Help with guide numbers when using a flash manually
Help with guide numbers when using a flash manually
Apr 14, 2013

Hi,
am trying to use my canon 420 ex flash with a newly purchased Panasonic fz 200 as I cannot afford a compatible flash at this time.
the 420 is a zoom flash, according to the manual with guide numbers from 23 (at 42 mm?) to 42 (105mm). There is a note that it is for ISO 100 meters.
I am totally confused. Can someone break this down for me into feet and tell me how to calculate the f stop for let's say 4 to 25 feet? There is a chart in the manual
http://www.usa.canon.com/app/pdf/lens/Speedlite_420EX_manual.pdf
thanks,
vito
Re: Help with guide numbers when using a flash manually
In reply to Vito_F,
Apr 14, 2013

Guide number From Wikipedia, the free encyclopedia
Living and loving it in Bangkok, Thailand. Canon 7D  See the gear list for the rest.
Re: Help with guide numbers when using a flash manually
In reply to Vito_F,
Apr 14, 2013

Vito_F wrote:
Hi,
am trying to use my canon 420 ex flash with a newly purchased Panasonic fz 200 as I cannot afford a compatible flash at this time.
the 420 is a zoom flash, according to the manual with guide numbers from 23 (at 42 mm?) to 42 (105mm). There is a note that it is for ISO 100 meters.
I am totally confused. Can someone break this down for me into feet and tell me how to calculate the f stop for let's say 4 to 25 feet? There is a chart in the manual
http://www.usa.canon.com/app/pdf/lens/Speedlite_420EX_manual.pdf
thanks,
vito
I do not understand the numbers you give ( (( 23 (at 42mm )) ) or the (( 42 (105mm) )) .
Very seldom would a guide number be given for 105mm or for 42mm .
and 42 meters would be way out there about ( 150 feet ) Nor a very probable guide number either .
You could find the real Guide number with test shots .
Try 10 feet and work backwards by changing the aperture on the lens until you have a perfect exposure .
Then you would have the two numbers you keep for finding the correct exposure .
Guide numbers usually are given for a certain ISO and at a certain distance .
ISO 100 at ( what distance ) would equal a guide number .
Read it again .
Re: Help with guide numbers when using a flash manually
In reply to DUSTY LENS,
Apr 14, 2013

DUSTY LENS wrote:
Vito_F wrote:
Hi,
am trying to use my canon 420 ex flash with a newly purchased Panasonic fz 200 as I cannot afford a compatible flash at this time.
the 420 is a zoom flash, according to the manual with guide numbers from 23 (at 42 mm?) to 42 (105mm). There is a note that it is for ISO 100 meters.
I am totally confused. Can someone break this down for me into feet and tell me how to calculate the f stop for let's say 4 to 25 feet? There is a chart in the manual
http://www.usa.canon.com/app/pdf/lens/Speedlite_420EX_manual.pdf
thanks,
vito
I do not understand the numbers you give ( (( 23 (at 42mm )) ) or the (( 42 (105mm) )) .
Very seldom would a guide number be given for 105mm or for 42mm .
and 42 meters would be way out there about ( 150 feet ) Nor a very probable guide number either .
You could find the real Guide number with test shots .
Try 10 feet and work backwards by changing the aperture on the lens until you have a perfect exposure .
Then you would have the two numbers you keep for finding the correct exposure .
Guide numbers usually are given for a certain ISO and at a certain distance .
ISO 100 at ( what distance ) would equal a guide number .
Read it again .
Hi Dusty,
Thanks for the suggestion, I will try it but I was looking for help with interpreting the GN chart from Canon. I did include the link but will add the chart at the end of this message.
What is confusing me is the statement about ISO in meters, the "Flash coverage in mm" and the how to use the different GN by distance.
The 420 EX is a zoom flash, although I am not using the TTL as it is not compatible with the camera.
Are these different GN in the chart based on the focal length I am using with my lens? Or is it because I am not using TTL I use one GN and stick with it?
Totally confused.
Thanks,
Viyo
Re: Help with guide numbers when using a flash manually
In reply to Vito_F,
Apr 15, 2013

