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Digital Picture Basics Lesson 3 - Image Size, File Size and Resolution

You have no doubt heard these three terms before, but do you understand what the difference is between them? Many people use the terms interchangeably and do not realize that they are in fact three separate ways of measuring a photo.

For the purposes of learning these differences, we will be working with an image file in Photoshop.

Please right-click here and save the image to your assignments folder.

Resolution - Pixels... Megapixels... What does it really mean?

The amount of detail in an image is called the resolution, and it is measured in pixels. Resolution is very important when editing images for print. The more pixels an image has, the more detail is displayed and the larger you can size pictures without blur or grainy results.

**** Important****

Your first concern should always be resolution. Once you are sure that a picture is the proper resolution for your purpose (print or web) then we can worry about the image size.

Resolution and Cameras

You can set your camera to take pictures at different resolutions. Depending on your camera, you can set the resolution to as little as 256 x 256 (number of pixels in height and width) or as high as 4064 x 2704.

Once you take a picture, the resolution for that picture is set and can only be changed using software like Photoshop.

It is fine to decrease resolution using software, but not a good idea to increase it (making the computer guess at filling in the missing pixels).

For this reason, it is usually a good idea to capture images at a higher resolution setting.

Some typical resolutions include:

The size of the same image taken at different resolutions.

High-end consumer cameras can capture over 12 million pixels. Some professional cameras support over 16 million pixels, or 20 million pixels for large-format cameras. It is estimated that the quality of 35mm film (non-digital camera) is about 20 million pixels.

Keep in mind that once you get past a certain point, you are paying for pixels you may never need.

Changing Image Resolution

You can increase the resolution for print, but we will have to sacrifice the print size to do this.

1. Open the sample file in Photoshop

2. Choose Image > Image Size


You will notice that the resolution of the photo is set at 72 pixels per inch.

This is a good time to mention that a resolution of 72 ppi is fine for the web because it will transfer quickly. However, this will produce terrible print quality. When we are looking to print images, they should be at a resolution of 300 ppi.

3. Uncheck the box that says "Resample Image"

Note: By doing this, you are allowing a decrease in the print size of the image to gain better quality in a smaller image.

4. Change the Resolution to 300.


Note: You'll notice that as you do this, document size will change, but the number of pixels has remained unchanged. This means that you now have a smaller document with the same number of pixels as before.

5. Choose OK

We could have increased the resolution without sacrificing document size, but that would mean the computer would have to guess at filling in the pixels added pixels.

Image Size - Document Size

The term image size refers to how large the picture will be when it is printed. Once you have set the resolution you need (72 for web or 300 for print) you can now change the Image Size.


Because of the high number of pixels, we can print this picture in excellent quality at a size of 10in x 6in or smaller.

Let's decrease the image size so that we can print it in a standard size.

6. Make sure the "Resample Image" and "Constrain Proportions" boxes are checked.

7. Change the document size height to 4 pixels.

Note that the height changes automatically. This is because you have the box checked at the bottom that says "constrain proportions". You should always have this box checked when resizing images.

8. Choose OK

Changing Image Size is not very difficult, but there is one rule we must follow:

Decreasing image size can be done to any degree, but when Increasing image size, you will lose quality and the image may become blurry.

This is because when you decrease the size you are simply cutting out some of the pixels from the picture. When you are increasing the size, you are asking the computer to stretch a picture and to take a best guess at filling in the empty pixels along the way. When a computer has to guess, the result is a more grainy picture.

File Size

Remember that when your image size is larger, it will contain more information and so file size will be larger. The same is true for resolution. This means that when you are capturing at a higher resolution on your camera, you will be able to store fewer images on your memory card.

This also means larger file sizes to store on your computer and slower transfer speeds if the images are stored online or emailed.

For example, when our sample file is resized the file size would change as seen below:

- Sized at 42 x 28 inches at 300 ppi the file size is 24.6 MB

- Sized at 42 x 28 inches at 72 ppi the file size is 2.9MB

- Sized at 4 x 6 at 300 ppi the file size is 1.3 MB

- Sized at 4 x 6 at 72 ppi the file size is 205 KB

**** Remember****

Your first concern should always be resolution. We always set resolution first and then we set the image size.