Photography Basics #6: Image File Types, Size and Resolution Explained

Michael Sand
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Image File Types

For Photographers: How Many Do You See Every Day?

On a typical day, you will probably come across these image file types:

‒ JPG: JPEG is the most widely used image file type. JPEG stands for Joint Photographic Experts Group. It can be compressed using both lossy and lossless methods. JPEG is a bitmap file format. JPG is best used for photos that contain a lot of detail and color variation.

‒ GIF: GIF stands for Graphics Interchange Format and it also uses lossless and lossy compression. GIF files are typically smaller than JPG files and thus, good for small images with a limited number of colors.

‒ PNG: PNG stands for Portable Network Graphics and is a bitmap image file type. PNG is an open-source image format that is more efficient than GIF but not really effective at displaying photo-realistic images.

Raster (Lossy) vs Vector (Lossless) Images

The terms raster and vector are commonly used in image editing programs.

Raster images are composed of pixels. Pixels are simply squares of color. Pixels give the image definition and the appearance of depth and texture.

Vector images are simply equations. The pixels in vector images are mathematically calculated, not rectangular in shape and they don’t have a defined size.

Vector images are naturally editable even after they are exported because the original data is still editable.

The raster images on the other hand have pixels that are hard to recreate once they are lost because of the complex nature of the pixels and the amount of time required to recreate them.

This nature of raster images is what makes them take up more space. This is because the mathematical equation that helps create them is also used to recreate them and this can be quite slow depending on the image that’s being processed.

Since pixel images are made up of pixels, the images tend to be less flexible in terms of editing after they are exported. This is because a lot of time and effort are required to create the pixels.

Another good thing about vector images is that they are often replicated in-line or at a fraction of their size at the same resolution. This is unlike raster images which tend to be less clear when they are resized.

Raster Formats

Raster image formats or bitmap image formats are file formats used to store digital images.

Bitmap formats directly represent a picture as a grid of pixels using a limited set of colors.

The pixels (tiny dots) in a bitmap image can be organized in various ways or simply "scattered" which makes it difficult for machines to auto-interpret it, impossible for viewing and editing by humans.

This is why your digital images need to be "rendered". This process converts the image from a bitmap format to a vector format before printing or viewing.

The most used raster format for images is .jpg (JPEG), which is a lossy format, based on the Joint Photographic Experts Group (JPEG).

Here is the difference between a JPEG image and vector image:

A JPEG image is a bunch of tiny dots saved in a bitmap file; a vector image is comprised only of lines, graphics and fonts saved in a vector file.

Another popular bitmap image format is .png (Portable Network Graphics) also known as a "lossless" format. Much like JPEGs, PNGs are easy to use and maintain, allowing you to crop, scale and re-size your image.

So when should you use a JPEG and when a PNG?

Vector Formats

  • Open-source Adobe Illustrator: AI
  • Open-source Corel Draw: CDR
  • Commercial Corel Draw: CPT
  • Microsoft Visio: VSD
  • Open-source Xfig: XFIG
  • Adobe Systems Freehand: FIG
  • Adobe Systems Photoshop: PSD
  • Commercial Macromedia Freehand: SWF
  • Commercial Macromedia Flash: FLA
  • Commercial Macromedia Flash (old): SWF
  • Commercial Macromedia Flash (old): MNG

Graphic formats that retain vector information as opposed to image information.

Used for print resolution images.

Image resolution is not important.

Converting Between Different File Types

What’s the difference between a RAW file and a JPEG file, or why do they have different compression ratios?

To know the difference between file types, you first need to learn about the file size and resolution of an image.

The size of an image file is measured in pixels. You can find the pixel dimensions of your image by going to the image properties or right-clicking your image and selecting Properties.

This will give you the width and height in pixels of the image.

The image resolution is a measure of the density of pixels of the image (the pixel count). For example: if your image has a resolution of 300 pixels per inch, then it is half the resolution of a 600 ppi image. Plus the 300 ppi image is half the size of a 600 ppi image.

This is important because it determines the picture’s sharpness, which is also linked to the file size. For example a high resolution image with a small file size is going to be very sharp, while a low resolution image with a high file size will look dull

Image Size and Resolution

If you want to be sure to achieve the optimum quality, the most important thing to think about is dimensions (the width and height of your image). When you upload an image to a website or photograph it, it is automatically resized, whether it’s to make the image fit better on the page, to make it more compact, or otherwise change the proportions, most likely to be something related to the height.

It is important to understand the difference between image dimensions and image proportions, because they are not the same. Changing the proportions of an image will typically also change the size of the image, but changing the dimensions will not.

Image dimensions are written in pixels, and represent the number of pixels arranged in rows and columns in your photo. A higher pixel count means your displayed image will be sharper. JPEG images support a maximum of 2.1 million pixels, which is very high. You should avoid resizing large images. Large images load faster and are often better on the screenshot.

Image proportions, sometimes called the “aspect ratio,” are expressed as ratios, and are either noted as a ratio or worded as a fraction. There are three common ratio sizes: widescreen (16:9), standard (4:3), and cinemascope (2.35:1).

Why Size Is Important When Printing

In order to display your images appropriately on screen, or print them out on paper, you need to make sure that you have selected correct image file types, image dimension (size) and resolution.

As I have explained in my previous photography basics articles, the number of pixels that make up an image is called its resolution.

Resolution is a measurement of how sharp an image will appear. The higher the resolution, the sharper the image will appear. The number of pixels used to make an image (in each direction) is known as the Size. When I say size, I mean the physical dimensions of the image.

Use the chart below to help you translate pixel dimension (size) into real-world, printable dimension.

Do remember that there are exceptions to the rule. For example, a 4″x6″ print may not contain 300 pixels (width) or 600 pixels (height). That’s because there are indeed a number of variables in the printing process. Hence, a 4″x6″ print may contain only about 270 pixels in width and 540 pixels in height.

The table below should help you with the general conversions.

Sizing for Screens

Now that you have your digital images, you need to figure out what to do with them.

That could mean uploading them to a web gallery or having them printed on different paper sizes.

So, it’s important to have a basic understanding of image file types, image size, and image resolution before you start to process your images.

Image size is measured by pixels and how large you want the "picture" to appear, whereas image resolution refers to the amount of detail, or sharpness, that’s in the image. The resolution is measured in pixels per inch.

Image file types are different kinds of files that store your digital images. File types will determine how you are able to manipulate the image.

If you’re going to be printing your photos, it’s important to understand how image files are displayed on different formats before you send anything off for printing.

This chapter will take you through all the basics information on image size, image file types, and image resolution.

It’ll also cover the varying paper sizes and explain how to figure out what size you need for your photographs when you print them at home.