The "At ISO 100 in meters" means if you set the ISO of your camera to 100 you can figure the aperture by using the guide number at the flash (and lens) zoom setting and the distance in meters if the flash fires at full power.
If you had read the reference I gave you then you should have been able to understand this and how to do the calculations yourself but here is one example just to help you along.
Assuming you are at ISO 100, using a 50mm lens, the flash has zoomed to 50mm, and the subject is at 13' (about 4 meters).
Aperture = GN/Distance = 34 meters/4 meters = 8.25 which is closest to f/9.1 (i.e. f/8.3).
If you increase the ISO to 400 (two stops greater sensitivity) and reduce the flash power to 1/4 (two stops less power) you would still be shooting at f/9.1 but you flash batteries would now last four times as long, the flash would recharge much faster, and the flash would be much less likely to overheat if you took multiple flashes as fast as possible.
Living and loving it in Bangkok, Thailand. Canon 7D  See the gear list for the rest.
Re: Help with guide numbers when using a flash manually
In reply to Vito_F,
Apr 15, 2013

Vito_F wrote:
DUSTY LENS wrote:
Vito_F wrote:
Hi,
am trying to use my canon 420 ex flash with a newly purchased Panasonic fz 200 as I cannot afford a compatible flash at this time.
the 420 is a zoom flash, according to the manual with guide numbers from 23 (at 42 mm?) to 42 (105mm). There is a note that it is for ISO 100 meters.
I am totally confused. Can someone break this down for me into feet and tell me how to calculate the f stop for let's say 4 to 25 feet? There is a chart in the manual
http://www.usa.canon.com/app/pdf/lens/Speedlite_420EX_manual.pdf
thanks,
vito
I do not understand the numbers you give ( (( 23 (at 42mm )) ) or the (( 42 (105mm) )) .
Very seldom would a guide number be given for 105mm or for 42mm .
and 42 meters would be way out there about ( 150 feet ) Nor a very probable guide number either .
You could find the real Guide number with test shots .
Try 10 feet and work backwards by changing the aperture on the lens until you have a perfect exposure .
Then you would have the two numbers you keep for finding the correct exposure .
Guide numbers usually are given for a certain ISO and at a certain distance .
ISO 100 at ( what distance ) would equal a guide number .
Read it again .
Hi Dusty,
Thanks for the suggestion, I will try it but I was looking for help with interpreting the GN chart from Canon. I did include the link but will add the chart at the end of this message.
What is confusing me is the statement about ISO in meters, the "Flash coverage in mm" and the how to use the different GN by distance.
The 420 EX is a zoom flash, although I am not using the TTL as it is not compatible with the camera.
Are these different GN in the chart based on the focal length I am using with my lens? Or is it because I am not using TTL I use one GN and stick with it?
Totally confused.
Thanks,
Viyo
Vito_F ,
The Lens focal length will not effect the actual exposure , Only the aperture setting of the lens is important .
The Guide Numbers are a power output number and they are used for manual exposure control and for you to determine what distance and aperture to use when shooting in manual exposure mode .
it is important to know that only the actual flash to subject distance is used to determine aperture setting , when using the manual exposure mode .
So if the flash is not mounted on the camera , ( which makes much more natural lighting ) you do not need to measure the camera distance to the subject , you are free to move to any shooting position you like with the camera and it will not effect exposure brightness ) .
The numbers given in mm are only a measure of the flash covering area diameter , but they make no sense to me , because 24mm is only one inch in diameter . 1 inch = 25.4 mm .
This chart makes no sense to me .
This is what I would do : Place the subject 23 feet from the flash , and take a photo of the subject , then see how it looks ( use ISO 100 and use guide number 23 on the flash head .
If the photo is too bright in the shot change the aperture on the camera to a higher number , ( so it allows less light to reach the sensor.
If you can use a lower guide number and test again . or for under exposure use a higher guide number . or a larger aperture opening on the camera .
The conversion for meters to feet is .3048 . if you measure in feet you can multiply the number of feet by .3048 to convert the distance to meters .
Be sure to set the camera shutter speed to the correct flash synch speed , Usually 1/200 ,You can also try Shutter speed of 1/100 to see how that works .
Dusty
Re: Help with guide numbers when using a flash manually
In reply to DUSTY LENS,
Apr 15, 2013

