trying to grasp guide numbers... question?

[photosoto]

So first off, I don't have a light meter.

I'm trying to learn this the manual way for right now. In my following question I am applying the formula

GN=aperture X distance from light to subject

My questions is -
when I am applying this formula I do not have control over the factory given guide number. I do have control over aperture and distance to subject. So to adjust my flash output for a relatively accurate exposure can I just multiply my aperture setting, for example, 4.5, by 10 feet to get a guide number of 45 and then turn this into a ratio of 45/90 (90 being the factory given GN) and then equate this into a 1/2 power flash setting?

note: my sb600 has actually a GN of 98 but I know they aren't exactly accurate

 

[KmH]

You do have control over the guide number, by varying the SB-600's output power and zoom head position.

See pages 34-35 of your SB-600 Users Manual.

 

[photosoto]

You do have control over the guide number, by varying the SB-600's output power and zoom head position.

See pages 34-35 of your SB-600 Users Manual.

I didn't phrase my question very well. I just meant that each flash has its own guide number, as in output capability. I was just trying to dumb this down for myself. There are a lot of variables and I am trying to do this with no light meter. I read those pages in the manual already, but I'm not making much sense of it. I'll re-read. As for my original question, am I far off base?

 

[mikehaugen]

That equation can be translated to: gn/distance=aperture. The way that I do it (and I am not sure if this is right) is divide the gn by the distance to get what the aperture should be. If that is too high, since each power level decrease (on my flash anyway) is 1/2 of the previous 1/2,1/4,1/8,1/16... etc then that would be 1 stop of aperture. So if the flash is 10ft away with gn of 90, the aperture would be 9 (or whatever is close). In this example we will use 8 because it is easy. If you turn the flash down 1 step to 1/2 then you should increase the aperture to 5.6 to get the same flash exposure. I may be way off base here, but seems to work fairly accurately. Sometimes I will tweak a little based on the results. I use the LCD much more than I probably should, especially when using flash.

 

[Derrel]

mikehaugen is giving you the right kind of information. Guide Number DOES go up, or down, based on the beam width of a zoom-head speedlight, or based on the beam spread of a studio flash reflector. Guide NUmber also goes down, as the power levels of both speedlight and studio flash are reduced. The narrower the area the flash output is distributed over, the higher the guide number, and the wider the area the flash is distributed over, the lower the Guide Number. So, each flash has, potentially, MULTIPLE guide numbers (if the flash zooms, or has interchangeable reflectors, OR if it has variable power adjustment.)

Guide Number is usually abbreviated GN, and it also has to be specified for each ISO value AND in meters OR feet. As the ISO levels go up, the GN increases; as ISO levels go down, GN drops. European and UK flash makers typically list their GN in Metres (aka Meters).

 

[photosoto]

mikehaugen is giving you the right kind of information. Guide Number DOES go up, or down, based on the beam width of a zoom-head speedlight, or based on the beam spread of a studio flash reflector. Guide NUmber also goes down, as the power levels of both speedlight and studio flash are reduced. The narrower the area the flash output is distributed over, the higher the guide number, and the wider the area the flash is distributed over, the lower the Guide Number. So, each flash has, potentially, MULTIPLE guide numbers (if the flash zooms, or has interchangeable reflectors, OR if it has variable power adjustment.)

Guide Number is usually abbreviated GN, and it also has to be specified for each ISO value AND in meters OR feet. As the ISO levels go up, the GN increases; as ISO levels go down, GN drops. European and UK flash makers typically list their GN in Metres (aka Meters).

Excuse my newbness, I just don't get how to approach this by calculating with an already given guide number. It just seems to me the GN would be the variable to figure out if you know the aperture and distance from object/light and ISO. It'll eventually click...

I understand what mikehaugen saying partially. what I don't understand is if I find my aperture with the GN/distance=aperture formula and it is f22, what kind of increments do I use for each stop on the camera vs. the flash. If I step down on the flash from 1/1 to 1/2, how much do I open up the aperture?

 

[Big Mike]

Excuse my newbness, I just don't get how to approach this by calculating with an already given guide number. It just seems to me the GN would be the variable to figure out if you know the aperture and distance from object/light and ISO. It'll eventually click...

If you understand it well, that would work but I agree with Mike & Derrel. The 'normal' way to do this, is to take the given GN, then divide by the distance to get the aperture. This usually requires you to know the GN for the different zoom settings on your flash, and of course, you can take the power settings into account when using lower than Full power.

 

[photosoto]

Excuse my newbness, I just don't get how to approach this by calculating with an already given guide number. It just seems to me the GN would be the variable to figure out if you know the aperture and distance from object/light and ISO. It'll eventually click...

If you understand it well, that would work but I agree with Mike & Derrel. The 'normal' way to do this, is to take the given GN, then divide by the distance to get the aperture. This usually requires you to know the GN for the different zoom settings on your flash, and of course, you can take the power settings into account when using lower than Full power.

