Measuring & Exposing for Dynamic Range

Reader, friend and fellow photographer Todd B asked me to go into more detail on how I measure the dynamic range of a scene and then decide how I will shoot the exposures for that scene. 

What I do is quite simple. I set my camera to Aperture Priority mode and the aperture and ISO I will be shooting with. I then set my metering mode to spot. I use aperture priority for this instead of Manual because I am just looking for numbers (shutter speeds) right now. I may, and probably will, end up shooting in a different mode, most likely manual. 

I then seek out the brightest and darkest areas of my scene. If the sun is in the shot, don’t measure it for many reasons. First off it’s not good for your eyes or your camera and secondly because of its brightness you will end up with exposures that in reality have very little use. If the sun is just at the horizon line you may be OK, but anything above that you are asking for trouble. But in most circumstances if the sun is in my image I will meter slightly to the side or above it. 

Also make note of one phenomenon, just as the sun hit the horizon it is not always the brightest region of the image and the clear sky above or a reflection off a cloud may actually register higher 

So setting my spot meter on the brightest area I will take note of the shutter speed at that point, I then search my field for the darkest area of the scene and take note of that shutter speed. Now the only thing I need to do is connect those two with a number of exposures 

So we know that using our camera’s reflective meters (or even a handheld reflective meter) that meter is going to meter for a midtone and that’s OK because we really don’t want that brightest or darkest area to be anything but a midtone at the most. There is no point in taking a highlight and making it a shadow nor a shadow and making it a highlight. 

Shooting exposure beyond those two measured midtones will lead us to shooting way too many exposures which lead to problems with alignment and processing speed later. Also, shooting exposures beyond what we measured leads to problem with noise in shadows and loss of detail in highlights (Bloom)

Deciding on Exposures

 The first thing I will think about in my head is, how many stop range is the dynamic range of this scene. Luckily since we are using shutter speed it’s easy to think in your mind how many stops. A doubling or Halving of Shutter speed is one stop. 1/100 to 1/200 = one stop. 

If I have less than 4 stops, I probably won’t shoot an HDR at all. There really is no point; our cameras are able to cover that dynamic Range. 

If I measure at or around 4 stops I’m probably just going to use  AEB (Automatic Exposure Bracketing) with  a shutter speed in the middle of the two ends as my starting point and just do 3 exposures +-2EV(stops). This is really sufficient for 1/.2 of the scenarios we come across outdoors. 

If it is beyond that I will shoot 1 step exposures going from one end to the other, no matter what that number of exposures is. And I don’t sit there and calculate it out. I start with my camera set at one end of the range and then just turn my shutter exposure dial 3 click for every stop (my camera is set for 1/3 stop intervals) Until I see my final exposure is at the other end. 

Some of the newest camera on the scene now allow for 7 or 9 exposures AEB, if you’re lucky enough to have one you can set yours up instead of the method I use. This also helps eliminate and camera shake even on a tripod as moving a dial can cause. 

After I shoot my first series, I will go through and check my histograms and make sure I have pretty much pegged each end. If I see one or either end not quite to the end I may add an exposure or two 

The only thing left to do if you are shooting close to sunrise or sunset is to keep an eye on the starting point of your exposures as the light changes every 10 minutes or so. This is when I have my handheld exposure meter handy so I don’t need to take the camera off the tripod to get a reading. 

Then after the sun sets,  you should measure the entire range again as it will change quite dramatically shooting into the Blue Hour as the Dynamic Range lowers considerable although the first 10-15 minutes of Blue Hour the DR of the sky is still quote high. 

And that’s all I do