How do you fix a dull sky of a digital photo (on Mac OSX)?

• first, the caveat. the technique I'm going to describe is something I consider heavy editing (contrast this with light editing, which is no more than cropping and rotation of an image). what you get out of this may not be an honest reproduction of the scene you photographed and depending on your ethical alignment, it may seem like cheating to you. and the problem this technique fixes can always be avoided if you use the right exposure and white balance while shooting the photo, at least in theory.

it's common to shoot a digital photo which has a very dull looking sky, devoid of colors. this happens most often when you shoot it on a bright sunny day and let the camera decide the exposure for you.

the good news is, the dull sky can be fixed using a photo editing software. Internet is ripe on tutorials which teaches you how to accomplish this using Adobe's PhotoShop. however, I couldn't locate anything that tells me how to do something similar with the applications that came bundled in my Mac OSX (Snow Leopard). ok I was somewhat lazy and didn't look beyond the first page that google returned for a handful of queries I formulated.

ultimately, I figured this out using Preview.

• first, use Preview's instant alpha tool to select just the sky from your photo. select instant alpha and then click on the sky. 

here's the photo with the sky selected by intant alpha.

• next, cut (cmd+x) the selected sky off the photo.

• copy the sky as a new image using File > "New from clipboard". 

• switch to the new Preview window with just the sky. 

• so you have just the sky as an image which you can play with as you like. in this case, we'll fix it's color. select Tools > "Adjust colors".

• the small "Adjust color" window gives you a bunch of sliders to vary exposure, contrast, saturation, color temperature, etc. 

• play with the sliders till you're satisfied with the color. in this case, I've reduced the exposure and the contrast a bit and pushed the temperature towards blue. 

• here's how the sky looks like after the adjustments. now select the entire image (cmd+a) and copy it to the clipboard (cmd+c).

• switch back to the Preview window which has the original image sans sky. paste the sky back to it (cmd+v). and that's it. save the edited image.

Before

After

Here's a good tutorial on exposure that'll help avoid the problem to start with.

Happy clicking!