diablopilot 2 #1 January 8, 2004 Ok. Everytime I think I have it figured out, I rethink it and I'm back to not understanding it. What does "guide number" mean in relation to flash units? My Canon 550EX is reported to have a guide Number of "180'/55m at 105mm Another that I've looked at is "160'/49m at 50mm" How do those two compare? Does it mean that a subject 160 feet from the camera with a 50 mm lens will be lit to a specific amount by the second flash? I don't get it, and haven't found a resource to answer the question yet.---------------------------------------------- You're not as good as you think you are. Seriously. Quote Share this post Link to post Share on other sites
quade 4 #2 January 8, 2004 http://photonotes.org/articles/eos-flash/index2.html#guidenumber http://www.vividlight.com/articles/1214.htm http://www.minoxlab.com/dk051597/dkflash.htm http://www.shortcourses.com/how/guidenumbers/guidenumbers.htm All the same information, but in slightly different formats. You should be able to understand at least one of them.quade - The World's Most Boring Skydiver Quote Share this post Link to post Share on other sites
diablopilot 2 #3 January 8, 2004 Somehow I knew you'd be the first one dropin' knowlage..... You big ol' camera pimp!---------------------------------------------- You're not as good as you think you are. Seriously. Quote Share this post Link to post Share on other sites
diablopilot 2 #4 January 8, 2004 So in the case of my 550EX does that mean that at a distance of 15 feet from the flash I should be safe using f/11 or so with ISO 100? What does the 105mm bit refer to?---------------------------------------------- You're not as good as you think you are. Seriously. Quote Share this post Link to post Share on other sites
quade 4 #5 January 8, 2004 I'm not sure where you're getting the 105mm from? The focal length of the lens -shouldn't- have a whole lot to do with this. Aperture, yes.quade - The World's Most Boring Skydiver Quote Share this post Link to post Share on other sites
quade 4 #6 January 8, 2004 Ok, I have it figured out . . . duh! The 550EX has a flash head that "zooms". It concentrates the light for various lenses. Ok, so NOW I understand.quade - The World's Most Boring Skydiver Quote Share this post Link to post Share on other sites
quade 4 #7 January 8, 2004 So, anyway, this is going to be a little confusing. When the 550EX flash head is zoomed into the area that would be covered by a 105mm lens -on a normal 35mm film camera- the guide number is 55 (in this case meters). When the flash head is zoomed out to a wider area, the guide number goes down. http://www.usa.canon.com/html/cameras_speedlite/550exspec.html And scroll down to almost the very bottom of the page. Now, all of this said, if you use -any- of the automatic flash metering, none of this matters as the flash unit itself will work things out. The guide number is really just for complete manual stuff if you don't have a flash meter handy and also as a "guide" for comparing the relative strength of various flash units. More is "better".quade - The World's Most Boring Skydiver Quote Share this post Link to post Share on other sites
blazetailman 0 #8 January 8, 2004 In the first article, the inverse quare law is a good expantion on light and how flash units work in manual mode which the 550 has and is easy to use. this will also give a good working expereince with test shots to see how light works. How things going JP???www.canopyflightcenter.com www.skydivesac.com www.guanofreefly.com Quote Share this post Link to post Share on other sites
quade 4 #9 January 8, 2004 Quote So in the case of my 550EX does that mean that at a distance of 15 feet from the flash I should be safe using f/11 or so with ISO 100? So, to answer that -specific question . . . With the 550EX zoomed into the 105mm setting you'd have a guide number of 55. 55(guide number) / 4.572 meters(15 feet) = f12 Assuming ISO 100 speed film.quade - The World's Most Boring Skydiver Quote Share this post Link to post Share on other sites
blazetailman 0 #10 January 8, 2004 from my expereince in the phtoto world, guide numbers were a gauge to see how powerful the flash was. The bigger the number, the more powerfuk the flash. Not much more interest in it than that. But this is a good educational thread. The automatic flash funtions help the average shot, I use a little flash exposre compenstion to tone it down so it dosen't become overbearing. It gives a more natural fill instead of the flood light wash out.www.canopyflightcenter.com www.skydivesac.com www.guanofreefly.com Quote Share this post Link to post Share on other sites
psw097 0 #11 January 8, 2004 Along the same lines: Does anyone know if the flash head accounts for the digital crop factor? On my 420EX - it reports the lens setting as true - like 50mm on my Sigma 50/2.8EX. But with a crop camera that example would be 80mm, I would think the zoom head would need to go up to the next stop (75mm I think) for E-TTL to be correct. I don't know if it is an oversimplified user interface and it just knows what its doing or its all based on a film camera and Canon decided not to worry about the effect the smaller sensor has. A wider flash circle is not a bad thing but would mess up the range and power significantly. Since the 420EX is E-TTL only I don't get much choice, but its nice to know. Quote Share this post Link to post Share on other sites
diablopilot 2 #12 January 8, 2004 QuoteHow things going JP??? Can't complain. Well I can about how much money camera gear, the tunnel, and skydiving take out of my pocket, but then the enjoyment and benifits I get from them probably outweight the cost. In short: GOOD! Come on down and visit.---------------------------------------------- You're not as good as you think you are. Seriously. Quote Share this post Link to post Share on other sites
diablopilot 2 #13 January 8, 2004 So I'm guessing to make an accurat comparison between two units they have to have the guide number measured at the same ISO rating AND the same focal length.---------------------------------------------- You're not as good as you think you are. Seriously. Quote Share this post Link to post Share on other sites
quade 4 #14 January 8, 2004 Well, the Guide Number is one of those dirty little secret things between manufacturers. To do a real comparison, you break out a flash meter and see if the manufacturers are telling the truth.quade - The World's Most Boring Skydiver Quote Share this post Link to post Share on other sites
bmcd308 0 #15 January 8, 2004 The 420 EX was developed to be coupled with the Elan 7 / 7e series of 35mm bodies, and there is no 1.6x crop factor there. This should not make a difference, though. Remember, your 300D really just displays the center part of the image from the lens, and E-TTL meters on the fly, so to speak. So your flash shots will be correctly exposed (if you understand Canon flash) regardless of what lenght lens your flash thinks your camera has. The only real disadvantage that you face is that your flash is always lighting up a wider area than it really needs to, so you are losing some reach. Brent ---------------------------------- www.jumpelvis.com Quote Share this post Link to post Share on other sites
psw097 0 #16 January 8, 2004 Thanks, that is what I thought - the reach is reduced since the zoom head might be in the wrong spot. Flash works fine - I bounce it most of the time anyway. It is good to know how the guide number is effected on a digital camera in case I'm trying to get some reach with it. Anyway I guess I'm asking too much for comsumer products. I would have thought the logic would be in the camera - thought that was the point of E-TTL. And, that the camera would report the correct effective focal distance to the flash-head. Quote Share this post Link to post Share on other sites
