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Tuesday, August 21, 2012

Look Into My Iris

Understanding Apertures • An Eagle-Eyed Reader • Calibrating The Color Of An iPad • Speed Up Your Laptop

Labels: ColumnTech Tips
In summary, the photographer needs to have an idea of how much depth of field is needed to cover the subject and achieve the best composition, where the best sharpness of any particular lens is, and what aperture will still give the correct exposure. These variables are combined with shutter speeds (time of exposure) and ISO settings (sensitivity of the sensor) to achieve the best possible rendering under varying conditions. The very best way to become comfortable with these principles is to practice various combinations with a variety of subjects, and carefully observe the results they give you until the choices become instinctive.

For an excellent tutorial on the relationships between depth of field and aperture settings, see www.cambridgeincolour.com/tutorials/depth-of-field.htm.

An Eagle-Eyed Reader

Q In the June 2012 issue of OP, you published a photograph of a bald eagle taken at 2912mm using a Canon EOS-1D Mark IV (1.3X crop factor), EF 800mm f/5.6L lens and EF 2X and EF 1.4X tele-extenders. You indicated your exposure was 1/1000 at f/16 with ISO 400. The sunny-day rule (f/16 rule) indicates that in full sun, which it looks like the eagle is, the ISO would be the equivalent of the shutter speed at f/16. Was there a misprint, or am I missing something?
B. Guiliani

A Good catch! The correct ISO for the eagle image featured with the column was 1600, not 400. The Sunny 16 rule, which is a standard basic exposure for film, still has merit with digital capture, although we have more control now. In this situation, the 1000 ISO that the rule suggests offered insufficient light on the subject, as was demonstrated on the histogram of a previous exposure; I therefore manually selected 1600 ISO for a better result.

Photographers in the digital age should make good use of expanded ISO for exposure control. While higher-speed films were (are) available for use in low-light situations for film photographers, the resulting loss of quality (graininess and noise) is far from acceptable. Now, each new generation of digital camera offers increasing capability for expanded ISOs with excellent results. Manipulating the capture ISO is no longer a last-ditch effort to save the day; it has become a critical third factor in calculating exposure—as important as choosing the shutter speed and aperture.

There are a number of ways to arrive at the optimal exposure settings for any given photographic situation, but because ISO does have quality implications, I tend to first determine the optimal shutter speed and aperture setting and then adjust the ISO to enable them, keeping in mind that there are upper limits. For example, when photographing flying birds, I choose a shutter speed to stop the action, an aperture for needed depth of field and then adjust the ISO until the shutter speed/aperture combination is achieved. Many DSLRs can make the ISO adjustment automatically, calculating the ISO needed to capture under the prevailing light conditions, shutter speed and aperture settings. But you can still override these settings to fine-tune the exposure based on the results you're getting.


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