DUSTY LENS wrote:
Vito_F wrote:
DUSTY LENS wrote:
Vito_F wrote:
Hi,
am trying to use my canon 420 ex flash with a newly purchased Panasonic fz 200 as I cannot afford a compatible flash at this time.
the 420 is a zoom flash, according to the manual with guide numbers from 23 (at 42 mm?) to 42 (105mm). There is a note that it is for ISO 100 meters.
I am totally confused. Can someone break this down for me into feet and tell me how to calculate the f stop for let's say 4 to 25 feet? There is a chart in the manual
http://www.usa.canon.com/app/pdf/lens/Speedlite_420EX_manual.pdf
thanks,
vito
I do not understand the numbers you give ( (( 23 (at 42mm )) ) or the (( 42 (105mm) )) .
Very seldom would a guide number be given for 105mm or for 42mm .
and 42 meters would be way out there about ( 150 feet ) Nor a very probable guide number either .
You could find the real Guide number with test shots .
Try 10 feet and work backwards by changing the aperture on the lens until you have a perfect exposure .
Then you would have the two numbers you keep for finding the correct exposure .
Guide numbers usually are given for a certain ISO and at a certain distance .
ISO 100 at ( what distance ) would equal a guide number .
Read it again .
Hi Dusty,
Thanks for the suggestion, I will try it but I was looking for help with interpreting the GN chart from Canon. I did include the link but will add the chart at the end of this message.
What is confusing me is the statement about ISO in meters, the "Flash coverage in mm" and the how to use the different GN by distance.
The 420 EX is a zoom flash, although I am not using the TTL as it is not compatible with the camera.
Are these different GN in the chart based on the focal length I am using with my lens? Or is it because I am not using TTL I use one GN and stick with it?
Totally confused.
Thanks,
Viyo
Vito_F ,
The Lens focal length will not effect the actual exposure , Only the aperture setting of the lens is important .
The Guide Numbers are a power output number and they are used for manual exposure control and for you to determine what distance and aperture to use when shooting in manual exposure mode .
it is important to know that only the actual flash to subject distance is used to determine aperture setting , when using the manual exposure mode .
So if the flash is not mounted on the camera , ( which makes much more natural lighting ) you do not need to measure the camera distance to the subject , you are free to move to any shooting position you like with the camera and it will not effect exposure brightness ) .
The numbers given in mm are only a measure of the flash covering area diameter , but they make no sense to me , because 24mm is only one inch in diameter . 1 inch = 25.4 mm .
This chart makes no sense to me .
This is what I would do : Place the subject 23 feet from the flash , and take a photo of the subject , then see how it looks ( use ISO 100 and use guide number 23 on the flash head .
If the photo is too bright in the shot change the aperture on the camera to a higher number , ( so it allows less light to reach the sensor.
If you can use a lower guide number and test again . or for under exposure use a higher guide number . or a larger aperture opening on the camera .
The conversion for meters to feet is .3048 . if you measure in feet you can multiply the number of feet by .3048 to convert the distance to meters .
Be sure to set the camera shutter speed to the correct flash synch speed , Usually 1/200 ,You can also try Shutter speed of 1/100 to see how that works .
Dusty
Hi Dusty,
Thanks for responding and for validating that the chart is confusing.
The 420 REX has no manual controls, so I am unable to change any settings on the flash.
What is confusing to me, is that since I cannot use TTL with my current camera I don't see how the flash will zoom as it is not communicating with the camera. Therefore I assume it is firing at maximum flash.
I will use the maximum guide number and flash exposure to see if I can get the right formula.
Thanks again for responding.
Vito
Re: Help with guide numbers when using a flash manually
In reply to Vito_F,
Apr 18, 2013

