Many people struggle with the use and transfer of the digital files obtained from their digital cameras. Most people don’t realize that the image that comes off their camera-even if it was a low-priced unit are far too large for many uses. Time, space and trouble can be saved by understanding image size and properly rightsizing your image before conveying it to its next destination.
The Two Uses of Images:
Image use falls into two basic categories: For Screen and For Print.
For Screen means the image will be ultimately output only on a computer monitor, projector, television screen or some other temporary electronic visual conveyance.
For Print means that the image will be transferred through some semi-permanent process (printed) to a piece of paper, a T-shirt, a coffee mug, or any other fixed size.
Images destined for the screen are measured in pixels. Monitors display in pixels, projectors display in pixels, televisions in lines. Each of these, regardless of size, has a limitation upon its image size imposed by the hardware of which it was built. When a video card on your computer sends an image to the screen it sends it in pixels. You may have your video card set to output in 1024 px wide by 768 px high (a very standard resolution) but your video card does not really know or care how many inches across your monitor is. It sends pixels and your monitor places them on the screen for you. It could be a 13″ monitor or a 24″ monitor but you still only get 1024×768 discreet pixels of information. A projector, similarly gets the same number of pixels but does not know how large the screen may be or how far the projector is to throw a certain size image. Regardless of the projected image size, there are still only 1024×768 pixels.
Print is a different story. Printing has to be concerned with something called resolution. Resolution is commonly expressed through software as a DPI, LPI, or PPI standing for Dots, Lines, Points/Pixels per inch respectively. In print, the number of bits of data per inch determines the quality (or lack thereof) of the image being printed. 100 PPI is less quality than 200 PPI and so on. 300 PPI is usually specified at most quality labs for making photographic prints.
The problem comes in when we take that digital photograph on our X.X megapixel camera and then try to shoehorn the file into various purposes without resizing it for the particular media in which we are placing the picture. The image above was taken with a 10 MP Camera and represents a file some 3888 px by 2592 px. It is a huge 6 MB JPG file. For comparison, I have taken the 10 MP File and overlaid file sizes of typical use in the image below:
The various image sizes compared to capture size.
The largest image represents the capture size of the Canon 40D camera at 10 MP.
- The next smaller image is a 8×12 inch print enlargement at 300 PPI
- The next smaller size is a 4×6 inch print such as you might make for your photo album at 300 PPI
- The next reduction represents a size suitable for insertion into PowerPoint or a software solution for eventual output on a monitor or projector.
- The next reduction is the approximate size of an image suitable for a web site usage or to send as an email attachment.
Email/Web: Speed is of the essence. If I am traveling on the road, I don’t want to wait for a 6 MB file to download to get to the messages behind. Chances are I am going to look at the picture, react, send you a message and delete. I only need to be able to see the picture on my screen in a preview window. If I need bigger, I can always ask for it.
When it comes to web, download speed is critical to keeping your viewer from getting bored and surfing on. Browsers display slightly less real estate than the screen so anything too big is going to spill over and make you scroll. Then there is the issue of theft and reproduction. Keep your images small and people can’t easily steal them, duplicate them and use them with any quality retention.
Your longest edge should be under 600 pixels.
Projection: PowerPoint and Keynote presentations are usually prisoners of the projector. Projectors tend to max out at about 1280×1024 pixels. PowerPoint, when optimizing for screen, will create a graphic that is roughly 1500×750 pixels. Anything else is really overkill and just using system overhead with no real gain.
1200 pixels on the long edge-larger if you are zooming or panning the image.
Prints: Prints are physical output. Here inches and points per inch rule. If the print is 4×6 inches, you are talking about a 1200×1800 pixel image. Print quality suffers in side-by-side comparisons between 200 PPI and 300 PPI. Prints at 100 PPI are visibly lower in quality even without the side-by-side comparison.
300 Pixels multiplied by the inches in your dimension for photographic quality.
Resizing your image before you use it is going to make your software run faster. Your web pages and email will download faster. Your chance of theft will be less. You will also ensure the quality you expect in your final output.
A more detailed 45 minute presentation is available on this topic and includes resizing strategies in Adobe and Corel Imaging Products. Contact me if you would like a group presentation.
Rikk Flohr © 2008