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Forum: Troubleshooting and Beginner Help
07-18-2011, 07:18 PM
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Here is a graphical guideline for selecting sweet spot for aperture for digital cameras of all types of sensors. This graph is established based on Digital Camera Sensor Sizes: How it Influences Your Photography.
To find out the sweet aperture for your camera, first identify the type of camera/sensor you have. Then find out its sensor resolution in term of mega-pixel counts. From the horizontal axis, draw a vertical line from the sensor resolution to meet with the curve that corresponds to the camera/sensor you have, and then draw a line left to meet with the vertical axis. The intercept will be the so-called diffraction limited aperture, which is the smallest aperture before triggering the diffraction that degrades the image quality. Typically, the sweet range of the aperture is within two stops larger than the limited aperture. As also noted in the reference, this is a theorectical limit and the diffraction may not kick in at this limited aperture right away.
For example, for Pentax Kx or Canon Xsi (12 MP), the diffraction limited aperture is around f/10. The range of sweet spot is typically between f/5 - f/10 theoretically. Some readers may wonder how is this guideline applied in the field, since there is no f/10 setting in camera. Experiments (DIY lens test) actually show the camera (Xsi) is still able to resolve well at f/11. See Lens Tests - Stepwise Photography. We could therefore conclude that the sweet aperture for most of APS-C with 12 MP max. resolution is between f/5.6 and f/11.
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