Shooting underexposed, or “shooting under”, has pros and cons. I’m going to make the case for why I like to shoot under in some situations. You may have varying results with your camera. If you are unfamiliar with histograms and how to use them, please go catch up on those posts first.
First, different makes and models of cameras have different sensors. These sensors have varying abilities to handle high ISO and varying amounts of destructive noise present in shadows. Unfortunately, those are typically tied to price point. An entry-level dSLR won’t handle big number ISO without the introduction of more noise than an investment dSLR’s sensor would introduce at equal ISO.
Noise (or grain for the film peeps) is more common in underexposed shadows. If you underexpose an image too much, the result will be destructive noise.
So if you have a situation that is mostly dark – or your pixels in your histogram are not covering a wide dynamic range – the light and shadows are all sort of flat and bunched together rather than spread across the spectrum of tones on your histogram – then shooting under wouldn’t make much sense, and you’d have little to gain by shooting underexposed.
But many times, your histogram will show you that you have pixels across the entire dynamic range. Your image has pixels in the shadows and in the highlights.
Your camera cannot see the same range your eye can. It can expose properly for the highlights or for the shadows, but not both well in the same click. There are ways to compensate for this – HDR (high dynamic range) uses bracketing, images at several exposures + and – a middle exposure, then combined in post processing. But that’s another post for another day. 
When you have a dynamic range greater than your camera can capture – where you are choosing to lose highlights or shadow, this is when I encourage you to shoot under. And not blacked-out under, I mean only one or two stops.
Why?
I’ve found that having some information in the highlights makes a better exposure and is easier to work with in post. I also believe that clipping (losing pixels because it’s outside the dynamic range of your camera’s sensor) of the shadows is less distracting in a photo than clipping the highlights – which look 255 white and blown out.
When you shoot a stop or two underexposed, you can bump up the mid-tones and shadows in post and you might still have some pixels. The reverse is not true – if you blow out the highlights, those pixels are gone and irretrievable. The result is pure white with no details.
When I’m shooting in a situation where the range of luminance is greater than what my sensor can capture, I prefer to shoot a stop or two underexposed and salvage as many pixels in the highlights as possible. If you can’t tell by looking at your subject, a quick test shot then a glance at your histogram will let you know if there is any benefit to your image by shooting a stop underexposed.
There’s nothing worse than blown out highlights – especially on faces or skin. Try it see if it helps your images!
Tomorrow, I’ll talk about when it’s better to shoot over.
Don’t forget about these awesome 31-dayers, too!
Great post, and I like the histograms. Some examples photographs of the results would be very helpful.
Keep up the great work.
Steve
Thanks Steve!
I’ve found that with varying monitor brightness, varying (and usually not calibrated) color temperatures in screens – that the best way to teach this effectively is to make people rely on the histograms. Providing images makes those who are new to histograms judge the image, relying on how it appears – like a crutch, instead of learning to read the histogram.
Leaving the images out was an intentional strategy to force the readers to judge the pixels based on the histogram alone.
Thanks for stopping by!
this is great. I have recently changed my thinking when shooting weddings to shoot a tad under- mostly in shots involving the bride. in sunny CA its far too easy to loose large areas of the dress from shooting with exposure for the skin, or shooting over at all.
Awesome! I’m DEVOURING your blog – I finished the 31 Days in a little over a week. I couldn’t put my camera down! This series is just as good. I really need a little more on metering and THIS. IS. IT. Great job – thank you!