I've decided for now just to leave the zoom at 24mm to keep it simpler. As for power settings on the flash, how many increments do I open up my aperture if I decrease my flash power from 1/1 to 1/2, etc.

 

[Derrel]

One takes the GN, in advance of a shooting situation, and computes the needed distance to position the flash unit from the subject for a desired f/stop. For example, a Vivitar flash with a 200 ISO Guide Number in Feet, at Standard Zoom head angle....your GN is 110 with this flash unit...you focus on a group shot,and the lens's focus scale reads 10 feet...110 divided by 10 equals f/11.

At 7 feet flash-to-subject distance, 110 divided by 7 equals 15.71, or roughly f/16

At 40 feet flash-to-subject distance, 110 divided by 40 equals 2.71, or roughly f/2.8

I understand from your OP that you are working from the premise that "GN=aperture X distance from light to subject ", which is accurate, but is typically not the way GN information is used. You're approaching it sort of bass-ackwardly, if I might say so.

One thing that the SB 600 flash unit LACKS, that the more-expensive Nikon speedlights have is a GN or A- type flash output control, where you pick a SPECIFIC aperture you'd like to use, and then the flash unit will adjust the Guide Number upward or downwardly to match the Aperture in use, from near to far, depending on focusing distance...that is the type of feature that makes the SB 800 and SB 900 flash units so useful for more advanced users. So, I can understand your desire to have a GN or A-type of control over flash output, but the old-fashioned Guide Number method really is not typically used in the way you're putting forth in your OP.

 

[mikehaugen]

I think I understand what you are saying, but with a given guide number (though as derrel pointed out can change), it is much easier to find aperture from distance than to go backwards. Are you familiar with the inverse square law? I am not going to try to explain it off the top of my head, but basically light changes exponentially with distance, not linearly. It would a lot of algebra (maybe even calculus, I'm tired) to try to figure out light output needed for a given distance. That is why I explained it using the guide number that you have and what it will give you (aperture) at whatever distance, then adjusting flash output in whole stops to alter the aperture back to where you wanted it originally.

I think I just gave you a really bad explanation, but basically they give you a gn to try to simplify everything. It is saying "this has enough power to give you f:whatever at whatever distance at full power. And if you know what aperture you want and what distance you want, then you can lower flash power in however many stops to brings the aperture to where you want it. People already hate formulas and trying to think of flash power (a fraction of full power) directly from the distance and aperture desired, would be a hell of a formula that 99.999% of people either wouldn't understand or wouldn't take the time to utilize.

Hopefully I haven't confused you more, but I feel that you are thinking too hard about this. I used to have an old canon flash with which you could set the iso, then you had a choice of 2 f-stops, 4 or 5.6, and it would give you a distance scale within which that was supposed to give you an accurate flash exposure. It was so simple but I could never really understand the theory so I never used to use it. Not to mention it gave you very few options, and since it was on a film camera you couldn't see if you were even close until you got the film back. Anyway, I am going to bed (I work 3rd shift), good luck and sorry again if I confused you. Maybe tomorrow I will be able to give an explanation that makes more sense.

 

[mikehaugen]

I took a really long time to write that and derrel's post (in the mean time) probably makes more sense. Also I was not aware that Nikon flashes did that, it's kinda neat. Do you know if the 580ex will do that? I am pretty sure my 550 won't, maybe I will have check the online manual again since I don't have mine.

 

[mikehaugen]

One more thing to add, going back to your op and maybe better explaining my other explanation about the inverse square law. No, 1/2 power does not mean 1/2 the guide number- well, I don't think anyway. I could be wrong.

 

[Big Mike]

I've decided for now just to leave the zoom at 24mm to keep it simpler. As for power settings on the flash, how many increments do I open up my aperture if I decrease my flash power from 1/1 to 1/2, etc.

If you double (or half) the amount of light, that is one 'stop'. So from 1/1 (full power) to 1/2 Power is one stop difference. So adjusting your aperture, you would adjust it one stop (say from F8 to F5.6)....which is probably three click of your dial.

 

[photosoto]

I took a really long time to write that and derrel's post (in the mean time) probably makes more sense. Also I was not aware that Nikon flashes did that, it's kinda neat. Do you know if the 580ex will do that? I am pretty sure my 550 won't, maybe I will have check the online manual again since I don't have mine.

mikehaugen, thank you very much for taking the time to reply. And to BigMike and Derrell as well.

 

[photosoto]

I've decided for now just to leave the zoom at 24mm to keep it simpler. As for power settings on the flash, how many increments do I open up my aperture if I decrease my flash power from 1/1 to 1/2, etc.

If you double (or half) the amount of light, that is one 'stop'. So from 1/1 (full power) to 1/2 Power is one stop difference. So adjusting your aperture, you would adjust it one stop (say from F8 to F5.6)....which is probably three click of your dial.

ok thanks much. that clarifies one of my misunderstandings. I've got some pics here that I attempted today. Let me know what you think. I know they are all the same shot basically but this is my first attempt. All are sb600 camera right thru umbrella and a yn460ii behind subject camera left. The light was constantly changing in the background too.

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