Monday, July 19, 2010

Size versus Quality

Size versus Quality

I'm trying to figure out the relationship between the file size and the quality of digital images.
To start, I've selected a JPEG image (a photo taken of an oil painting by a great!)
The following series of images are all the same size, but I've progressively decreased the quality. IrfanView allows me to select options for saving JPEG or GIF images.

The only option I changed for each of the following examples is that of quality, starting with 100% and decreasing to a minimum of 1%.
Image #1: firefighter-400x312-100.jpg, 84 KB

Image #2: firefighter-400x312-90.jpg, 28 KB

Image #3: firefighter-400x312-80.jpg, 20 KB

Image #4: firefighter-400x312-70.jpg, 16 KB

Image #5: firefighter-400x312-60.jpg, 14 KB

Image #6: firefighter-400x312-50.jpg, 12 KB

Image #7: firefighter-400x312-40.jpg, 10 KB

Image #8: firefighter-400x312-30.jpg, 9 KB
Image #9: firefighter-400x312-20.jpg, 6 KB

Image #10: firefighter-400x312-10.jpg, 4 KB
Image #11: firefighter-400x312-01.jpg, 2 KB

Severe degradation is not seen until the quality is reduced to about 20%.
A huge reduction in file size is seen at the 90% level: from 84 KB down to 28 KB.
Now, let's use a sharper image to start with.
soda-pop-400x533-100.jpg, 172 KB
soda-pop-400x533-90.jpg, 62 KB
soda-pop-400x533-50.jpg, 29 KB
soda-pop-400x533-20.jpg, 18 KB
soda-pop-400x533-10.jpg, 14 KB
soda-pop-400x533-01.jpg, 9 KB
Again, at about 10-20% the quality is unacceptably degraded. But even at 50%, the image is fine, at least for small images. Probably, the larger the image, the more critial resolution becomes. But for purposes of a blog, even as low as 30% is fine, and the file size is greatly reduced, making site load times much faster.
For larger images, I plan to make several trial versions of the image, selecting the one that is lower in file size, but still acceptably sharp.

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