows doesn't always deal with custom PPI settings well, so even if it's not perfectly accurate, it may well be your best choice to keep the default setting (I don't know about Macs).

When it comes to a print, the resolution and DPI settings of an image are combined to get a result.  Reversing an example that padas gave - you could have an image that is 2400 x 3000 pixels (these are the image pixels, the physical pixels of the monitor used to display the image could be more or less depending on zoom settings, etc.).  2400 x 3000 pixels can also be called the image's resolution.  Now depending on DPI that image's size could be very different.  Without resampling (changing the resolution of) the image in Photoshop, you could set that 2400 x 3000 image to have a DPI of 300, in which case it would have a size of 8"x10".  If you set it to have a DPI of 200, it would have a size of 12"x15".  In both cases, the image file has exactly the same pixel information.

You may ask, "what is the right DPI to use?"  The answer varies, but generally you'll be fine with 300.  Too much above and the extra information goes to waste, too much below and the print quality will suffer due to not enough information.  There are various techniques to resample an image (either increasing or decreasing the number of pixels) but there are limits as too how much this can be done and still maintain good information, as essentially this resampling is sophisticated guessing.  Also, as I recall, certain inkjet printers may have there own native DPI, which would be the optimum DPI setting to use in images that are submitted to them - however this gets into such nitty-gritty that you're unlikely to see any difference between a print submitted at 300 DPI vs. one submitted at the printer's native DPI.

Last it may be worth noting that the DPI setting for an image is different than the dots that a printer makes when actually printing an image.  Using inkjet technology as an example, the size of the ink droplets sprayed onto the paper is many times smaller.
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Expert Comment

ID: 39702758
Saying DPI is resolution is the same like saying that meter is the length. Length is dimension (physical property), and meter is a physical unit.

Please first look at the answer footech gave for the same question and correct this article.

There are: actual size (in pixels), print size (in inches or cm), print resolution (used always when we deal with physical size, e.g. also when scanning; in PPI), display resolution (property of a display in PPI; but be aware that we can use projector instead of monitor).

Another problem arises when we see that pictures are composed of pixels (picture elements :-)), and printer make colored pixels combining many tiny dots. In the past unit DPI was used when scanning although we make pixels and not dots and PPI is correct :-(. One good resource for clarifying these concepts is http://99designs.com/designer-blog/2013/02/26/ppi-vs-dpi-whats-the-difference/

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