tag:blogger.com,1999:blog-6851855957680540394.post955835456649329019..comments2023-03-23T13:10:37.803+00:00Comments on LightStudio photography and studio lighting blog: Guide to Guide Numbersn506http://www.blogger.com/profile/06611267218004453911noreply@blogger.comBlogger4125tag:blogger.com,1999:blog-6851855957680540394.post-24970943969837166592009-09-07T23:37:46.215+01:002009-09-07T23:37:46.215+01:00Thanks for pointing out that very silly mistake! I...Thanks for pointing out that very silly mistake! I've updated the blog post to correct the error.n506https://www.blogger.com/profile/06611267218004453911noreply@blogger.comtag:blogger.com,1999:blog-6851855957680540394.post-11279719789972473802009-09-07T23:03:54.432+01:002009-09-07T23:03:54.432+01:00It's not a good idea to teach things you don&#...It's not a good idea to teach things you don't understand. Your claim that "360 feet @ ISO 200 = 180 feet @ ISO 100" is incorrect.<br /><br />The ISO number is a linear brightness scale, so ISO 200 is twice as light as ISO 100 (and ISO 800 is 4 times as light as ISO 200 (800/200=4), etc). However, light intensity fall-off is exponential. Light intensity is inversely proportional to the square root of the distance.<br /><br />So, you should multiply your ISO number by the square of the number you multiply your distance by.<br />E.g., if you multiply your distance by 2 then you should multiply your ISO number by 4, or if you multiply your distance by 4 then you should multiply your ISO number by 16.<br /><br />Let's say you get <b>F</b> feet @ ISO <b>I</b>:<br /><b>A</b>) The distance at ISO <b>X</b> = <i>'the square root of (<b>X/I</b>)'</i> * <b>F</b> feet<br /><b>B</b>) The ISO number at <b>Y</b> feet = ISO <i>'the square of (<b>Y/F</b>)'</i> * <b>I</b>.<br /><br /><b>A</b>) 360 feet @ ISO 200 = 'the square root of (100/200)' * 360 feet @ ISO 100 = 0.707 * 360 feet @ ISO 100<br /> = <b>255 feet @ ISO 100</b>.<br /><b>B</b>) 360 feet @ ISO 200 = 180 feet @ ISO 'the square of (180/360)' * 200<br /> = <b>180 feet @ ISO 50</b>.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-6851855957680540394.post-71421712993256739932008-04-06T01:00:00.000+01:002008-04-06T01:00:00.000+01:00Hi JanFirst of all, it wouldn't surprise me at all...Hi Jan<BR/><BR/>First of all, it wouldn't surprise me at all to find a large manufacturer overestimating their product's abilities. That said...<BR/><BR/>A lot of things could be going wrong with the test. Here's a quick list of things which could be wrong:<BR/><BR/>What batteries are you using? If you're using rechargeables, they are only 1.2V rather than 1.5V. This could mean that the capacitor doesn't get "juiced up" quite as much, and results in a lesser power output when the flash is tripped. You might also have grabbed a set of alkalines to try the 420EX, thereby giving it a little extra juicing up?<BR/><BR/>If that's not the case, then how sure are you of your flashmeter? How old is it, and therefore how long since it was calibrated? If you're finding your 420EX giving a slightly low value, then you might find that the flashmeter is also giving a non-linear response resulting in a value even further out for the more powerful 580EX. Might be worth grabbing someone who has another flashmeter and doing the test again and comparing results before you blame the 580EX for sure.<BR/><BR/>Lastly, I believe I've mentioned before in my blog that guide numbers are pretty much just literally a guide. The big bit of hazyness is in the "correct exposure" definition of what the exposure is. What Canon believes is a correct exposure might well be darker than what your flashmeter was set to believe. Not only that, but Canon's marketing dept. could well have changed the corporate view of what is a correct exposure in the generation since the 420EX was built...after all, what's the odd 2/3 of a stop between friends?n506https://www.blogger.com/profile/06611267218004453911noreply@blogger.comtag:blogger.com,1999:blog-6851855957680540394.post-87272808328166457742008-04-04T11:49:00.000+01:002008-04-04T11:49:00.000+01:00I havejust bought a new Speedlite 580EX II...It se...I havejust bought a new Speedlite 580EX II...<BR/><BR/>It seems that the flashoutput i a little to low.<BR/><BR/>I set up my Konica Minolta Autometer VF mountet with the optional flat diffuser on a stand at exately 3 meters.<BR/><BR/>I then mountet the camera on another stand at exately the same hight as the flashmeter.<BR/><BR/>With the settings ISO 100, 50 mm, the guidenumber is specified ba canon to be 42.<BR/><BR/>So I was expecting to mesure an output of f:14<BR/><BR/>But I mesured only f:11 ???<BR/><BR/>So I took my old EX420 wich has a guidenumber of 34 i the 50 mm range.<BR/><BR/>And here I mesured f:11 (11x3=33) wich i pretty close to the specified 34.<BR/><BR/>I contacted the dealed who sold me the 580EX II and he send me a new one...<BR/><BR/>It does have a different serielnumber (old=252902, new=259432).<BR/><BR/>But the new 580EX was not performing better...<BR/><BR/>Infact the output in manual, ISO 100, 50 mm was still f:11 at 3 meters.<BR/><BR/>That gives (in my calculation) a guidenumber of as low as 33 instead of the specified 42.<BR/><BR/>And exately the same as my old 420EX ???.<BR/><BR/>Does anyone have had a similar expirence ?.<BR/><BR/>Then please mail me at :<BR/>mail@mobilfotografen.dk<BR/><BR/>Best regards<BR/><BR/>Jan HunicheAnonymousnoreply@blogger.com