First, let's look at the 35mm film negative. The aspect ratio of the negative is 2:3 (24 x 36 mm). The theoretical size of a full frame print would be 3.5 x 5.3 inches -- delivered as 3.5 x 5, with an enlargement ration of 370% (the printed image is 3.7 times the size of the negative). Jumping to the 4x6 image size, at least in theory, this print size should be perfectly enlarged from the 35mm negative since the aspect ratio is exactly the same (2:3).

NOW, let's confuse you by discussing the digital world.

Let's first start by looking at cameras from Canon. The current high-end consumer models, the 300D (Digital Rebel) and the 20D both have slightly different sized censors. [The sensor is what captures the image.] The sensor in the 300D is 22.7x15.1 mm (3072 x 2048 pixels) and the 20D sensor is 22.5x15 mm (3504 x 2336 pixels). Both of these models follow the 3:2 aspect ratio of the 35mm film camera.

Now let's switch track over to a mass market class camera, also from Canon. The Canon Powershot S1 IS is a standard 3.1 Megapixel camera with a comparably tiny image sensor. The image sensor in this camera is only 5.27x3.96 mm with a maximum resolution of 2048x15336 pixels. This image size does not follow the standard 3:2 aspect ratio, it follows a 4:3 aspect ratio.

What the hell does all this mean. Well, what it means is that when you go to print the images on some online service, there's a really good chance that you image will be cropped. What will happen to your image is going to depend entirely on the dimensions (in pixels) of the image that you provide to the printer. Why does this matter? Well, if you upload a 4:3 image and expect a beautiful 4x6 photo with no borders, then a piece of your image is going to get chopped off -- probably from the top and bottom. There are some printing companies (like MyPhotoAlbum.com) that provide a print called a 4D or a 4xD. This image is actually only 5.33 inches wide, but does NOT crop any of your photograph away since it follows the 4:3 aspect ratio.

Confused? If so, you can remember it this way, if you have a digital SLR camera (removable lenses), you can print the 4x6 size but if you have a fixed lens, you will need to print 4D or 4 x D.

To be certain here's what you can do:

• Open your camera manual and look up 'maximum resolution'.
• Turn to that page.
• Look for the pixel dimensions (something like 2048x1536).
• Divide the smaller number (1536) by 2.
• Multiply this result by 3.
• If the answer to this is NOT exactly equal to the bigger number, then you should print the 4D size.