Canon Speedlite 580EX I M-Mode confusion...

Discussion in 'Beyond the Basics' started by lextalionis, Apr 14, 2008.

  1. lextalionis

    lextalionis TPF Noob!

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    Okay,
    I'm confused...after reading pgs. 22 and 23 here: http://eosdoc.com/manuals/?q=580EX and using the flash I'm confused.
    In M-Mode the power starts at 1/1 and I assume this is full power. Now what's confusing me is when I can select 1/2, 1/2 -0.3, 1/2 +0.3, 1/2 -0.7 and 1/2 +0.7 huh? I know I'm missing something simple so some help would be appreciated.

    Generally, I want to really dial down the output of this guy since he's on my camera body and then use 430EX off camera as my primary light. Which btw, even after reading, if I use a 580EX I as a master on and a 430EX as a slave both in M-Mode, the power of the 430EX is controlled via the 430EX and not the 550EX right? I could have sworn that I read somwhere that you control the pwr of the slave from the master. And, I can safely say that manual pwr adjustments on the 430EX behave differently than the 580EX.

    Thanks,
    Roy
     
  2. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    You can set the master to 'not fire' and still have it trigger the slave. So you don't need to dial it down if you don't want it to affect the photo.

    If you have the slave (430EX) in M mode...then I don't think you can control it's power from the master unit. If it's in M mode...it will fire at the selected power...that's it.

    When you are using E-TTL mode, the output is read by the camera, so the output (relative to the reading) can be set.
     
  3. Tiberius47

    Tiberius47 TPF Noob!

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    Regarding the use of a 430 as a slave to a 580, it's not a straight optical set up. There are flashes with digitally encoded information that are sent by the master flash (on the camera, this should be the 580) to the slave (the 430). I've never used the set up, but you can find a huge amount of information about flashes for Canon EOS cameras HERE.

    As for the other bit...

    When you halve the power from the flash, you are decreasing it by one stop. If you double the power, you are increasing it by one stop.

    So you have 1/1 which is full power. Then you have 1/1 -0.3, which is a third of a stop less than full power. Then you have 1/1 -0.7, which is two thirds of a stop less than full power. Then you have 1/2, which (being half full power) is a complete stop below full power. Same thing going up. 1/2 +0.3 is the same as 1/1 -0.7, which is a third of a stop more than half power.

    Think of it like this...

    1/1 * 1/1
    1/2 +0.7 * 1/1 -0.3
    1/2 +0.3 * 1/1 -0.7
    1/2 * 1/2
    1/4 +0.7 * 1/2 -0.3
    1/4 +0.3 * 1/2 -0.7
    1/4 * 1/4​

    The higher on that list, the brighter the flash. Each level is the same, but the flash will display it in two different ways, depending on whether you are increasing the power or decreasing it. Increasing the power it will display it as adding a third of a stop, decrease it and it will display it as reducing a third of a stop. 1/1 is maximum power. 1/2 is a stop less - half maximum power. 1/4 is a stop less again, a quarter. And so on.
     
  4. Village Idiot

    Village Idiot No longer a newbie, moving up!

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    They're just 1/2 stop increments. The + and - just show which way you're going.
     
  5. lextalionis

    lextalionis TPF Noob!

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    Wow, I love this place. Thanks for the prompt replies. Now, there are a few begging quesions. First, why the geryatric scale? Must be a Canon thing eh? Also, why in the hell would you even offer two settings that mean the exact same thing? 1/2 +0.7 is synomous to 1/1 -0.3 right? I also get 1/3 stop as .3 but 2/3rds and .7? Again, must be something elementary I'm missing here.
    Another confusion I have is like in E-TTL I can adjust FEC to 0EV and +N EV, what about Manual, why can't I make more than full power output, or wait, I just answered my own ?, full-power is full power right...can't add more HP to a Hemi right?
    Last, I'm just now learning what an actual stop is considered as so the following practical example is speaking aloud for my benifit and for anyone to critique.
    Lets say I'm indoors and I expose for a portrait with the ambient light given and the flash on manual 1/2 +.3 and at ISO 200 I have a stop of f/4 and shutter speed of 1/125. Lets say I stop down one stop with the aperture and use 5.6 (is this right?). To compensate for this I would have to change the flash manual setting from 1/2 +.3 to 1/2 +0.7 or if I'm thinking with the left side of my brain 1/1 -0.3?
    Whewww...time to read (I mean go to sleep) the Canon EOS Flahs articles.
    Thanks,
    Roy
     
  6. Tiberius47

    Tiberius47 TPF Noob!

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    It's just rounding to one decimal place. A third is 0.33333..... and two thirds is 0.66666..... But because the flash can only display one decimal place, it has to round.

    And you are right. 1/2 +0.7 is the same as 1/1 -0.3.

