With todays digital SLR cameras, one would think that the problem of underexposure would be a thing of the past. "Not exactly". The chances of underexposure are still a click away where:
1)deliberate underexposure in manual mode are not uncommon;
2)auto-bracketing at 2 stops or greater will generate a lot of underexposed images;
3)to avoid noise at high ISOs or camera shake at low shutter speed, light has to be given up = more underexposures;
4)Murphy's Law dictates that the best shot has to be underexposed, badly. Just below is an example of case (2) and (4) - autobracketing at the Buskerfest had Murphy providing the best shot badly underexposed:
Note the Histogram of colors is sharply shifted to the left
Fortunately, Photoshop CS4 provides a wealth of exposure corrections tools and we look at four methods that provide robust recovery of underexposed images:
1)Exposure command - gets close but needs selected second application
2)Levels command - also gets very close but needs second approach, the Colormancer plugin
3)Screen Duplicate Layers - is a bit tedious but works well, color balance correction
4)Shadow/Highlight command - Photoshop's software "Flash attachement" did better than expected
5)Some other Methods tried

The Image | Adjustments | Exposure command has become my personal favorite for fast exposure corrections. But I did not expect the command to get close to recovering a badly underexposed picture. I expected it to add too much noise. But look at the results here:
True the Exposure command starts to add too much noise and the photo is shifted to the blue cyan spectrum as can be seen in the histogram; but the results were a lot better than Brightness/Contrast command and most attempts with the Curves commmand. However, a correction for color shift required a foray with the Image | Adjustments | Color Balance command. And often a third step required some local masked corrections for either Filter | Blur | Smart Blur to smooth out noisy splotches or the Image | Adjustments | Curves to correct for contrast and/or specific color shifts.

In sum, the Exposure correction for underexposure got close enough in one try sometimes; but sometimes it took two or more additional corrective steps.

I use the Image | Adjustment | Levels command when an image has a badly shifted Color histogram - which is of course the case with an Underexposure. So this was my second choice for underexposure correction - and the results were impressive:

It is important to note that I used levels' white eyedropper on the poster in the upper left to establish the white sample. Then [and the order is important] I brought the Output Levels black shader up to as high as I could without introducing to much gray or noise in the overall image. Note the highlights and whites are not blue-shifted as in the Exposure and Screen methods; but there is a notable red shift - this can be corrected in the Levels' Channel dropdown. Just choose the Red channel and watch the histogram as you correct for the overdose of red.

Again, like Exposure and Screen corrections, two or more additional adjustments were required. I used the Image | Adjustments | Shadow/Highlight command to great effect. In general I find that in most images [but not ones with large expanses of a flat color], Levels gets very close and then only one additional correction is required. Perhaps that is because color shifts can be handled within the command itsself. the net result is I have tended to try to fix underexposures with the Levels command first.

I learned theScreen trick at the Toronto Camera Club from a member who used it with Corel Paint Shop Pro. The trick - duplicate the starting layer and converts its color mode to Screen. Then repeate this duplication process until the image becomes too light [usually in the highlights]:.
As you can see in the screen shot the crossover point was adding the sixth layer. Then scale back the Opacity of the layer [in our example it is reduced to 46%]. Note that the highlights are not bleached out and the people standing at the bottom of the image are not lost in gray darkness. The downside to the screen method is a blue/cyan shift in color and the breaking of smooth colors or gradient colors into splotch artifacts. A smoothing operation like Filter | Blur | Smart Blur is often required. The Screens method of underexposure correction is remarkably workable. The clear upside is that the the resulting image is that clear all over - no lingering grays in the highlights or shadows. The downside is that two added corrections are often required - smoothing the finish with Image | Adjustments | Smart Blur and color corrections with Image | Adjustements | Color Balance. But unlike other underexposure methods, rarely did Screens require a local correction for lingering darkness/grayness on specific portions of the image.

The Image | Adjustments | Shadow/Highlight command is often touted as Photoshop's Software Flash Attachment.And Adobe has rigged up Shadow/Highlight with enough controls that it certainly reminds users of the complexity that can be using real hardware lighting and flash.

So given the above screenshot, I admit that it is very difficult to explain how to use the Shadow/Highlight exposure correction tool in detail - the command will be the subject of an upcoming review in detail. Here is what I found in working with Shadow/Highlight with underexposed images.
1)You often have to pump up the Amount by 10-30%;
2)Also increasing the Tonal Width brought up midtones;
3)Highlight settings were all over the place - just use a full sweep to see what works best. Note how the whites are reasonably true in the screenshot;
4)Saturation and MidTone Contrast were the most contentious settings introducing noise and/or dramatic shifts- use these carefully.
In sum, I worked quickly to get good enough in Shadow/Highlight but to coax superb adjustments requires a lot of extra work.

Other UnderExposure Adjustments

Photoshop has 21 commands in its CS4 roster of Color and Exposure Adjustment commands [just open Image | Adjustments menu item and see for yorself]. The two most prominent exposure adjustment
tools are Image | Adjustments | Curves and Image | Adjustments | Brightness/Contrast . Both tended to produce colorshifted and contrasty or noisy adjustments. In the case of Curves I was able to save some presets that got close; but still the number of secondary corrections was at least 2 or 3. In sum, I found Levels and Exposure able to get me closer, faster and with less "distress". But rember my Uncle's proverb: "to each his own poison" because rescuing badly underexposed images is a dirty business.

This tip is proof that photofinishers should not give up on underexposed images - even ones badly underexposed. As shown there are a lot of methods for correcting underexposed images in Photoshop. And everyone can be "made to work". However, the two that worked with the least secondary corrections required were the Image | Adjustments | Levels command and then the Image | Adjustments | Exposure command . As noted above Levels consistently required the fewest additional adjustments - and as can be seen below produced 'rescued' images that are a vast improvement over the original.

However two warnings are clearly required. First, underexposed images are to be avoided, all the 'rescued images' had colors that were inevitably shifted, a grey veneer in many cases, more noise, and sometimes outright splotchiness. I now tend to narrow my bracketing to 1/2 stop especially in poor lighting conditions and try to avoid under [or over] exposures in general. The second warning is that underexposure can often be tolerably rescued - overexposure much less so. So if you have a choice err on the side of underexposure.