It’s a question I haven’t thought much about since the days when I shot JPG images only. Now that’s a long time ago. Back then if you lost the highlights in your image you’d basically lost the image, so it was general practice to underexpose the JPG by 2/3rds of a stop from the cameras meter reading. Yes every photo was a little dark but that could be fixed in Photoshop and you still had highlight detail.
With RAW there’s a lot more exposure latitude and worries about clipped highlights in day to day photography are almost a thing of the past. In fact there’s so much hidden image information in the highlights of a RAW file I know plenty of photographers who will deliberately over expose their images knowing they can recover it in post-production.
Why overexpose?
In a word, noise, and the theory goes something like this. Noise is very evident in the dark, shadowy areas of a photo that are made lighter when processing. But as RAW files have all this hidden highlight detail, if you start off with RAW files that are a bit too bright and make them darker, you won’t add any more noise.
Sounds crazy right? Well, believe it or not it really works… but keep reading because there’s a catch.
Take a close look at the image above, you’ll need to click it to really see what’s going on.
I shot the same scene at three different exposure (Top Row) I then brought all three RAW files into Adobe Camera RAW and edited them so they looked more or less the same (Middle Row). From a distance the three images in the middle row all look the same but a close up says otherwise (Bottom Row).
The bottom row confirms the theory. Brighten an under exposed image and they’ll be more noise evident but darken an over exposed RAW image and the noise is about the same as the correctly exposed image and may be even slightly less!
The catch
So you may be think that I’m going to say from now no you should over expose your images by a stop… well I’m not.
You see, although the lower noise is brilliant there are other things to think about.
First there’s the question of just how much highlight detail you can recover. Take a look at the photo above and you’ll see the problem. Where’s the sky gone on the right hand (overexposed) image? The correctly exposed image (middle row) struggled to keep highlight detail so over exposing is asking just a bit too much, even in RAW.
That’s not all. One stop over exposure means your shutter speed is twice as long and that could be the difference between a sharp shot and a blurred shot. You might be able to counter that by opening up the aperture if the lens / depth of field allows it and of course you could double your ISO but that rather defeats the purpose doesn’t it.
Over, under or as metered?
As is so often the case there’s no rights or wrongs that will apply to every situation and camera meters are prone to errors by design. However for me, I’m sticking to the as metered reason unless there’s a valid reason to do otherwise and I’d recommend you to do the same.
