Mike DelGaudio

For basic, quick photo editing you can’t beat Picasa, Google’s Photo software.

Many times, you may find your images underexposed, often as the result of weak batteries in your flash, or the flash simply not firing. Don’t fret. Picasa can usually salvage an underexposed image, if you pay attention the the histogram.

If you don’t have Picasa you should . It’s free and really worth it.

To see the histogram of your image, go to the photo detail view of a picture (by double clicking an image in the Library.) Then click the little propeller in the lower right

A histogram is a simple chart that shows you the quantity of each color in your photograph, with darks on the left and lights on the right. An underexposed image will have a histogram that us predominantly loaded up on the left:

whereas an over exposed image will be loaded up on the right.

And a picture that has good exposure generally has it’s histogram in the center:

Now, this is not a hard and fast rule, but for general purposes it holds true.

So, how do we correct exposure? First, you need to know that if a picture is really over exposed, there will be little that you can do. Once an area has gone all the way overexposed to white, you can’t get the details that were there back. But, underexposed? That we can work with.

Once you are in the photo detail view, click the Tuning tab on the left. There you will see four sliders: Fill light, highlights, shadows, and color temperature. For this tutorial we won’t need the Color Temperature slider.

Start by sliding the fill light slider to the right until the histogram stretches about 1/2 to 2/3 of the way across the available are in the graph. Then switch down to the Highlights slider and slide that to the right until the histogram goes from 3/4 of the way across to all the way across. Look at the image as you go, if it gets to bright or grainy, back off a bit until it looks better. Now, I like to slide the shadows slider just a bit to the right to deepend some of the shadow areas lightened by the fill light. Make sure the histogram does not bleed too far off either end of the histogram, it should be a nice distribution across the whole chart.

Here is a video to demonstrate on an underexposed picture.

I learned all about Histograms from Luminous landscape. It’s techie, but a worthwhile read. If you want the real in-depth goods on histograms, read it.

I hope you find this tutorial helpful, and if you have any tips or tricks regarding correcting exposure, I’d love to hear about them in the comments.