Auto Exposure

Beautiful rolling hills with patches of trees
Picture taken just north of Turner Valley along highway 22 looking east
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Modern digital cameras are excellent at automatically calculating the correct light exposure in most situations. Not only is the amount of light that enters the lens calculated, but the camera’s computer calculates the difference between the dark and light areas and takes color into account as well. In the more sophisticated cameras a statistical graph of the light distribution from dark to light is computed and displayed on the camera’s LCD screen. Even the different color distributions from dark to light can be displayed. These graphs are called “Histograms”. When the camera is set in one of the automatic modes, it tries to adjust Shutter Speed, Aperture and ISO in such a way to optimize this histogram for the best quality result. (Click on the images to enlarge, use the Browser Back Button to return here.)

Of course not all lighting conditions are ideal, and optimizing a histogram does not always produce the best aesthetic results.

The automatic exposure settings on most cameras are “A – Aperture Preference (Av for Canon)”, “S – Shutter Preference (Tv for Canon)” and “P – Program”. Aperture preference sets fixed opening size on you lens. This is most often used to control “Depth of Field” or how much of the near objects are in-focus. Shutter preference is most often used when you need to “freeze” fast moving objects. You typically use high shutter speeds for that. Or sometimes you might want motion blur pictures, where you would use a slow shutter speed. In A mode, the camera’s computer adjust the shutter speed (and ISO if that is set to Auto) to get the best exposure. In S mode, the aperture (and ISO) are adjusted automatically. The best exposure means the best distributed histogram. In P or Program mode the camera adjusts both shutter and aperture (as well as ISO when on Auto) for the best histogram distribution. The camera’s preference in this mode is typically the shortest possible shutter speed at the expense of aperture and ISO in order to prevent blurry pictures due to camera shake. This is almost the same a setting the camera to fully automatic. However there are a few items you still have control over, such as “program shift” and flash which remains manual.

The challenge is to try these setting out in different circumstances. I will try to do this in the space below this description.

If you have particular questions or problem examples, post them in the “forum” under “open discussion”, and I will try to incorporate them in this page.

Henri


Auto Exposure – Aperture Preference Exercise

May 30, 2017 – This evening we went out to photograph wild flowers on the Silver Springs Escarpment above the Bow River . The east facing slopes of the hilly terrain offer great opportunities, especially in the evening. There are many wild flowers to choose from in late May.  The light is warmer in the evening and the backdrop is relatively dark with the sun at a lower angle in the west. The sun rays come in at a particularly low angle in the evening around 7:00 pm on the steeper east facing slopes. This provides a great opportunity for back-light photography. Below an overview of the flower abundance on one of the slopes.

The objective is to start with a reasonable depth of field, say 2,5 cm (1″) which is achievable by setting the aperture to a relatively high value. I chose the Auto Exposure method –  “Aperture Preference” set at f/16. I tried my 100 mm Macro lens and got quickly frustrated with the fluctuating wind blowing the subject out of focus just as I was ready to press the shutter release.

The biggest problem with even a mild breeze, is that flowers move in the wind. Macro lenses  already have a shallow depth of field, which is a problem when you are trying to compose a picture at the relatively close distance of 50 cm, which gives you a depth of field of roughly 2 cm for a 100 mm macro lens, or even less for a standard 50 mm lens.

The solution turned out to be my big 100-400 mm zoom lens. At 3 meters distance with an aperture of f/11 and zoomed in at 400 mm, I got a depth of field of about 5 cm. By turning the ISO up to 1000 the Auto Exposure system (Aperture Preference at f/11) chose reasonably high shutter speeds that “froze” the moving flowers, and with my zoom at 400 mm I was able to compose decent pictures that had potential for post processing.

Star Flowered Solomon’s Seal
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