DPanswers
A primer on sensor sizes
Depth of Field (DOF) Examples
What happens to the DOF when he size of the imager changes? The answer depends upon on what you don't change. If you change the focal length to keep the field of view constant, theory predicts the DOF will increase as you move to a smaller sensor. If, on the other hand, you use the same focal length, the DOF will decrease as you move to a smaller sensor.
Same Field Of View
The two images below demonstrates how the DOF changes if the size of the sensor or film changes but the field of view is kept constant.
Figure 8 is taken with a Kodak DCS 460 digital camera with a sensor slightly larger that APS-C film. Figure 9 is captured with a Canon Powershot G5 which has small 1/1.8" sensor. Both images has been taken from the same position with respect to the the subject and with the same aperture and focusing distance. Both images have been converted from RAW with ACR, and no post-processing or sharpening has been applied to either image.
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As can seen from the images, the image captured on the largest sensor has the smallest DOF.
Same Focal Length
The two images below demonstrates how the DOF changes if the physical size of the sensor or film changes while focal length is kept constant.
The camera and lens used for this test is a Kodak DCS 460 and a Tamron SP AF 24-135 mm macro f/3.5-5.6. In figure 10 the image from the entire sensor (27.6 x 18.4 mm) is used. In figure 11 the image is cropped to simulate the tiny 1/1.8" sensor (7.2 x 5.3 mm) used in popular compact digicams such as the Canon Powershot G5. Using the same capture for both images ensures that position with respect to the subject, focal length, aperture and focusing distance is identical. There is therefore only one independent variable: sensor size. The dependent variable is magnification, which depends upon sensor size. The images have been converted from RAW with ACR, and no post-processing or sharpening has been applied to either image.
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As can seen from the images, when we magnify the crop simulating the smaller sensor to appear beside the image captured on the large sensor at identical size the observer translates this greater magification into a perceived more shallow DOF - i.e. image captured on the largest sensor is perceived as having greater DOF.
DOF is a subjective property, and may vary from individual to individual. As I see it, DOF in figure 10 stretches from roughly 7.5 cm to 8.5 cm (i.e. a span of 1.0 cm), while DOF in figure 11 stretches from 7.7 cm to 8.2 cm (i.e. a span of 0.5 cm). Other may set these limits larger or smaller, but it is pretty obvious that figure 10 exhibits a larger perceived DOF than figure 11.
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