Simple introduction to Diffraction
Simple introduction to Diffraction | ||
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In order to maximize depth of field, you should stop down the lens as far as possible, and if necessary, add more light to the subject, or give a longer shutter speed to allow a small aperture to be used. There is a major problem with this argument, though—the lens resolution is limited by an optical concept known as diffraction. Most lenses suffer from various chromatic and spherical aberrations when used at full aperture. These aberrations are gradually reduced as the lens is stopped down. However, the effects of diffraction increase as the lens is stopped down, and there will come a point where the image quality begins to fall. Most lenses give their optimum performance at two to three stops from their maximum aperture, perhaps f/8. Stopping down the lens to this region improves definition by eliminating off-axis rays of light and reducing lens aberrations such as chromatic and spherical aberration. Beyond this, resolution may be affected by diffraction, which limits the resolution of a lens. Diffraction does not suddenly cut in—it appears gradually as the lens is stopped down, and different lenses will have their own optimum apertures. The physics of diffraction are complex, but simply, when light rays touch the edge of any opaque material, such as the blades of the iris diaphragm in a lens, they are bent due to the wave nature of light, and the light splits into separate colors (as in a rainbow). Violet rays are diffracted more than red. This is similar to water being passed through a hosepipe. If the nozzle is closed too much for the amount of water flowing through it, the water 'spills out' around the edge. Thus, there is a compromise between depth of field and resolution. For any given situation there is an optimum aperture, where the balance between sharpness and depth of field is optimized. It is worth doing tests with your lenses to see where the diffraction limit is, and how greatly the image is degraded at various apertures. It may be that, in certain instances, resolution needs to be sacrificed at the expense of greater depth of field. A practical rule of thumb for minimizing the effect of diffraction is to keep the effective aperture (EA) you are using below the diffraction limit. This will vary according to the size of the image sensor in the camera. Suggested figures are: Full-frame sensor (24 × 36 mm): 32 The effective aperture of the system is derived from a simple formula: Effective Aperture = f no. × (M+1 ) where M is the magnification at the sensor. The f no. is, strictly, the relative aperture of the lens, and is the number engraved on the aperture scale. For example, if the aperture ring on the lens is set to f/16, and the subject is being photographed at a magnification of 1×, then the effective aperture is 16 x (M+1) = 16x2 = 32 Using a camera with an APS-C-size sensor, and this aperture/magnification combination, it is highly likely that image resolution will be reduced due to the effects of diffraction. To achieve a sharper result, it may be better to open the lens aperture by two stops, even though this will mean losing some valuable depth of field. Although these figures are approximations, and image quality will be dependent on other factors, they do serve to show that different sensor sizes will affect final image quality. It is one of the reasons why apertures on compact cameras only go to f/8 or f/11. If the lens was stopped down any more, the image quality would suffer from diffraction. The type of subject will also have an effect on the final image quality; for example, a subject with lots of high-contrast detail may suffer more than one with smooth tones and little detail. Several companies make depth-of-field calculators for field use, while others are available online where it is possible to select the sensor size, lens focal length, and focusing distance. They then give you an overall depth of field. Some of these are not designed for macro photography at magnifications over life size. | ||