to 71–60% of their unfiltered rating. Guide numbers diminish as the square root of filter attenuation in fstops, as per the following formula: …where equals the filter's rated loss in fstops. The following is a step-by-step example of using the above formula: Suppose your guide number is 32 (it is irrelevant if it is scaled for meters or feet for this purpose) and the rated filter loss is 1.5 fstops. Take 0.5 and raise it to the power of 1.5 (using the button on a scientific calculator), which equals roughly 0.35355. Take the square root of that (the button), which equals approximately 0.595, and multiply that by the guide number of 32 to obtain a filtered guide number of 19.0. The below table provides some common filter values. When a flash device is set to manual (M) or automatic (A) exposure mode and is not being controlled via the camera's through-the-lens metering, a convenient way to compensate for the effect of a lens-mounted filter is to set the ISO rating on a camera to a higher value than the flash device. For instance, if a polarizing filter attenuates by 1 fstop and the flash device is set to ISO 100, then the camera can simply be set to ISO 200. The extra camera sensitivity compensates for the loss due to the filter. The formula governing this relationship is as follows: …where equals the camera filter's rated loss in fstops. Here is a step-by-step example of using this formula: Suppose a filter attenuates by 11/3 fstops and the flash device is set to ISO 100. Take 2 and raise it to the power of 1.3333 (using the button on a scientific calculator), which is approximately 2.5198, and then multiply that by 100, which equals about 252. The nearest standard camera setting is ISO 250. Effect of shutter speeds With electronic flash When electronic flash devices based on flashtube technology are used with most modern cameras (those with focal-plane shutters), shutter speed has no effect on guide numbers. See also Shutter (photography). This is because even at the most powerful settings, flash durations seldom exceed a few milliseconds (thousandths of a second). With focal-plane shutters, a flash begins shortly after the shutter curtain has fully opened and must extinguish before the curtain begins to close. Selecting any shutter speed faster than the camera's rated Xsync speed, which is often between and of