Achieving Correct Exposure - Part Two

The spectacular Gullfoss waterfall in Southern Iceland photographed, through the spray, during the afterglow that followed a beautiful sunset.

Here's the second part in a series of posts dealing with correct exposure and how to achieve it with your DSLR camera. Today's post deals with the basic principal that determines how light meter's in DSLR camera's measure light, under what circumstances they get it wrong and how you can go about overriding the meter to achieve the desired result. 

Just Your Average Guru

I don't want to overstate my expertise in any area other than an ability, and a determination, to explain difficult concepts in simple terms and share that information widely. As correct exposure is such a difficult concept to grasp I thought it was past time that I attend to it here. I hope you find this series of articles helpful. If you do, please SHARE.

When it comes to exposure there remain certain situations when it's very difficult to achieve correct exposure. Reflections, predominantly light or dark toned scenes and certain fast moving subjects (e.g., bright racing cars) can easily fool your camera's exposure meter. Back in the day you'd employ various techniques to help ensure optimal exposure, including a Kodak Gray Card and a handheld incident light meter.

The Camera Doesn't Recognize Subject

One of the greatest revelations for me, as a photographer, came the day I realized that our camera's really have very little idea about what it is that we are pointing them at. Put simply, your camera doesn't know whether you are photographing a baby, a Bar Mitzvah or a birthday cake.

The Portrait, Landscape, Vivid and Monochrome settings within the menus of some Canon (Picture Styles) and Nikon (Picture Controls) DSLR cameras don't relate to how the photograph is recorded, but how it is processed. It's important to understand that a digital camera, when used in JPEG mode, is a kind of photo lab as well as a camera.

Interestingly a fairly significant departure from the above seems to occur with the sports, landscape, portrait, candle light, sunset, etc., settings within the "so-called" Intelligent Auto exposure modes of many point and shoot cameras. By engaging one of these settings within the Intelligent Auto mode you seem to be guiding the camera to make a range of exposure (i.e., shutter speed, aperture, ISO), and color temperature (i.e., white balance) adjustments in addition to the kind of processing (e.g., color saturation and sharpness) adjustments made within the Picture Style and Picture Control settings within many DSLR cameras.   

Once we accept that the camera has little idea about the specific nature of the subject or scene it is depicting it's easier to accept that it doesn't always achieve correct exposure. Confused? Think of a traditional near white (as it contains texture it's not quite as bright as pure white) wedding dress. The camera doesn't know you are photographing a bride, let alone a bride in a near white wedding dress. 

Because That's What Camera's Do

To make it worse our cameras incorporate built in light meters that measure light reflecting off surfaces back towards the camera. The trouble is that, while they do measure brightness, they don't know how bright the area they are reading should be. Why? Because they don't recognize subject. As a result manufactures design light meters to record what they see as a mid tone: a level of brightness half way between jet black and pure white.

If your meter reads the face of a Han Chinese bride (without too much makeup) you can expect a relatively accurate result. Why? Because the average Han Chinese person has skin which is mid tone on the brightness scale and, therefore, perfect for the camera’s light meter. Now take a close up photo of her dress, excluding skin tone and background from your composition, and be shocked by the result. The near white dress will be recorded as a mid tone. Why? Because that's what camera's do. Move back and compose your image so that it includes a full length of the bride against a dark green wall. Who knows what kind of exposure your camera will produce?

Spot Meter To The Rescue?

Have you ever heard the saying that a little bit of knowledge is a dangerous thing. This can very much be the case for folks who use their camera's little meter on Spot Metering mode. The idea is that, rather than being fooled by having to consider varying degrees of brightness throughout the frame, the meter can be pointed at the most important area (e.g., the bride's face) so as to achieve correct exposure. Anything lighter (e.g., bride's dress) or darker (e.g., dark green background, grooms black suit) in tone will, theoretically, photograph proportionally lighter or darker than the area your camera's spot meter based its exposure upon.

I say theoretically because the difference in relative brightness between the darkest and lightest parts of the frame (i.e., scene brightness range, contrast or dynamic range), as well as the particular sensor used to record the image, will effect how much detail can be recorded in those areas. But while contrast is intimately connected to exposure, let's table a discussion on it for another day.

While I use Spot Metering every time I make a photograph I understand that, fundamentally, spot metering suffers from the same problem that afflicts Matrix (also known as Evaluative) and Centre Weighted Averaging metering. All are only accurate when the subject they are pointed at is of mid brightness. If not, this is what will result.

  • Predominantly light tone scene photographs too dark
  • Predominantly dark tone scene photographs too light

Why? Because that's what cameras do. Fortunately, mid tone scenes photograph, at least in relation to brightness, as you might expect them to do.

In the case of our Han Chinese bride we're in luck. But what happens when you point the meter at the face of her fair skinned beau from upper Lapland? Under exposure, that's what. OK, how about use spot metering to measure light from the face of her Nigerian bridesmaid. That would produce an overexposed (i.e., overly bright) image as the light meter will try to render the dark skin as a mid tone. As a result every other part of the image will record brighter than it is in reality.

Solutions

If you're going to employ spot metering you need to take action by doing one of three things:

  1. Point the spot at a mid tone within the composition. That area will be correctly exposed and, as a consequence, other areas will record lighter or darker in relation to it.
  2. Point the spot at an important focal point (e.g., subject) within the frame and then adjust the camera's exposure, up or down, from the mid gray (US spelling) reference point the camera has chosen for you in relation to that subject's actual brightness.
  3. Photograph in RAW exposure mode, rather than JPEG, and employ the Expose to the Right (ETTR) method.

During my years as a film-based photographed option 2 became the practice by which I achieved correct exposure. It's a simplified version of the Zone System, made famous by the great American landscape photographer and educator Ansel Adams. That system still holds today, though to a lesser degree due to the reduced dynamic range of most DSLR cameras compared to that of black and white film, for folks who set their camera to JPEG mode.

These days I employ the Expose to the Right (ETTR) method for single image exposures. But this method, which I'II explain in a subsequent installment of this series, is dependent on the photographer processing their images in a RAW Converter such as Adobe Lightroom, Adobe Camera RAW or the proprietary (e.g., Nikon, Canon or Sony) software packaged with your DSLR camera.

JPEG versus RAW

The discussion as to whether RAW or JPEG exposure is best is, to my way mind, irrelevant. What's important is which method is most appropriate to your own individual situation. Here's My View on that particular debate.

Likewise it's inappropriate to include High Dynamic Range (HDR) photography in this article. HDR, my which I refer to its Tone Mapping capability, is not so much about achieving correct exposure as it is about managing contrast under circumstances where the dynamic range of the scene is beyond that which the camera can record in a single exposure.    

Kodak Gray Card

Before I got my head around the Zone System I employed, for a time, a Kodak Gray Card to arrive at a relatively accurate mid gray exposure. Generally available as an 8"X10" card it was mid gray on one side and white on the other. The idea was that you placed the gray card within the scene and, by moving in close (being careful not to cast a shadow upon it) and filling the frame with the card, you would be able to set your exposure based upon a mid gray tone. Correct exposure, theoretically, would then follow.

But, while handy, they were pretty unsophisticated tools. Over time they would fade, due to exposure to UV light. And while they worked quite well for still life, landscape and architectural images, they were inconvenient for portraits. It's bad enough holding up a hand held light meter in front of your subject. Imagine how they'd feel if you were to hold up an 8"x10" card in front of their face. These days a range of smaller, more portable and longer lasting accessories are available.

I hope you found this article to be of value. This series on achieving correct exposure will continue.

Glenn Guy, Travel Photography Guru