Digital Photo Resolution

What is resolution?

The closest analogy that digital photography resolution has to conventional film is grain. Normal film is not a continuously variable media it is limited by the underlying granularity of the light sensitive film. Grain tends to increase as light sensitivity (ASA rating) increases. Regardless of what you do you can’t get more detail out of a negative than what the grain permits.

The equivalent to grain in digital photograph is the pixel, the pixel is a single point in the image that can take on any one of many millions of colors. Image sizes in digital photography are measured by the number of pixels wide by the number of pixels deep. This is the same measurement as used for computer screens, for which typical measurements are 800 x 600, 1024 x 768, 1280 x 960.

To put it another way a digital photograph is an image that is made up of millions of dots, each one of which can take on any one of millions of colors. Got the picture?

PCs and resolution, is more better?

Like anything else to do with computers Yes and No.

Yes, the greater the resolution of the screen the more detail that can be displayed of a photographic image. Also associated with this is the dot pitch which simplistically is the diagonal distance between each pixel. The smaller the dot pitch, the sharper the image. However, very small dot pitches may also translate into decreased brightness and contrast.

A secondary affect of increasing the display resolution used on a screen is that the standard size of windows text characters decreases as you increase the resolution. It can get to the point where you can no longer read the screen and are forced to use big fonts in display settings. The problem here is that some programs won’t work when you use big fonts, their windows become garbled with missing text,  missing fields etc. In general life is easier if you can use the windows default font size.

Another problem is that all early screen sizes were in the ratio of 4:3 horizontal to vertical resolution, that is 640 x 480, 800 x 600 and 1024 x 768. Some recent video cards have options such as 1280 x 1024, a 5:4 ratio. It is wiser to stick to the 4:3 ratio choices as often graphics created or manipulated on a system running with a different ratio will not look the same when transferred back to the standard 4:3 ratio. It is also possible on a 5:4 ratio screen that the aspect ratio of photographs may be distorted.

How much resolution do I need?

You can do many tricks with digital images but the one thing you can’t do is add resolution to a photograph after it has been taken. This is the main thing you have to make sure is right before you take the next photograph with your digital camera.

You have to ask the same question when you scan images. Too much resolution takes time, consumes memory and rapidly fills your disk. Too little and you need to go back and scan it again.

The table below shows typical uses and suggested resolutions for both using a digital camera and scanning images.

Digital Camera Scanning Photos
Print a standard size photograph
(6″ x 4″ or 7″ x 5″)
1800 x 1200 to 2100 x 15002.2 mega-pixels to 3.2 mega-pixels 300 dpi scan. Higher scan resolutions generally achieve little to no visible improvement in printed image quality.
Full screen PC based slide show You can get away with half your current screen resolution for now but with the future in mind we suggest as a minimum at least 1800 horizontal pixels. See the note that follows.
E-mail to a friend or to post it on the Web 600 x 400 72 or 100 dpi scan. This saves disk space and time moving the image over the Internet.
Enlarging (10″ x 8″) Use a conventional camera You need at least 300 dpi in the output result. For ten inches that is 3000 dots. Divide the 3000 by the width of the input image to get the minimum scan resolution. If 6″ then the minimum scan resolution is 500 dpi.
Note:  A PC based slide show is different in that you can expect your display resolution to change several times over your lifetime. In many cases the photographs you put on CD today will be handed down to successive generations. You could be starting today on a 17″ screen with a resolution of 1024 x 768 and ultimately displaying on a full size wall unit of far greater resolution.The higher the initial resolution the less raggedness there will be when images are resized to fit any screen size. You cannot target image sizes to a specific screen size as the screen size will change with time. If possible work with a minimum of at least 1800 horizontal pixels.Fortunately images do enlarge on-screen quite well. Doubling the width and height increases the image area by 4 times. Provided software that uses resampling is used acceptable quality is obtained.

How big are digital images?

Digital images are BIG. They use lots of memory and consume lots of disk. Lets look at  some examples.

Image Size Uncompressed Size
2 mega-pixels 8 Mb
1024 x 768 3 Mb
3000 x 2200
(10″ x 8″ enlargement)
26.4 Mb