Deciphering Digital Images
|One of the major complaints that come this time of year is the fact that people are sending digital camera pictures that are huge, take forever to download, and tie up the internet line for hours. One reason is that we tend to set the camera to capture the best image it can, then forget that the resulting picture if you have a 10 plus megapixel camera can be quite large. So the question comes up, what size is that picture I just took. Well, it depends and unfortunately, is not often an easy question to answer. The first thing to understand is that a digital camera captures the image in pixels without regard to the physical size of the picture you just took. All it cares about is the number of pixels. My camera for instance, captures pictures at 2560 x 1920 pixels. Multiplied together, you get 4915200 total pixels or about 5 megapixels which is what my camera is. If you use a jpg image format for the pictures, they take up between 1.6 and 2.5mb of space on the disk. Don’t ask about a tif image, it can be 10 times bigger because this is all depending on compression used by the camera. What you end up with though is that each of those pictures will still be 2560 x 1920 pixels.
Now here is where some of the confusion comes in is in the question, "what size is that picture?". And it all depends on your image editing software package. When I use Paint Shop Pro from Corel, www.corel.com, to edit one of my pictures, it tells me that the picture is 35.6 x 26.7 inches in size! I would need a mural for that. But if you look closely at the details, PSP is assuming that this picture is only 72 dots per inch. If that same picture is 300 dpi, then the picture size becomes 8.5 x 6.4 inches. All again, keeping the same number of pixels that I originally captured. It is the image editing software that starts this confusion by introducing the dots per inch (DPI) into the equation but we need to consider dpi when preparing the image for the web, the printer, or just looking at the picture.
For instance, say you are using a 17 inch monitor set to 1024x768 screen resolution. Look at Control Panel's "Appearance and Themes" or Display Properties if you don't know your screen resolution. With about 14 inches of glass showing horizontally, that gives you (1024/14) 73 pixels per inch or dpi. That is why most software packages assume you are just going to look at the picture on your screen and so give you 72dpi and such large images. So what does this have to do with my picture that I want to print out at 4x6 inches? Lots. Take that 6 inches and multiply it by 72 pixels per inch and that gives you 432 pixels across by 288. That is all the pixels you need to view that huge picture on your monitor. 432x288. The picture will look great, and if you email it to someone, they can see it at 4x6 inches on their screen as well and it will only take up around 126k in size versus the original 2.4mb. That you can email to someone still stuck in the dark ages of dial up internet service.
To do that, use your image editing program to resize the picture. You probably need to make two changes, one is the dots per inch (DPI) setting, make it 72. The other is to change the image size to 4x6 inches. This will give you your image for emailing.
(Tip - for a really easy to use quick image resizing program, look at a Microsoft Power Toy called "Image Resizer".)
For web sites, I often use 90 dpi as more and more people are getting larger monitors but you go through the same process of changing both the dpi and image size. A 90 dpi 4x6 picture is only 540x405 pixels, still looks great, still is small enough for emailing and web usage at 156k in size. My personal preference for web pictures is about 3.5 inches wide especially if you will have a lot of text around them. That puts the picture under 100k in size. Try this with your own image editing software and see what you come up with.
|So what you see here is the 3.5 inch 100k picture that is 315x236 pixels in size at 90dpi. Don't pay attention to the contents of the picture, just the size of it.|
Now to print that picture, you need a better dpi resolution out of it and from the help files I have read on printers, 200 to 300 dpi is about all you need. Doesn’t matter if the printer says it is a 1440dpi printer or not, what you send to a printer and what it says it is rated at unfortunately, have nothing to do with each other. So, resize that picture at 300 dots per inch and use the same 4x6 inch format, that picture now becomes 1800x1350 pixels. In my case, the file becomes 750k in size. Because dpi is more of a matter of what the software thinks it is, if I take that 1800x1350 pixel size picture and send it to another program, guess what might happen. It might think that you have a 72dpi picture that is 25x18.7 inches in size !
What you need to remember is that it’s the number of pixels that matters at all. What makes it more frustrating is that now that you may have the picture resized just right for printing, your printing software may forget all that you did because it looks at the pixels differently. What you have to remember is that after you have resized it to 4x6 inches, when you send it to the printer, be sure to make sure that the printer knows that it is 4x6 inches in size.
Here are a couple of articles from the archives that will give you more information on digital cameras and the problems if dealing with sizes and pixels.
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