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Controlling Exposure


Canon 5D mark II camera and Canon 24-105mm f4 L series USM lens

Canon 5D mark II camera and Canon 24-105mm f4 L series USM lens at 45mm Exposure Details: 1/2 second @ f22 ISO 1600

Here’s an image made last Saturday night at a photographic workshop I was running in Central Victoria. Following on from a wonderful dinner, prepared by my good friend Joseph, I had a few minutes to play prior to the evening session.

This image is a fairly straightforward shot of the wonderful wood fire that helped provide a comfortable atmosphere for our lecture space. I always use manual exposure and spot metering, as I believe they provide me with maximum control over exposure. When the camera is set to spot metering the camera calculates what it believes to be correct exposure by measuring light only through the very centre (spot) of the viewfinder. But, just like all other metering modes, spot metering suffers from a major flaw that can adversely affect exposure.

All light meters, including the one built into your camera, are designed to record what they see as a tone that is exactly half way between jet black and pure white. In a black-and-white image that tone is referred to as mid gray. When photographing a scene compromising predominantly mid tones your light meter should produce an accurate exposure. However, when photographing a light or dark tone scene the light meter will render what it sees, regardless of whether it is light or dark in tone, as a mid tone. As a result the light tone scene will photograph too dark, while the dark tone scene will photograph too light. It’s important to understand that this happens with every single camera that incorporates a light meter measuring light reflected back towards it from the subject.

The idea of spot metering is that you can determine what the most important part of the scene is and, by pointing the centre of the viewfinder at that area, you can tell the camera to render that area as a mid tone. But, if your goal is accurate exposure, this approach would only be appropriate when the meter was pointed at a mid tone.

As the above image contained no mid tones I pointed the camera’s spot meter at the flames in the back centre of the image. I decided I wanted to record those flames at two stops brighter (one stop brighter would make them twice as bright as a mid tone) than a mid tone. That would make the flames four times brighter than a mid tone, rendering them as very light tones containing only subtle highlight detail. To achieve this result I set my camera’s light meter to +2 stops above the meter as read (MAR) mid tone setting. I then adjusted my framing to achieve the desired composition and made the picture.

With the camera set to manual exposure it’s quite possible that the meter will jump around as the composition changes. The meter’s spot is simply reacting to changes in brightness as it passes over different areas within the scene. However, on manual exposure the Shutter Speed and Aperture had already been locked in (manually, by me) when I set the camera to the MAR +2 exposure. The meter will go up and/or down, depending on where the spot is pointed, but the actual Shutter Speed and Aperture will remain constant, unless I choose to manually change them.

I added a little extra contrast while processing the image in Adobe Lightroom 2 and Adobe Photoshop CS3, where the original color image was rendering into black-and-white via a Black and White Adjustment Layer. By the way, in case you’re wondering why I needed to shoot at ISO 1600. I didn’t, I was just doing some testing to see how the camera performed at high ISO settings.

Please check out tomorrow’s post where a more subjective rendering of the fire was produced, only a few short moments after the above image.

© Copyright All Rights Reserved

Glenn Guy, Blue Sky Photography


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