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Ian Bailey; Magnification, Field of View and Depth of Field in Low Vision Aids and Optical Instruments. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2776.
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Clinicians prescribing low vision aids or using visual instruments to examine eyes often need to understand what fine details can be seen, how much of the object can be seen, and how visual access depends of the viewing system. Some new simple concepts can allow clinicians to estimate the magnitude of magnification effects, fields of view and depths of field.
There are several different methods used to quantify magnification effects at near and this often leads to erroneous predictions of optical performance. As an alternative, we use Equivalent Viewing Distance (EVD) which is the distance at which the object would subtend the same angle that is being subtended by the image. The size of just-resolvable detail is simply proportional to the EVD. For collimating systems, the EVD is equal to the equivalent focal length. For images at finite distances, the EVD is calculated by dividing the eye-to-image distance by the enlargement ratio (ER) . To understand such systems, the clinician needs to know the image location and ER. The Field of View (FoV) is determined by a “field cone’ whose angle is determined by the field limiting aperture and its distance from the eye. The Image Field Aspect Ratio (IFAR) is a diameter-to-distance ratio defining this angle. The width of the FoV is equal to the product of EVD and IFAR. Slit lamps, and most other microscope systems are analogous to Kelperian telescopes and then, IFAR = approx. 1.0, and the EVD and the FoV will be about equal. A practical estimate of the Depth of Field is given by the square of the EVD.
These methods can provide an immediate and accurate estimate of EVD, FoV and DoF in hand held magnifiers, stand magnifiers, telescopes for distance or near. It is easy for clinician observing a patient to mentally visualize the field cone and EVD plane, and immediately estimate the FoV. The same approach can easily be applied to the slit lamp and most other visual inspection instruments. Once the geometric concepts are grasped, only minimal calculation skills are required.
The concept of Equivalent Viewing Distance simplifies the quantification of magnification effects and prediction of resolvable detail. The image field cone concept when combined with the EVD, directly and intuitively predicts the width of the FoV. Once the EVD is known, the depth of field can be estimated by squaring the EVD.
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