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Back in February of 2012 I wrote an entry that was posted here discussing the rudiments of the exposure triad.  I want to take the next step by discussing the process of applying the flexibility of the exposure triad to real world, in-field situations.  I’ve adapted a piece that I posted on my own blog earlier this year that discusses these matters, and that adaptation appears below.

I’ve received a number of e-mails asking how I deal with exposure settings in the field.  These aren’t “how do you meter” questions; the queries fall more in line with the notion of “what part of the exposure settings triad do you prioritize” when approaching a particular scene, and why?

The broad answer (and I’m sure you saw this coming) is, “it depends.”  And, to respond to a question that often accompanies the one detailed above–“how will I know what to do?”–it comes with experience.

I’ve had enough of these questions that I thought a dedicated entry covering the subject was warranted.  What I’m laying out here is second nature to experienced photographers, so I apologize to those of you reading this who already know this stuff, but for relative newbies–and even some intermediate photographers who have largely let their cameras choose their settings–this may be somewhat helpful.  Note that, as alluded to above, this isn’t a primer on how to meter a scene per se; it’s more about how to go about prioritizing the process of setting your exposure benchmarks.

The Exposure Triad

Just to clarify, when I use the term “exposure triad,” I’m referring to the basic set of three adjustable settings that impact exposure in combination with one another.  They are, in no particular order:

  1. Shutter Speed
  2. Aperture (f-stop)
  3. ISO

You should be familiar with all of these concepts already and understand that, in the most fundamental sense, all three can be used interchangeably, in equal increments, to impact image exposure in either direction (i.e. make the image lighter or darker).

(That’s as much as I’m planning to say about the true exposure basics in this piece, because I sense that most readers of this blog already understand the fundamentals.  If there are readers out there who would like a basic primer about how the exposure triad functions, let me know, in the comments and/or by e-mail, and I’ll prepare something on that subject down the road.)

The “Correct” Exposure:  A Brief Cautionary Tale

I want to briefly touch upon a related topic:  the notion of the “correct” exposure.  The entire phrase is a bit of a misnomer because the correct exposure for any given scene is the one that establishes the look you’re trying to achieve, whatever that may be.  The classic example would be trying to render a silhouette of some foreground object against a bright sky, or trying to show detail in a foreground object against that same sky.  (Even leaving out the possibilities of multi-image blends and HDR; with many, if not most, current DSLRs, both options are typically available to you.)  The “correct” exposure in this instance depends on your intent.

So don’t buy into the notion that there’s a single “correct” exposure for a given scene.  The exposure of choice is the one that enables you to render your vision in the resulting image.

Examples

Here are some images that will illustrate the point of different exposure triad priorities.  I’ll show the image and then explain what my priorities were and why, and how I went about implementing them.  Note that in many cases there are multiple priorities.

Background:  It was a windy, overcast day.  The breezy conditions meant that the foliage was blowing around quite a bit, and my goal was to freeze the leaves.  Based on a bit of brief experimentation, that meant a minimum shutter speed of 1/30 of a second.  It could be faster than that, but no slower.  The problem was that, given how relatively dark it was (remember, it was completely overcast), achieving those shutter speed at base ISO (200, with the D700 I was using at the time), meant a fairly wide aperture–less than f/8.  But in this instance, f/8 didn’t provide an adequate depth of field to render the entire scene acceptably sharp from front to back.  I needed f/11…and I couldn’t even get f/8!  Focus stacking, using a wider aperture, was out because the movement of the foliage would make compiling a single image from multiple frames impossible (i.e. the elements wouldn’t line up).  What to do?

Raise the ISO.  I always shoot at base ISO when I can…but generally speaking with the last few generations of digital sensors, almost nothing in the way of image quality is surrendered by raising the ISO a few stops.  If you need to do it, it’s there.  In this case, I was able to boost the ISO to 400 and achieve the minimally acceptable shutter speed (though I also waited for a lull in the breeze, just to be sure).

So again…the scene required a minimum shutter speed (to freeze the foliage) and a maximum aperture (for depth of field purposes).  I knew the minimally acceptable settings.  When I found that they couldn’t be achieved with the base ISO, I adjusted the third element of the triad to achieve the required settings.

This one was fairly easy.  I was trying to achieve a simplified hard (rock)/soft (water), yin-yang contrast.  I wanted to slow the shutter speed down a great deal, to get as detail-free a look in the water as possible.  Depth of field was no consideration at all here.  So, I dropped the ISO to it’s lowest level (the equivalent of 100).  I closed the aperture down to f/16.  I put a polarizing filter on the lens.  I still wasn’t getting a slow enough shutter speed.  So I pulled out one of my neutral density filters (a three-stop) and, removing the polarizer, placed that on the lens, then stacked the polarizer on top of that.  Bingo.

Again, this was all about obtaining a very slow shutter speed (in this instance, 15 seconds)–that was essentially the only priority here–and I used all three elements of the triad–plus filters–to realize the goal.

In this case I was just trying to get an exposure that would retain detail throughout the image, so the goal was to expose this as brightly as possible without blowing the highlights (with the possible exception of the sun itself).  I used a middling aperture (f/8), the base ISO (100, with the D200 I was using at the time) and whatever shutter speed (1/20 sec. in this case) would do the trick, with f/8 and ISO 200 as the other components.  I was perfectly happy to let the foreground ridge go black since this was fundamentally a layered image.   There were plenty of other exposure setting combinations that would have provided an equivalent result.

This situation is similar to the first example, but not identical because I was looking at a narrow range of acceptable shutter speeds, rather than a minimum.  Two equally important priorities played a role here.  Because of the foreground objects (the leaf and the rocks), depth of field was a major consideration.  I was looking at f/11 or so.  This image was shot with the D800E and, frankly, diffraction becomes apparent at f/8 with this camera, but sometimes, you need something narrower than f/8 to achieve the desired depth of field.  I’d rather shoot for the necessary DOF and deal with any elements of diffraction, but I do try to keep the aperture as close to f/7.1 as possible.  This scene might have lent itself to focus stacking, but I chose not to go that route.

Every bit as important as depth of field here was the chosen shutter speed.  I was looking for some water blur, but not too much (i.e. nothing like what I was aiming for in the Virgin River example above).  I had the polarizer on the lens and, with a bit of tweaking, I was able to get a shutter speed in the 1/6 of a second range at f/11, using base ISO (100 on the D800E) which was more or less what I was looking for.  That allowed me to render the water the way I wanted to and obtain the necessary depth of field.

Summing Up

You can see that prioritization of the exposure triad depends on what you’re trying to accomplish (which itself will vary depending on the scene you’re dealing with).  With a minimal amount of experience, these kinds of decisions will become instinctive.  If they’re not second nature to you yet, spend some more time in the field.  What better assignment could a photographer ask for?