Digital Photography vs Traditional Photography
resolution vs. enlargement

Digital Photography Fundamentals

Rule One - all digital cameras capture information at 72 dots per inch (DPI) regardless of their total pixel count and overall resolution. 72 DPI defines digital photography and yes it is both with still video cameras and digital camcorders. ( Just as a point of information, digital camcorders use 720 x 480 pixels as their standard resolution, but still present data at 72 DPI. )

The 72 DPI was chosen because digital play back happened originally on Television sets, and guess what. TV sets play back at 72 DPI. Computer monitors for the most part also playback at or about 72 DPI. For the purposes of the remainder of this article I will treat 72 DPI and 75 DPI as totally equivalent. This is because I hate left overs when doing math. Just remember for the remainder of this document 72 DPI = 75 DPI.

Ok here is the part nobody likes, the math. Camera resolution is measure by the total number of pixels (picture - elements) used in the X and Y direction. It is usually written as a numerical pair each separated by a "x" : 640 x 480, 1024 x 768, 1280 x 960, or 1600 x 1200 etc. etc. But according to rule 1 above, all digital video capture systems capture at 72 DPI. Therefore ...

ccd cell
Total Pixels
Manufacturer HYPE
Size @ 72 DPI
640 x 480
First hi-res Digital camera
8.88" x 6.66"
1024 x 768

14.22" x 10.66"
1260 x 960
1 Mega Pixel
17.5" x 13.33"
1600 x 1200
2 Mega Pixels ???
22.22" x 16.66"
2048 x 1536
3 Mega Pixels
28.44" x 21.33"
v2240 x 1680
First 4 Mega Pixel ???
31.11" x 23.33"
2272 x 1704
4 Mega Pixel ???
31.55" x 23.66"
2560 x 1920
Effectively 5 Mega Pixels
35.55" x 26.66"
2832 x 2128 ???
6,062,000 ???
6 Mega Pixels ???


total Width about 4 inches
total Width about 40 inches
total Width about 110 inches

Figure 1.

So if each camera captures at 72 DPI and each of these cameras captured exactly the same image, then the ones with more pixels would resolve more information. Since they display their images at 72 DPI on the computer screen, the ones with higher resolution will display at a larger size on a monitor.

The last column shows the size of each image as it is displayed on a TV or computer monitor. All are 72 DPI, and some are very large indeed.

Lets look at a couple of examples. We'll take a picture of an object using most of the resolution of the camera, then move the camera further back and shoot again. The same object will appear smaller in the image and thus will be represented by fewer pixels.

Figure 1. shows three camera positions.

Below are the three resultant images and each is blown up so the coins are roughly the same size. Notice the loss in resolution as fewer and fewer pixels are used to define the area. This is a direct example of what happen when a digital photograph is enlarged with software.

The particular camera used had a resolution of 1600 x 1200, so the first image (Photo 1) used 1600 pixels divided by 4 inches or 400 pixels per inch. Each of the coins was approximately 1" in width so there were about 400 pixels defining each in the first image.

In the second photograph the total width of the image was about 40", so there were 1600/40 or 40 pixels per inch. The coins were then enlarged so they were the same size as the first photo (Photo 2.) Here each coin must be resolved by about 40 pixels.

In the final image the camera was moved so that the image was a about 110 inches wide. Hence the calculation is 1600 / 110 or 14.5 pixels per inch. The coins were then enlarged to the same size as the first picture and are displayed in Photo 3. As you can see there is even less detail now.

You can always reduce a picture in size using software, but you cannot enlarge a digital image larger than its original size. If you do you will lose quality because it is already at its maximum resolution.

Rule of Thumb: Enlarge with hardware, reduce with software.

In a nutshell if you need to make an image bigger, then reshoot or rescan the image. Do not try to enlarge with Photoshop or other digital software. If you need to reduce the size of an image, then go ahead and use an editing program. Reduction maintains quality although there is still loss of content.

The three images below were all reduced in software from Photo 1. Although each one reduces the total number of pixels, the quality is still pretty good.

Photo 1. no enlargement

Photo 2. enlarged from 40" wide photo

Photo 3. enlarged more from 110" wide photo