Help with guide numbers when using a flash manually

Started Apr 14, 2013 | Discussions thread
Veteran MemberPosts: 3,616
Re: Help with guide numbers when using a flash manually
In reply to Vito_F, Apr 15, 2013

Vito_F wrote:


Vito_F wrote:


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



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.



420 EX GN

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 .


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