F-stop

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The aper­ture set­ting on a lens. The num­ber is the fo­cal length of the lens di­vided by the di­am­e­ter of the aper­ture. As a re­sult, larger f-stop num­bers rep­re­sent nar­rower aper­ture sizes. F-stop num­bers are used so that ex­po­sure set­tings for a par­tic­u­lar scene can be ex­pressed with­out hav­ing to know the fo­cal length of the lens used. The f-stop num­ber is the size of the lens’s max­i­mum aper­ture, mea­sured as a frac­tion of the fo­cal length of the lens. On some zoom lenses there may be two aper­tures quoted f/4-5.6, for ex­am­ple. This means that the max­i­mum aper­ture gets nar­rower as the lens is zoomed in. The max­i­mum aper­ture on the lens bar­rel may also be ex­pressed as a ra­tio, such as 1:4-5.6.

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