Originally designed as an auto-focus flash for the F-501, the Nikon Speedlight SB-20 Shoe Mount Flash allows you to expand your reach using compatible cameras. Because you can capture the smallest detail possible with each shot, the Nikon Speedlight SB-20 Shoe Mount Flash is perfect for both experienced and amateur photographers alike. This flash runs on 4 x AA batteries and gives photographers up to 140 flashes from a single charge. By selecting automatic mode operation, the SB-20 allows you to choose from five alternative apertures as seen on the back panel. The manual mode of operation lets you choose from five different levels of flash light output: full, 1/2, 1/4, 1/8, and 1/16. The TTL flash mode is simple to use and lets you control the exposure, and as long as the subject is within the flash working range, you have the widest selection of aperture to control depth of field. Need to set up for those multi-flash occasions like the exciting end of a football game? Just follow the simple layout steps and straight-forward directions. Do not fear if you have a situation that calls for great photos in total darkness. This flash will automatically deliver the precise amount of light you need to snap clear, in-focus pictures. When the light where you are photographing drops below the ideal level, the focus-assisted LED AF illuminator that is built into the SB-20 will automatically turn on as long as your camera is set up to run in the single-servo AF shooting mode. The integrated illuminator works to deliver contrast to the subject so the F501 and other camera autofocus systems to function as though it were daytime. Exposure control is automatic also, and all you need to do is program the SB-20’s selector to the TTL setting. And when you set the exposure mode on auto, you can rest assured you will not experience any shutter speed error. Extra features like being able to choose the flash coverage angle and set the bounce flash are features that make the SB-20 even compatible with the autofocus system. You even have the option to select from your choice of three different flash angle coverages without optional accessories, and the SB-20 revolves for wide coverage of 28mm, 35 and 85mm lenses. This flash is perfect for that close-up bird on a branch or butterfly shot, and should be used outdoors for improved lighting. While it may feel a bit old school, the Nikon Speedlight SB-20 Shoe Mount Flash is very user-friendly for people who don’t want to squint at an LCD, and can be used as a dedicated flash for the F-501, or used safely as a companion flash in situations where you need a secondary flash in a multiple-flash setup.