How to calculate proper image resolution.

This email question comes from Luke, an architecture student, who intends to make posters of individual images taken with his digital camera.

"On Jan 4, 2009, at 10:39 PM, Luke wrote:

Hey Andy, ...I have a lot of images on my flash drive that I want to make into individual poster images. What resolution should I use for each finished poster image?"

The answer to this question can be very complicated. I have not been able to get any particulars from Luke, so I am writing this as my immediate response. Raster images such as photographs are resolution-dependent, meaning they will deteriorate as they are made larger. This means the number of pixels that occupy a given space must be defined. Resolution is actually specified in pixels per inch (ppi) because they are typically viewed on a computer monitor. However, image resolution is commonly referred to in dots per inch (dpi). For our needs we can use the two interchangeably.

Dpi is more appropriately attributed to output resolution, where the number of dots an output device is able to produce within an inch represents the resolution of the device. While a computer monitor can display approximately 72 dpi, each printer model has a different maximum resolution.

The number that we need for each image is the effective resolution in order to print on the intended printer. When placing and scaling images, you are changing what is known as the image effective resolution. Effective resolution is a calculation of the actual resolution factored for the scaling performed in the page layout application.

The math works out so that if you reduce the scale of an image in the page layout, you increase the effective resolution. As you increase the amount of scaling, you will decrease the effective resolution.

The answer to this question is complicated, but due to the easy availability of storage on a flash drive, we do not necessarily have to calculate the final resolution needed for each picture.

Black and white or grayscale images do not require more than 160 dpi as the final resolution for each image. Color photographs typically require a minimum resolution of 300 dpi (I will explain this in a post very soon).

The bottom line; The effective resolution of black and white or grayscale images is 160 dpi. The effective resolution of color photographs is a minimum of 300 dpi. You can only scale images to the maximum size that will give you the desired effective resolution.