Lenses (or most of them, anyway) are not limited to full-stop steps in aperture. An f/1.8 lens is two-thirds of a stop slower than an f/1.4 lens.The difference between lenses of f/1.8 and f/1.4 is not really in the physical diaphragm itself.As the aperture opening gets larger, lenses tend to need a more complex (and hence more costly) optical design in order to deliver good image quality when used wide open.
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