Understanding JPEG File Sizes & Image Resolution

JPEG File Sizes and Image Resolution

There is sometimes confusion between image resolution and JPEG file size.   This post is going to take two identically sized family portrait images to highlight some common misconceptions.

images for comparing jpegs sizes and image resolution

Basics

Both of these family photos were taken with the same camera. Image 1 is a studio image containing a high proportion of uniform, white background.  Image 2 is a location shot with a much more detailed background. Both have been sized at 14 x11 inches @ 300 dpi. This means that for every inch of print there are 300 pixels (this is the industry standard for photographic printing), resulting in the images containing 4200 x 3300 pixels (or nearly 14 million pixels).

The other thing they have in common is that they both have the same full uncompressed file size of 39.7mb. The reason they are the same is that they contain the same number of pixels; and each pixel’s colour takes up 3 bytes of information (one byte – or 8 bits of binary information – for each value of Red, Green and Blue – RGB.). A white pixel is still represented by the same number of bytes of data as any other colour.  A camera producing this number of pixels for each image is known as a 14 Megapixel (or 14MP) camera, but each image would be 40 megabytes (40Mb) in size.

Image sizing in photoshop

Studio JPEG Files Are Often Smaller – Because of the Plain Backdrops

Both images are suitable for printing at high resolution at 14×11”; they have the same number of pixels and the same uncompressed file size (data information). But what about JPEGs? This is how they start to differ. The table below shows that at the highest quality (100%, lowest compression, Photoshop Level 12)) JPEG, the file size for the studio image is 3.34Mb, whilst the location JPEG 100% image is much larger 8.21Mb. We have also included the images saved as Photoshop levels 10 and 8 for comparison (involving more compression, but still regarded as good/decent quality JPEGs).

Although we have used the studio image as an extreme example, there is still quite a surprisingly big difference. Remember that both family images are equally sized pixels from the same camera, have the same high resolution and are both optimal for producing a 14×11” print. (In fact, the 3.34Mb Studio JPEG would actually produce a better, clearer, sharper print, because of the low ISO and high-quality studio lights and camera lens).

Chart explaining difference in uncompressed file size and jpeg file size

So why the difference? JPEG compression is actually a very technical process and well beyond the scope of this post (you can read in more detail at this Wikipedia article). But it might be helpful to think (in loose terms) of compression as being a series computational algorithms or shortcuts.  And then think how easy the shortcut process would be for row after row of the same white pixels (or any uniform background colour).  In contrast, virtually every pixel in the location image is a different colour, so requires much more individualised data attention.

When web designers or printing firms request images of a particular quality or image resolution by denoting the file size, this does NOT make sense when it comes to JPEG image files (only uncompressed image files). They should specify pixels dimensions instead.

Pros and Cons of JPEGs

JPEGs are sometimes described as lossy compression.  But at 100% quality, the loss in information saving a large file to a JPEG format is VERY negligible. You would be very hard-pressed to find anyone who would be able to discern a difference in the file quality saved in full lossless format (eg TIFF) versus JPEG format (viewing either in a large print or magnified on a computer monitor).

The major benefit of JPEGs is, of course, their size and practicality.  In this example, it is much easier to send 3Mb or 8Mb image files over the internet for a top quality 14×11” print compared to a 40Mb file. They will also take up much less storage on your hard drive.

It is true that if you open a JPEG image and then save it again, a tiny amount information is technically lost.  Again, this is negligible if done once, but if done repeatedly, there might start to be a noticeable effect. (Note that this is NOT true if you simply move or copy JPEGs across different media, without opening and saving the file). So, it is often sensible to keep one file as the master copy and create other JPEG versions for other purposes (for example, smaller versions for using on the web or for smaller prints).

Summary

  1. The studio JPEG image (3.3Mb) is not in any way inferior to the location JPEG image (8.2Mb).
  2. The JPEG image files have the same (high) pixel resolution and (low) data compression.
  3. If opened in software package such as Photoshop, both JPEG image files would open up to the full 39.7Mb files, with no compression.

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