Based upon the chart you supply, the manufacturer of the flash unit presumes that you will be shooting at 100 ASA (100 ISO).
“Guide Numbers” are nothing more than a number the manufacturer supplies to make proper calculations when you are trying to figure out which F/stop to use at which distance. They bare no relationship to how far away your subject is standing from your camera. You have to figure out that distance by using this Guide Number that is supplied. To use these Guide Numbers you must first have a formula to plug them into. The basic formula used is:
Guide Number = Subject Distance x F/Number you want to use (GN=Distance x F/number)
The “Distances” the manufacturer has given you in their chart, are their suggested distances that you might want to shoot at. Unfortunately, they have put them in “meters”. So, you must first convert their “meter” distances to “feet”. Do that by multiplying the number they give you by .3048. Once you have converted their “meter” numbers to “feet”, you now have two parts of the formula. The “F/Stop” to use at that distance, is the number you must now figure out.
Let’s take one of these, and see how it works. Let’s use their number of 105 meters. By multiplying 105 meters X .3048, we come up with 32.004 feet. Round it off to “32” for use in the formula. So far, that gives you:
32 (the GN) = distance x F/number
Now, you must figure which multiple of the F/stop you want to use, that will calculate out to the Guide Number of 32, when you multiply it by the number of feet your subject is standing from your camera. So, “what” times “what” will equal 32? The answer is 8 x 4. Now, plug those two numbers into the formula, and you get:
32 (the GN) = 8 (feet) x 4 (the F/stop to use).
So, if your subject is standing 8 feet away, you must use an F/stop of F/4 to properly illuminate the subject.
You must do this, using each “Guide Number” the manufacturer supplied in their chart, in order to figure out which F/stop you can use at what distance, when you use that flash.
truview
Re: Help with guide numbers when using a flash manually
In reply to Island Golfer,
Apr 18, 2013

Island Golfer wrote:
Based upon the chart you supply, the manufacturer of the flash unit presumes that you will be shooting at 100 ASA (100 ISO).
“Guide Numbers” are nothing more than a number the manufacturer supplies to make proper calculations when you are trying to figure out which F/stop to use at which distance. They bare no relationship to how far away your subject is standing from your camera. You have to figure out that distance by using this Guide Number that is supplied. To use these Guide Numbers you must first have a formula to plug them into. The basic formula used is:
Guide Number = Subject Distance x F/Number you want to use (GN=Distance x F/number)
The “Distances” the manufacturer has given you in their chart, are their suggested distances that you might want to shoot at. Unfortunately, they have put them in “meters”. So, you must first convert their “meter” distances to “feet”. Do that by multiplying the number they give you by .3048. Once you have converted their “meter” numbers to “feet”, you now have two parts of the formula. The “F/Stop” to use at that distance, is the number you must now figure out.
Let’s take one of these, and see how it works. Let’s use their number of 105 meters. By multiplying 105 meters X .3048, we come up with 32.004 feet. Round it off to “32” for use in the formula. So far, that gives you:
32 (the GN) = distance x F/number
Now, you must figure which multiple of the F/stop you want to use, that will calculate out to the Guide Number of 32, when you multiply it by the number of feet your subject is standing from your camera. So, “what” times “what” will equal 32? The answer is 8 x 4. Now, plug those two numbers into the formula, and you get:
32 (the GN) = 8 (feet) x 4 (the F/stop to use).
So, if your subject is standing 8 feet away, you must use an F/stop of F/4 to properly illuminate the subject.
You must do this, using each “Guide Number” the manufacturer supplied in their chart, in order to figure out which F/stop you can use at what distance, when you use that flash.
 hide signature truview
I G ,
You have the meter to feet conversion backwards .
Remember a meter is 3.2808 feet . Another way to remember the difference is to know the number of inches in a meter , which is 39.370078
So since it takes 3.28 fet to span the same distance as one meter You will need to divide the meters by the .3048 , not multiply . you will always end up with more than three times the number of feet to span the same distance .
105 meters is the same as 344.488 feet . ( another reason the chart makes no sense ) .
Dusty
Re: Help with guide numbers when using a flash manually
In reply to DUSTY LENS,
Apr 18, 2013

Thanks for catching that. I think I was trying so hard to explain it to him in a way he would understand, that I ended up confusing myself.
truview
Re: Help with guide numbers when using a flash manually
In reply to Island Golfer,
Apr 20, 2013

Island Golfer wrote:
Thanks for catching that. I think I was trying so hard to explain it to him in a way he would understand, that I ended up confusing myself.
 hide signature truview
You are very welcome , I make this mistake all the time , that's why I caught it .
Also , You explained the reasons for these numbers and how to use them better than I could have .
Dusty
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