JPEG Jaggies

What are those blocky chunks in the clear blue evening sky? It's a JPEG "artifact". Except, unlike most ancient artifacts, these aren't worth any money, not even on the black market.

These artifacts are the result of the technique JPEG uses when compressing images. The higher the JPEG "quality" when you save it, the fewer artifacts you'll see. (And the bigger the file will be—that's the tradeoff!)

Good quality (85%) JPEG.

Low quality (20%) JPEG.

Not only do artifacts show up around gradients, but also at the boundries of high contrast objects, such as black text on a white background.

The best way to combat these is to have the camera record the image in highest quality. Also save at higher qualities after you've finished messing with the image in Photoshop.

SLR cameras often save in RAW mode or RAW+JPEG. Raw images are direct dumps of the data from the sensor without any lossy compression applied. As such, the images are huge (8-12 MB), and you have to load them up with special software before you can save them out in another format.

(Adobe Photoshop comes with support for RAW format images from a variety of different cameras, and your camera itself should have been shipped with some barebones software for doing the conversion from RAW to something else.)

So what is the advantage of RAW? One is that you can save the image in a non-lossy format such as TIFF that won't suffer from banding issues like JPEG will. Also, it's easy to fix things like white balance with a raw image. Finally, you can rest easy knowing that you have a copy of every single bit of data the camera picked up for the image, and not just the ones that were saved in the lossy JPEG.