Can someone explain picture dpi, when not talking about printing, to me because surely the ‘dpi’ of a picture is irrelevant on a computer, because everything is the dpi of the screen. the resolution.
thanks
That is pretty much it. Screens display in the 60-75 dpi range. If you are not printing (for example in photoshop) the dpi setting doesn’t really matter.
It is something I have to pay attention to when I scan maps to digitize datums off them. I need to have the dpi dense enough that I can tell what’s what on screen. And it can be a concern when file size is critical - say you’re going to attach a picture to an email.
If you are discussing dpi of a monitor, it will make a difference if you use AutoCad or some other software that uses small fonts.
James
So. DPI is important when transfering to and from the computer from a physical format. but once IN the computer, the DPI is completely irrelevant.
The only thing that is important is image size. If I take a picture with my camera set at 1 megapixel, and a picture with my camera set at 6, and then transfer them both to the comp, they will both be exactly the same dpi, but the 6megapixel one is huge (more than twice the size of the 1280x1024 screen)
so the answer to this thread should be - “You can’t. unless you are printing it”. but it isn’t - apparently you can change the dpi. why?
It is confusing. the guys pictures are going to the internet. now in my understanding - the dpi of the picture will depend completely on the computer which is viewing that webpage, and cannot be ‘set’.
DPI is important when you are not printing because it dictates the size of the file. A picture with a large number of DPI will look no better than a picture with around 72 DPI on the computer screen, but it will take a long time to download because there is more information contained in the file. So if you are using pictures for an internet page then you may as well use the lowest DPI.
Image files can contain the resolution information in the header. So if you have a 900x600 pixel image and the header says it’s 150 psi, software are supposed to print it at 150psi, resulting in a 6x4 inch printout. Of course, you can use a utility to change the resolution flag, or force the printing program to print at a different size.
It’s good to know the DPI of images that will eventually represent any physical product, such as photographs or holograms.
The picture width (measured in dots) divided by the screen resolution (dpi) equals the width (in inches) of the picture on-screen. That size can be changed by enlarging or reducing the pic before display, but doing so takes computation.
Thus if you’ve scanned a breadbox at 300 dpi, it’ll appear roughly 4.2 times (300/72) larger than life when you display it on your monitor. If you scan the same breadbox at 4800 dpi, it’s image on the screen will be enlarged 67X. The dpi value of a picture gives you an idea how much work your computer must do to present the picture at 1:1 with the original object.
The DPI for SVGA is 96dpi.
I’m sorry, but this is just not accurate. Web browsers couldn’t possibly care less about the DPI of an image. All they care about is the actual pixel dimensions of the file. The pixel dimensions are what dictate the file size (along with compression level), the DPI only specifies the size of the physical printout. If you have a 640x480 image at 72 dpi and a 640x480 image at 300 dpi, they will look identical in a web browser and the files will be the same size. One pixel in the file is generally mapped to one pixel on your screen. The only exceptions would be if you were using software that magnifies the image or compensates for print size specifically (accesbility software and paint or design software are examples). If you are using pictures for a web page, the DPI is completely irrelevant.
Think of it this way: DPI relates the pixel dimension to the print size. It’s an equation with 3 parts.
(pixel dimension) / DPI = (print size)
So a 640x480 image at 72 dpi will print at about 8.89"x6.67" while a 640x480 image at 300 dpi will print at about 2.13"x1.6". However, the files sizes will be the same and they will display onscreen at the same size in a web browser.
Like any 3 part equation, you must know 2 of the values to calculate the third. Knowing only the DPI of an image is nearly uselss. The file could be any size at all! You also need to know either the print size or the pixel dimension to really know anything about the file. This is a very important point, and one that seems to create a lot of confusion. I’ve heard fellow graphics people say something like, “We can’t use any digital camera photos for our products, because they are all 72dpi.” But they’ve forgotten that the print size also matters. If you’re going to be printing the photo at 3"x5", then you only need 900x1500 pixels to achieve 300 dpi on your printout. If you’re making a poster that’s going to be 4’x6’, then you’re going to need a hefty 21,600x14,400 pixels to get to 300 dpi. That’s a world of difference in file size, but either way you’re using a 300 dpi file. It’s not all that important what dpi your file is, you can re-set it (with programs like Photoshop and Paint Shop Pro) to whatever you want, but you have to have enough raw pixels work with.
I am confused. How can this be true. I can display SVGA and many different sized monitors. Surely the DPI changes with each monitor.
That is a pretty arbitrary statement.
Consider that:
- Most monitors will display more than one SVGA resolution.
- and most of the SVGA resolutions will display on monitors of many sizes.
- SVGA is “technically” 800x600 but generally considered to be anything over 640x480.
My monitor has a viewable horizontal width from 16 inches.
at 800x600 I am displaying 50 dpi (the technical SVGA standard resolution).
1024x768 - 64 dpi
1280x1024 - 80 dpi
1600x1200 - (my standard resolution) 100 dpi.
To display 800x600 at 96 dpi would require a monitor with a viewable horizonal size of 8 and 1/3 inch.
Lobsang said:
Basically, you can change the DPI so that the file will printout at the size you want. The file has an internal tag that includes the dpi setting. That way, you can send a file to someone, or copy it to a new computer, and it will retain knowledge of it’s internal dpi setting. So, you can set the dpi at anytime, but it isn’t relevant untilyou print it. I’d say the answer to that thread is ‘You don’t need to.’ Sivalensis’s employer should’ve specified a maximum pixel dimension for the files, not a DPI number.
ftg’s response in that thread was exactly right. This is an all too common error.
This is another instance of the missing bit of info, namely: What size monitor? The standard SVGA setting of 800x600 would only be 96dpi on a monitor with about 10.5" diagonal screen size. A pretty small monitor by today’s standards (yesterday’s too! :)) Remember, a DPI number relates two other things and you have to know one of them to figure out the other. Imagine if you asked someone how long a particular drive is and they said, “65 mph.” Well, that didn’t tell you much, just like the DPI number alone doesn’t tell you much. However, If they told you that the trip took 1 hour and they were driving 65 mph and you would then be able to figure out that the trip was 65 miles.
Thanks Enigma42
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And everyone else.