Wednesday, July 29, 2009

Digital Resolution

There is a lot of confusion concerning resolution in digital photography. In the days of film, we just tried to get as much resolution as possible, figuring the more the better. With digital imaging, the subject is a lot less subjective and a whole lot more specific.

Cameras are sold based on the number of individual pixels, or picture elements, they can achieve. 35mm film, under ideal conditions, could possibly show the equivalent of about 10 million pixels. In reality, a five-meg, or 5 million pixel camera, is about equal to 35mm film in resolution.

But the number of pixels is far from the whole story in digital photography. With virtually every camera outside of camera phones, producing images with more than five megapixels, other qualities become more important. The quality of the lens, the type, size, and quality of the sensor, and of course the skill of the photographer are all more important than the number of pixels.

In practical use, the final use of the image determines the resolution necessary. The number of pixels in an image is determined by the media and the size of the image. The most common resolution of an image to be printed on paper is 300 dpi (dots per inch.) If the final result is an 8x10 print the resolution of the image would be 300dpi x 8 inches by 300dpi x 10 inches, or 2400 X 3,000 pixels. 2400 times 3,000 equals 7.2 meg. If the camera produces less than 7.2 megapixels, additional resolution must be interpolated by the computer. This is relatively easy and gives good results. A camera with more than 7.2 megapixels has excess capacity for an 8x10 print. The additional resolution is not only unnecessary, but can degrade the image and only increases the file size.

If the image is going to be viewed on a computer screen, the resolution is about 72 dpi. Web designers usually think of the image size in terms of pixels. If the resolution of a computer monitor is 600x800 pixels, a 400-pixel wide image will cover half the screen. An image in the corner of a web site might be 100 pixels wide. If I post a nice size 200 x 300 pixel image on my web site it has 60,000 pixels, it is a 60K image. If I shot the photo with a 10-meg camera, I must throw away more than 99% of the information.

I often get requests for “high-resolution” photos. I later find out that they are going to be used on a business card, brochure, or even a web site. The size of the final image is the determining factor in the needed resolution.

A good analogy might be to compare camera pixels to horsepower of a car. A 500 horsepower car might be cool, but if all of your driving is in traffic under 40mph, you have a lot of excess capacity and are probably wasting gas. The arguments in favor of a 500HP car are about as ambiguous as they are for a 20-Meg camera, based more on bragging rights than actual necessity.

The confusion over resolution may result from confusing sharpness with resolution. Sharpness is a relative term and has to do with what a picture “looks” like. The factors affecting sharpness are camera movement (the biggie), focus, exposure, and the subject. Maximum sharpness is not always necessary and is often undesirable as in the case of portraits. Professional photographers learn how to use relative sharpness to best advantage.

The format of the file has no affect on resolution. JPEG, TIFF, PSD, RAW, etc are all ways of storing data and have no affect on the resolution. The only affect they have on quality is in the way they are used.

I have seen publications request specific file formats under the guise of better quality. I presume that they had received low quality images in the format of JPEG, or whatever, and concluded that the format was the problem. Poor images can be any format. I will address these formats in a later post.