Lens f-number – what is it and why is it significant? - Resolve opticsThe f-number (f#) of a lens is the ratio of its focal length to entrance pupil and is used to indicate its aperture thereby determining how much light will pass through the #lens. The higher the f-number of a given lens the smaller the aperture will be, restricting the amount of light that will pass through. In contrast the same lens with a lower f# will allow more light to pass through and provide a brighter image. However, the aperture of a lens will also affect its depth of field. The larger the aperture (smaller f#) the shallower the depth of field will be and vice versa. Choosing the correct f# is therefore important to balance these two criteria.

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