Image Resolution

Understanding image resolution properties can help you
create better digital images

What is Image Resolution ?

An image resolution refers to how much detail an image can hold. It is controlled by the number of pixels and bit depth of each pixel that your camera uses to produce a photo.

Understanding Pixels and Bit Depth

A pixel is the smallest element of an image and displays one colour. When grouped together, they form an image.

The more pixels there are, the sharper your image is, and consequently the larger it can be printed.

Because so many of them create even a small image, pixel count is often expressed in megapixels (millions of pixels). For example, if you multiply 2,888 x 1,712 pixels, you get roughly 3.9 million pixels, or 3.9 megapixels.

Your camera likely has three or more resolution settings (image size settings), each producing a different pixel count.

The bit depth of the pixel determines its colour and brightness range.

Increasing the bit depth of the colour channel, exponentially increases the number of colours each pixel can express and the sharper the image is.

To understand the effect of bit depth on an image, look at the picture below which is an 8-bit grayscale image :

The initial bit depth of an image is controlled by your camera and it offers several image file types which all have different bit depths. Choosing the image file format you shoot your images in is an important decision as it has a dramatic effect on the visible tones in your images.

Mapping Resolution to Different Devices

The reason resolution is often confusing is because its measurements change from camera to display to printer. But I cannot stress enough how critical it is to understand these changes in order not to compromise the quality of your images.

Each digital camera has a potential resolution measured in megapixels. If we think of a film negative - the larger the negative, the more you can enlarge the image without losing sharpness and detail.

Similar connection can be made with the number of megapixels - the more pixels you capture, the more information is stored in the image and the larger you can print it without a noticeable loss of quality.

Looking at the two images below, they are both shot with the same camera. The difference is that the one on the left is a 3-megapixel image while the one on the right has its resolution reduced to 1-megapixel image.

At a snapshot size, the two do not look much different.

  

However, if we enlarge a portion of each image, the 1-megapixel image displays a stair-stepped appearance along the curved lines and subtle details are lost.

  

On the downside, the more pixels the larger the file size of the image, so you can fit fewer pictures in a given amount of camera memory card. Here are some examples :

Megapixels	Print Dimensions at 200 dpi	Approximate uncompressed file size
1	4" x 3"	1 MB
1	4" x 3.5"	2 MB
2	6" x 4"	3 MB
2.5	10" x 6"	7 MB
4	12" x 8"	12 MB
5	14" x 9"	15 MB
7	16" x 11"	21 MB

In addition, larger files take longer to process in an image-editing program.

The quality of a print is determined by the combination of two factors. One is, as we saw, the resolution of the image as determined by the bit depth and count of the pixels in the image. The greater the bit depth, the more colours a pixel can display and the more pixels an image has, the more detail it is capable of displaying.

The second factor is the printer resolution. It is determined by how closely together it is capable of placing dots on paper within a square inch and is measured in dpi. So in a printer we must look at the maximum dpi value it has as this determines the highest-quality image it can print.

On the other hand, for displaying pictures on a computer screen, television or a digital projector, the number of pixels does not affect the image quality, just the display size. The maximum resolution of a screen is determined by the maximum number of pixel it can display. Most screens have a variety of resolution settings from which to choose from.

As a photographer, you will want to set your screen at its maximum resolution setting as this will ensure that you see as much of the image as possible.

What Resolution to Use ?

The answer to this question will depend on what you intend to do with the image, how important is image quality to you, how much camera storage and computer storage space you have available.

If you want to make the best and largest print possible, then shoot in the highest image resolution setting your camera offers.

Shooting at a high image resolution also gives you an additional flexibility when editing the image. You can crop and enlarge a portion of the image to good result as we saw above with the 3-megapixel sunflower image.

This benefit becomes especially important when photographing moving targets or spontaneous events such as parties. In such instances, it can be challenging to achieve the perfect composition and shooting at high resolution enables you to correct the framing later without a loss of detail.

If, however you intend to only print 4" x 6" photos, then you might choose a lower image resolution setting. This will allow you to store more images on your camera memory card as well as on your computer hard drive.

When in doubt of how you will be using your image, shoot in a higher resolution setting. If necessary, you can eliminate extra pixels later in the image-editing program. You can also add pixels later too, but usually it does not result in a noticeable improvement in the quality of the image.

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