Originally Posted by HDNitehawk
At narrow apertures, a loss of sharpness due to diffraction begins to show up. In theory, anything beyond the diffraction limited aperture (DLA) for your sensor will be affected. For the 7D, the DLA is f/6.8. Within a few stops ofthe DLA, it's not that noticeable - but once you get down to f/14 or so it becomes more evident.
Most of the time, you're not stopping down that far - but in the 'quest for DoF' with macro shooting it's more common. It's also seen in landscape shots (another area when you want deep DoF), but since those are often taken with wide angle lenses apertures don't need to be quite so narrow.
Personally, with my 7D andEF 100mm f/2.8<span style="color: red;"]LMacro IS, I generally try to keep the aperture at f/11 or wider.
One other point for those using a 7D and a Canon macro lens for handheld shooting - if you set your autofocus mode to AI Servo AF, the 7D'sIntelligent Macro Tracking function will help counteract any back-and-forth movement you make while handholding the camera. That feature combined with the Hybrid IS in the 100mm L Macro IS lens means you're controlling for camera shake in all three dimensions.
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