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As distance or the size of the acceptable circle of confusion increases, the depth of field increases; however, increasing the size of the aperture (i.e., reducing f-number) or increasing the focal length reduces the depth of field. Depth of field changes linearly with f-number and circle of confusion, but changes in proportion to the square of ...
By contrast, the minimum aperture does not depend on the focal length – it is limited by how narrowly the aperture closes, not the lens design – and is instead generally chosen based on practicality: very small apertures have lower sharpness due to diffraction at aperture edges, while the added depth of field is not generally useful, and ...
In microscopy, NA is important because it indicates the resolving power of a lens. The size of the finest detail that can be resolved (the resolution) is proportional to λ / 2NA , where λ is the wavelength of the light. A lens with a larger numerical aperture will be able to visualize finer details than a lens with a smaller numerical ...
The f-number N is given by: = where f is the focal length, and D is the diameter of the entrance pupil (effective aperture).It is customary to write f-numbers preceded by "f /", which forms a mathematical expression of the entrance pupil's diameter in terms of f and N. [1]
With no modification to the microscope, i.e. with a simple wide field light microscope, the quality of optical sectioning is governed by the same physics as the depth of field effect in photography. For a high numerical aperture lens, equivalent to a wide aperture, the depth of field is small (shallow focus) and gives
Memorial in Jena, Germany to Ernst Karl Abbe, who approximated the diffraction limit of a microscope as = , where d is the resolvable feature size, λ is the wavelength of light, n is the index of refraction of the medium being imaged in, and θ (depicted as α in the inscription) is the half-angle subtended by the optical objective lens (representing the numerical aperture).
A high numerical aperture (equivalent to a low f-number) gives a very shallow depth of field. Higher magnification objective lenses generally have shallower depth of field; a 100× objective lens with a numerical aperture of around 1.4 has a depth of field of approximately 1 μm. When observing a sample directly, the limitations of the shallow ...
A real diaphragm when more-closed will cause the depth of field to increase (i.e., cause the background and the subject to both appear more in-focus at the same time) and if the diaphragm is opened up again the depth of field will decrease (i.e., the background and foreground will share less and less of the same focal plane). [4]