    What do you mean by geryatric scale?

    Yeah. The flash can't get any brighter than full power.

    A stop is just half the amount of light or twice the amount of light. if you increase the exposure by a stop you double the amount of light that is used to expose the image, and if you decrease the amount of light by a stop you are halving the amount of light.

    So, in photography, you can change the amount of light in three ways: Changing the shutter speed, changing the aperture or changing the ISO.

    If you double the shutter speed (1/250 to 1/125) then the shutter is open for twice as long and it lets in twice as much light. This increases the exposure by a stop. Halving it decreases the exposure by a stop.

    If you double the ISO, say going from 100 to 200, then it also increases by a stop. Again, halving the ISO rating decreases by a stop.

    Aperture is a little different, because it doesn't double when you increase by a stop. it increases by 1.4 times, because the aperture refers to the diameter of the hole that lets the light in, not the area. In order to make the area of the hole twice as large (which would increase by a stop), you need to multiply the diameter by about 1.4, not 2. Because the aperture is referred to as a ratio of the diameter of the hole to the focal length of the lens, the aperture value changes by 1.4 times rather than doubling or halving. So if you have an aperture of f4, you can increase it a stop to f2.8 (because 2.8 x 1.4 = 4, roughly).

    When it comes to flash, the flash output can double to increase the flash exposure by a stop, or it can halve in order to decrease the flash exposure by a stop.

    Notice that I'm saying flash exposure, not just exposure. Exposure is ambient light, but flash exposure is something different. For instance, imagine you are taking a picture of a person in front of the Eiffel Tower at night. Ambient light is going to give you the exposure for the tower, but it's going to leave the person as a black blob. So you use flash in order to expose them. So you have exposure (which shows the tower) and flash exposure (which shows the person). It's possible to have ambient exposure underexposed and the flash exposure over exposed. Just use a really short shutter speed, a wide aperture and a full power flash. It'll look terrible. And you can have the opposite as well. The tower might be overexposed and the subject underexposed if you set the shutter speed and aperture to overexpose the tower and use an under-powered flash.

    regarding your example, you are decreasing the aperture by a full stop, but only increasing the flash by a third of a stop.

    if you have the flash on 1/2 at f4, setting the aperture to f5.6 means you will need to increase flash to 1/1 to compensate.

    probably the best way for you to learn about this is to actually play around with it. Find something to shoot (a vase, a book standing up etc) with a background a few meters behind it. Set your camera to manual and the flash to manual. Take a photo and then play around with your flash power and aperture.

    A side note: flash exposure is not affected by shutter speed. The duration of the actual burst of light from the flash is so short that the shutter speed doesn't really have any effect on it.
     
  7. lextalionis

    lextalionis TPF Noob!

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    Okay, the mud is clearing some now. Maybe it was because I spelled geriatric wrong, never mind on what I said there...perhaps I shouldn't use that term ;-)

    It just seem like Canon could do away with redundant unit scale and make it more linear. Is there a reason that 1/2 +0.7 is the same as 1/1 -0.3 ?

    Back to the scale, my side notes are correct?

    1/1 * 1/1 /this means full power
    1/2 +0.7 * 1/1 -0.3 /this means 1/3 stop less power
    1/2 +0.3 * 1/1 -0.7 /this means 2/3 stop less power
    1/2 * 1/2 /this means 1 full stop less power
    1/4 +0.7 * 1/2 -0.3 /this means 1 1/3 stop less power
    1/4 +0.3 * 1/2 -0.7 /this means 1 2/3 stop less power
    1/4 * 1/4 /this means 2 full stops less power

    Now I just need to go back and look at my 430EX as I believe it's scale is different in manual. I recall somethin like 1/1, 1/2, 1/4, 1/16, 1/32

    Thanks for the great explanation and example too, it really helps.

    -Roy
     
  8. Village Idiot

    Village Idiot No longer a newbie, moving up!

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    Because it kind of tells you which way you're going. If you started at 1/4 and are at 1/4 -.7, you know you went 2/3's of a stop down from 1/4. If it was 1/8 +.3, you would know you went up 1/3 of a stop. Kind of useful when you're dialing in your flashes. 1/3 of a stop is not a lot of light.
     
  9. Tiberius47

    Tiberius47 TPF Noob!

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    I spelled it wrong too, nevermind. :p

    It's based on the way you are going. If you decrease the power and there's a fraction of a stop, it will display it as a minus fraction (such as 1/2 -0.3). if you are increasing the power, it will display it as a positive fraction (1/4 +0.7). Both are the same power, just written in different ways depending on if you increase power to get there or decrease it.

    Looks right to me. :)

    Looks to be the same as the 580 scale, just in full stops, not fractions of a stop.

    Glad it helped!
     

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