…which is now 7.5 meters at ISO200, compared to 5.5 meters at ISO100. ISO200 is a one-stop increase from ISO100, which would be the same as increasing our aperture one stop from f/11 to f/8.
F stop explained iso#
Well, ISO isn’t in our guide number formula. Improve your photography with more tips like this delivered to your inbox monthly - click here (opens new window). In our example above, if we needed to have our flash further than 5.5 meters from our subject, we could increase our ISO. That makes sense because the smaller aperture of f/11 lets in much less light than a wide-open aperture of f/1.0.īumping up your ISO will make your sensor more sensitive to light, meaning you’ll get more out of your flash. We’d get a (hopefully) perfect exposure at 5.5 meters now, not 60, with our aperture at f/11. …which is 5.5 meters, or 18 feet for us stubborn Americans. How far would our flash reach for a correct exposure? Now, what if we wanted to shoot at f/11 to get some depth of field, to have our foreground and background more in focus. How do you use a flash guide number? Finding the maximum distance for your flash when changing aperture It seems simple, right? It would be if we were always 60 meters from our subject, shooting at ISO100 and an aperture of f/1.0. So if our guide number is 60, that means that at ISO100 and an aperture of f/1.0 we’d get a correct flash exposure at 60 meters. Shutter speed doesn’t have much to do with it until you get into sync speeds but that’s another topic for another day.
The flash guide number formulaīefore we can understand anything further we need to know how the flash guide number (GN) is calculated.įlash exposure on your subject is dictated by aperture, ISO, and distance ( see Inverse Square Law).
We’ll use a Guide Number of 60 meters in all of these examples. Pay attention to these specifications when looking at flash units. The more expensive Sony units, for example, have the same guide number but are calculated at a flash zoom setting of 105mm.
F stop explained full#
They have a guide number of 60 meters, calculated at ISO100 and a full flash zoom setting of 200mm. The specifications will also show the flash settings at which the guide number is calculated, including the ISO and flash zoom setting.
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