Digital Photo Resolution
AbstractDo you have a favorite picture? When you have a photo that you treasure, you want it to look as good as possible. Find out how the resolution of a digital photo effects the way it looks when you print it out. How does the quality of the image relate to the resolution?
ObjectiveIn this experiment you will investigate the relationship between the resolution of a digital photo and the quality of the image.
Digital photography has revolutionized the way we take and make pictures. With digital photography, you can take more pictures, see them right away and share them with more friends and family. The problem with digital photography is that it took a long time for the technology to provide a quality image competitive with one from a traditional film camera.
A poor quality digital photo can be painful to look at. The edges of the lines look like tiny little dots. You cannot magnify, or zoom in on parts of a poor quality digital image without your photo looking grainy. The reason for the grainy texture is that digital photos are made out of pixels. Each pixel contains one piece of digital information, the color of the pixel. Each pixel is one box of color, and all of the pixels put together next to each other create your image, much like a puzzle.
A poor quality digital photo has very large pixels, which means that there are less total pixels in the entire image and the image looks grainy because the individual pixels are large enough to see. A high quality digital image has the smallest pixels, and can fit more pixels into the total image. The image will not look grainy because the individual pixels are too small for your eye to see.
A way of measuring image quality is to measure the number of pixels in a small space of your photo. If there are many small pixels it is a high quality image, if there are few larger pixels then it is a lower quality image. This measurement is called the "resolution" of the photo and is measured in "dpi" or dots per inch. A high dpi has more pixels and is a better quality image we call this a high resolution image. A low dpi has fewer pixels and is a lower quality image, called a low-resolution image.
What if you have a low resolution image that you want to look better? One way to make the image look better is to compress the pixels into a smaller space, this will cause there to be more pixels per inch. However, this will also make the total size of the image smaller. Because resolution is measured using pixels and space in inches, there is a relation ship between image size and quality.
In this experiment, you will change the resolution of a photo, and print out a series of high to low quality prints in different sizes. Then you can look at the photos with a magnifying lens to see the effect on pixilation. Can you determine a relationship between the resolution of the photo, the size of the photo and the image quality?
Terms and ConceptsTo do this type of experiment you should know what the following terms mean. Have an adult help you search the internet, or take you to your local library to find out more!
- digital photography
- image resolution
- low resolution
- high resolution
- image quality
- 2005. "Adobe Digital Kids Club." Adobe Systems, Inc. San Jose, CA. 12/30/05.
- Wilson, Tracy V., Nice, K., and Gurevich, G. 2005. "How Digital Cameras Work." HowStuffWorks, Inc. Atlanta, GA. 12/30/05.
Materials and Equipment
- a digital photo taken with a digital camera on high quality setting
- photo editing software like iPhoto or Adobe Photoshop
- a color printer, or an adult to take you to get your photos printed at the store
- magnifying glass
- Choose a photo for the experiment. Pick a photo that you like, and that was taken as a high quality image from a digital camera.
- Open the photo in a photo-editing software package like Photoshop.
- You are going to save many copies of the same photo, so choose what names you will save the photos with, for example "mydog1.jpg" each with a higher number.
- Make a data table to keep track of your experiments. In each experiment you will need to record the new file name, the new resolution, dpi and image size:
|File Name||Resolution||Dpi||Image Size
(in x in)
|Print Quality and Observations|
- Save your first version of the photo unchanged, just the way it was imported from the camera. Use the number 1 in this file name to keep track of it (Mydog1.jpg).
- You will need to find out what the resolution and size of your photo are. To do this click "image" menu bar and select "image size" in the drop down menu:
A cropped screenshot of Adobe Photoshop on a computer running MacOS. When Photoshop is open a navigation bar at the top labeled "Image" is selected. When "Image" is selected a drop-down menu will appear and an option labeled "Image size" will appear near the center of the list.
- Before you start to change resolution settings for your experiments, you will also need to change some other settings:
- Change the pixel dimensions from pixels to percent of image size. This will allow you to keep all of your images at 100%.
- Be sure the chain is linked beside the pixel dimension and document size fields.
- Make sure that the box is checked to constrain proportions. This will keep the proportions of your photo constant so make sure that they stay linked.
Screenshot of the image size window fin Adobe Photoshop. In the image size window, pixel dimensions are changed from a value of pixels to a percent based value. A small image of a chain link next to both pixel dimensions and document size must remain linked.
- Next turn OFF the Resample Image Box. Set the Width (or Height) to the size you want. You will see what Resolution (which means information) the starting image has without adding or removing any pixels. See if you have enough information to start at 300 PPI, if not try for the next highest possible number (like 240 or 200 PPI) by changing the dimensions.
- When you have the highest number of pixels turn the Resample Image box ON, then type the chosen PPI into the Resolution box. The image will be resampled according to those numbers.
- Now you are ready to enter the data about your picture into your data table. Find the dpi, the image size in inches x inches. Write these into your data table:
Screenshot of the image size window in Adobe Photoshop. The width and height in pixel dimensions are both changed to 100 since the values are now percentage based. This ensures that pixels are not lost or cropped out of the original image.
- Next, you will need to calculate the image area from the image size. The formula for Area is height x width, so multiply the number of inches of height times the number of inches of width. For the control photo example that would be:
- Now start setting the Resolution lower, with the Resample Image box still ON. If you start at 200, try then 160, 100, 72, 50, 24 and 10. The image will continue to print at the same size, but with fewer and fewer pixels to work with. At 160 it should still look good, but once you get down to 72 PPI and lower, it will start to look bad as there's too much space (too little information) between the pixels.
- Each time you will need to re-set the pixel dimensions to 100%. Write down the new data for image size in your data table and click on OK:
Screenshot of the image size window in Adobe Photoshop. The resolution is changed to 300 pixels per inch which causes width and height to change to 4 inches and 5.973 inches respectively. No pixels will be lost when changing the image resolution.
- Next, print and save each variation. When saving, choose the "save as" option from the file menu. Save each image with a new number (2, 3, 4, etc.) in the file name (Mydog2.jpg).
- After you have printed the images, use a magnifying lens to examine the photos. What differences do you see? Write down your observations in your table.
- The last thing that you should do is prepare a graph. You could graph the resolution against the image area with a line or bar graph. Remember to label your graph with units of measurement. The resolution will be measured in dpi and the image area will be measured in square inches (in2). What do you notice?
Ask an Expert
- Sometimes you have a photo that you want to make bigger, but the picture may not look as good. You can try a similar experiment to find out how changing the image size affects the image quality. What happens if you change the image size instead of changing the resolution?
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