Are you ready for the latest in my sporadic series combatting equivocal, ambiguous, or downright fallacious information that has made it's way around in the popular media about photography? Let's go…
For this chapter, I'm taking on something I heard on Photofocus podcast # 85, released on August 25th, 2011. (Photofocus is an excellent podcast, by the way…) In this podcast, professional photographer Scott Bourne and a rotating guest host (pro photographer Joseph Linaschke, in this case) take questions about photography sent to them via email or twitter. The question I'm going to tackle here is one about the best way to "expose to the right," a common technique in digital photography. "Exposing to the right" is a way to use the histogram on your camera to increase (lighten) your exposure as much as you can, to allow you to maximize the amount of information you capture without losing highlight detail. I'll describe this technique in more detail in a future post, as well as something on the basics of exposure.
The questioner asked if there was a quality difference between using exposure compensation, adjusting the ISO and changing the aperture/shutter speed combination. Scott started his answer by saying "When you start talking about ISO, then ISO should be ISO and has nothing to do with exposing to the right." Misinformation alert!
In this answer, Scott appears to be reminiscing about his days shooting film. In any photo, exposure is a combination of three factors: ISO (the sensitivity of the sensor), aperture (how wide the hole in the lens is that lets in light), and shutter speed (how long that hole stays open). In the days when film was king, you really only had control over aperture and shutter speed; while you were out shooting, you were stuck with whatever ISO film you had loaded in your camera, unless you wanted to rewind it and put in a new roll partway through a shoot. One of the advantages of digital is that you can change the ISO from shot to shot, if you want; many cameras also include an "auto ISO" feature, where you decide on the lowest shutter speed you're willing to accept and the camera dynamically sets the ISO to ensure you don't go below that speed.
So lets put this in practical terms… Say you're out shooting, and you take a shot at f/8, 1/250th of a second and ISO 100. You check your histogram, and notice you have a little bit more room for highlights, so you decide to take another shot, this time to expose to the right, and you want to increase the exposure by one stop. You could open up the aperture to f/5.6, but f/8 gave you the perfect depth of field that you envisioned for that shot… You could also change the shutter speed to 1/125th of a second, but then you might get a little motion blur from some elements of the shot that are moving around… ISO to the rescue! In this case, to increase the exposure by one stop, you could keep your aperture and shutter speed the same and increase your ISO to 200. That would quite effectively slide your exposure over to the right without changing the other artistic elements of your photo. You do, however, run the risk of increased noise with an increased sensitivity. (Didja really think you could get something for nothing? There ain't no such thing as a free lunch.)
The bottom line is that you CAN use ISO to adjust exposure in order to expose to the right. Take a look at your camera manual to see how to adjust the ISO, and go forth and photograph!
Looking for more great photography related info? Check out the PhotoFocus blog and podcast and theThis Week in Photography blog and podcast.
