A Quick, Simple Guide to Image Management

A long long time ago, say, 15 years, when an advertiser asked you for a photo for an ad, you simply sent them a photo or a slide and you were done with it. Nobody asked for a high res or low res image and you certainly weren't concerned about pixels or DPI (dots per inch). These days, each photography request comes with a set of specifications. While it may sometimes seem that advertising production folks are simply entertaining themselves at your expense, there's a reason behind this, and it's a fairly simple one to understand.


A pixel is just a dot, and it can be any one of millions of colors. When you put hundreds (or thousands) of pixels together like microscopic puzzle pieces, you get a picture. When you hold the picture in front of you, it's just a picture. But if you use a powerful microscope to look closely at the picture, you can see all the individual dots.

Guess what else uses pixels? Your computer screen (and your television screen). Why was HD such a big deal when it first came out? Because an HD television uses way more pixels in every image, making those images clearer, crisper, and more life-like.

If you have a fairly new monitor, it probably has a resolution of 1920 X 1080.  All that means is that it will display 1,920 pixels left to right, and 1,080 pixels top to bottom.

So, does Resolution = Pixels? Not quite.  Resolution also includes the size of the pixels. If you think about older computer monitors, they weren't just square and/or smaller than today's monitors, their resolution was also lower. As you can see in the following image, these two monitors are the same number of inches from top to bottom. Yet the older (square) monitor displays less than half the number of pixels. The new monitor uses pixels that are much tinier, so you can pack in more pixels per inch. This makes for a much deeper, richer use of color and detail. This is why higher resolution monitors give you better images.

resolution example 


Once you understand how many pixels are in the monitors most people are using, you can start thinking about how your images will display on most monitors. Let's play around with this idea. Let's assume most people are using modern monitors, with an average monitor size of 1,920 Wide X 1,080 High.

400 image on 1900 screen
So if you have an image that is 400 Pixels Wide X 284 Pixels High (usually shown as 400X284), it will look like this on your monitor.

Now look at the monitor in our first example - the old square style. If you do the math you'll see that this 284 Pixel High image would take up more than half of that screen from top to bottom.



800 image on 1900 screen 
If you have an image that is 800 Wide X 568 High, it will look something like this. 



1920 image on 1900 screen
And if you have an image that is 1920X1362, it will look like this


As you can see, pixel display is really just a matter of math. Pixels can be large or small. You can cram more small pixels into the relatively small space of a higher resolution monitor, and more is better, because more means more color, more definition, more shadows, more light. 

Dots per Inch

Dots Per Inch (DPI) come into play when you decide to print something. It's really just another math problem. You see, the "pixels" in your digital images become "dots" at the printer. If you are running an ad in the newspaper, it may print at 200 Dots Per Inch. This means that your 400X284 picture will be about 2" X 1.5" (400 dots divided by 200 dots per inch, and 284 dots divided by 200 dots per inch). But what if you are running the same picture in a magazine? Most magazines print at 300 DPI. So your 400X284 image will be smaller, because the magazine will use more of those dots per inch. The final image result will be 1.3" X .9" (400 dots divided by 300 dots per inch, and 284 dots divided by 300 dots per inch). Here's a handy little reference for common print DPIs.

Very high quality prints 600 DPI
"A" quality book, magazine, calendar 300 DPI
"B" quality book, magazine, calendar 266 DPI
Newsprint 170 DPI
Billboard 20 DPI

So if you know how many pixels in your image, and you know the DPI of the print job, you can calculate how large your image will be.

Why do you think Billboards can get away with having such a low DPI? It's a matter of distance. Remember when we said earlier that if you used a microscope, you could actually see the dots in a digital photograph? The closer you are, the more obvious the dots become. You will always see a Billboard from a distance. If you get very close to one, it will look more like this:

Favorite iStock pixillated

Seems really easy. What's the catch?

Here's the scenario. An editor asks you to send an image. You look in your folders, and you see that you have an image that is 800 X 1200. Seems large enough, right? So you send it, and the editor says it's not "high res" enough. The problem may be that the image was originally saved on a lower pixel setting. Here's a peek a some of the options for saving an image in Photoshop:

Image Sizing in Photoshop

As you can see, the person processing the image can choose how many pixels per inch to pack it with. If that person set the size at 72 Pixels/Inch, the 72 Pixels/Inch are hard coded. This would be suitable for most websites, but not for print. Why would anyone want a 72 pixels/inch photo? Because nobody likes a website that loads slowly, so website developers set images at the lowest resolution possible to encourage the page to load faster. When you download images from the internet, they will typicaly be low resolution for fast-loading. If you have a photography program like Adobe Photoshop, you can resample the picture to add more pixels. Basically, you tell the software to put more dots back into the image. If you want to take a postage-stamp-sized image and make it large enough for a trade show banner, that may not work, because the image can become distorted. But most of the time you can increase both the size (inches) and the quality (pixels or dots per inch) with resampling by using a program like Photoshop.

So the next time you're asked to send a photograph, feel free to ask "where will this be run" and "what quality should I send?" You're now fully prepared to participate in your (photographic) image management.

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