Understanding File Formats and Sizes

Applications such as Photoshop give you many options when saving files.

For instance, a graphics file can be saved as a PSD, a JPEG, a PNG, a Bitmap, and many more formats.

Here’s an example, a photo saved form Photoshop. Each one looks about the same:

But when saving the same image, different formats produces different size files. Here’s what “Windows Explorer” (or My Computer) tells us:

The size varies from 36 Kb to 21,899 Kb (21.9 Mb)!!! What’s up?

· All the images we work with are “bitmaps”, which means it just stores the colour of pixels (tiny dots) that make up the image.

· If you simply store the colour of each pixel with no compression, this is a BMP or Bitmap file. It’s big because there are millions of pixels in a typical picture, and you need a few bytes to store the colour of each pixel

o This picture’s resolution (its dimensions in pixels) is 2130 pixels wide by 1880 high. That’s 2130 x 1880 = 4 million pixels!

o The chart above tells us that saved as a Bitmap (BMP) it takes up 11.7 million bytes.

· If you save it as a Photoshop file (PSD), it takes almost 22 million bytes. That’s because I added a background layer, and every layer of a PSD file is saved as a bitmap. Multi-layered PSD files can get HUGE!!!

· JPEG is a compression scheme, which makes the file only 5 to 10% of the bitmap size, yet most people can’t see any difference. The “standard quality” JPEG (quality set to 8 as I saved) has only about 432 thousand bytes – 20 times smaller than the bitmap!

· If you set the quality lower, JPEGs compress even more – a setting of 0 gives only 36 thousand bytes. But if you zoom in to 100%, you start to see the lower quality. The upper image is at quality 8 (“high”) while the bottom is at 0 (“lowest”).

· The PNG (Portable Network Graphics) file is way bigger than a JPEG, but half the bitmap size. It’s advantage is that unlike a JPEG, it can have transparent parts, which is good if you want to insert it into Powerpoint (or many other programs) without having the background rectangle showing. The PNG below is transparent to the right of the face, so it doesn’t block the other picture or the green background, unlike the JPEG.

· Notice the 2 Powerpoints have vastly different sizes too. I imported the same photo into both and saved them, but I first compressed the picture in the smaller one to nearly 10% of the size. Just double-click any picture in your powerpoint to see the Compress button and options.

· There’s one more format worth mentioning here: RAW. RAW files are bitmaps that come off the camera, uncompressed and split into Red, Green and Blue components. Good digital SLR cameras let you save your pictures in this format if you want. The files are HUGE, but allow you full control of how the image is processed, and you lose nothing through compression.

Summary:

1. All photo files are stored as bitmaps.

2. Uncompressed bitmap files (.BMP file extension) are huge.

3. Photoshop files (.PSD) are even huger, because they store each layer as a big uncompressed bitmap.

4. JPEG files (.JPG) are very compressed bitmaps. They are usually 5 to 10 % the size of a BMP.

5. You can set the quality of a jpeg as you save it. Low quality gives a smaller file, but the low quality shows if you look closely.

6. PNG files are smaller than bitmaps, much bigger than JPEGs, and allow parts of the image to be transparent.

7. Before saving a Powerpoint, compress the pictures in it.