Confused by guide numbers

[kchong]

Something that still stumps me to this day is guide numbers, I know I have to divide the GN by the aperture to get the distance but what about the shutter speed part of the equation? How does that factor in? Thanks in advance for the enlightenment!

 

[konicaman]

Shutter speed is not an issue here, as the flash burst is too short to be influenced by the shutter speed. Usually you just set the camera to the highest shutter speed with which the camera is able to sync with the flash. Unless you want to use a very long shutter speed to get e.g a rather dark cityscape behind your main target well exposed too. Last example would not have any bearing on your calculation for the target lit by the flash either.

Happy shooting
Henrik

 

[Chriscrawfordphoto]

THe flash burst itself is so short, only 1/1000 of a second, that shutter speed doesn't matter most of the time. In dim light, the flash will provide all the exposure at the usually shutter speeds used for flash (like 1/60 or 1/250) and the ambient light will be too dim to affect the exposure.

Guide numbers are only accurate if the flash points direct at the subject. If you bounce it off the ceiling, they won't work because the ceiling will absorb some light. For that, you need to use a flash meter or the autoflash mode, if your camera and flash have one.

 

[Sarcophilus Harrisii]

Something that still stumps me to this day is guide numbers, I know I have to divide the GN by the aperture to get the distance but what about the shutter speed part of the equation? How does that factor in? Thanks in advance for the enlightenment!

As others have said the flash exposure (in the context of fully manual operation) is determined by the flash power (guide number); subject distance; ISO; and aperture. The shutter speed will not have any bearing on the flash exposure providing, (particularly if it is a focal plane shutter) it is within the shutters flash sync range.

Where the shutter setting will of course have an effect will be with the amount of exposure created by any non-flash light sources in the scene. So it can be helpful for some shots to consider the proportion of flash fill and ambient light you want to achieve. As an example; combinations of: 1/60 f/11; 1/125 f/8; and 1/250 f/5.6 will all provide the same amount of exposure (Exposure Value #) from ambient light, but the different apertures would deliver different amounts of fill flash relative to the (same amount of) ambient exposure. Apart from the usual photographic considerations such as depth of field, blurred or frozen movement etc. choosing the best combination of shutter and aperture available within the metered EV of your scene enables you to achieve the balance between ambient and flash that you want for the shot.
Regards,
Brett

 

[oftheherd]

Up to the X-sync speed of a focal plane camera, in normal flash exposure, the flash is fired when the first curtain has fully opened. That means the second curtain is still open as well, and the short duration of the electronic flash allows proper exposure, assuming your aperture and distance are correct for the guide number of the flash.

If you exceed the x-sync speed, the second curtain may have begun to travel, cutting off part of the film plane before the first curtain is all the way open, and the flash fires. At a shutter speed above the x-sync, the camera shutter is trying to controll the exposure by a moving slit across the film plane, not how long the entire plane is exposed by a fully open shutter. The flash firing when the second curtain has partially closed will cut off part of the film plane from the flash exposure, resulting in the photo being very dark on the cut off portion.

So, as has been alluded to, the shutter speed has no effect on flash as long as you are using the x-sync speed or slower.

With in-lens shutters, the x-sync fires the flash when the shutter is completely open, and again, the shutter speed therefore does not affect the photo in any way, but does allow x-sync at all shutter speeds.

If that is too confusing, just go with what has been said above, that shutter speed at or below x-sync allows use of the GN formula without regard to the shutter speed.

 

[rbsinto]

As I was taught:
During flash exposures, the aperture exposes for the subject and the shutter speed exposes for the background.
The actual duration of the flash burst is much shorter than 1/1000th of a second.
It is probably closer to 1/15,000th of a second, which is why it isn't influenced by the shutter speed (as long as it is no faster than it's rated flash synch which is the fastest shutter speed at which the first shutter curtain travels all the way across the opening before the second curtain begins to follow it to close the opening and complete the exposure).
So a 1/15,000 or 1/20,000 sec burst of light is bright enough to illuminate the subject, and the balance of the time the shutter is open before and after the burst is what will expose for the background which has not been illuminated by the burst.
Long shutter speed and the background will be bright, and at a faster shutter speed, and depending on the ambient light it might be darker.
I hope this information is helpful, and, ahem.......illuminating.

 

[kchong]

Okay, it all makes much more sense now. Thanks for the clarification, gents.

 

[21-135 Apo All Good]

As I was taught:
During flash exposures, the aperture exposes for the subject and the shutter speed exposes for the background.
The actual duration of the flash burst is much shorter than 1/1000th of a second.
It is probably closer to 1/15,000th of a second, which is why it isn't influenced by the shutter speed (as long as it is no faster than it's rated flash synch which is the fastest shutter speed at which the first shutter curtain travels all the way across the opening before the second curtain begins to follow it to close the opening and complete the exposure).
So a 1/15,000 or 1/20,000 sec burst of light is bright enough to illuminate the subject, and the balance of the time the shutter is open before and after the burst is what will expose for the background which has not been illuminated by the burst.
Long shutter speed and the background will be bright, and at a faster shutter speed, and depending on the ambient light it might be darker.
I hope this information is helpful, and, ahem.......illuminating.

IMHO you're the only poster with the correct answer. In terms of flash, most folks working with Guide Numbers fail to take into consideration that shutter speed controls the background's exposure level, regardless of the speed of the film used.

Dragging the shutter is used by pros all of the time during indoor photos with bracket-mounted flashguns, as well as studio lighting on location to light a ballroom area for instance. The reverse is true for establishing correct exposure of an interior with windows to the outside world. A delicate balance of both shutter speed and aperture is required. I learned the hard way, then bought a polaroid back for the Hasselblad.

For the OP: Guide Numbers are easiest to use in practical application of the inverse square law: double the distance requires 4X the quantity of light (or 2 full F-stops).10 feet @ F-16, 14 feet @ F-11, 20 feet @ F-8 are all GN 160. Establishing a manual GN via a test roll is the best way to go. If you know a GN for a 100 ISO exposure (i.e.: 160), you can therefore extrapolate a GN for almost any ISO speed. In this example, switching to a 200 speed-rated exposure yields a GN of 220 and a 400 speed exposure value yields a GN of 320, or 10 feet @ F-32!

As far as flash duration, shoe-mounted strobes tend to have a shorter flash duration, whereas "potato masher" strobes like the Metz 60 CT-4 operate at around 1/400th sec. on a full-power manual dump. Check your flashs' manual for it to determine whether or not any exposure compensation is required if your shutter speed is shorter than the flash duration of the strobe in use, i.e.: 1/500 sec. in daylight with between-the-lens shutters.

Hope this helps the OP.

Dave

 

[Sarcophilus Harrisii]

IMHO you're the only poster with the correct answer. In terms of flash, most folks working with Guide Numbers fail to take into consideration that shutter speed controls the background's exposure level, regardless of the speed of the film used.

Dragging the shutter is used by pros all of the time during indoor photos with bracket-mounted flashguns, as well as studio lighting on location to light a ballroom area for instance. The reverse is true for establishing correct exposure of an interior with windows to the outside world. A delicate balance of both shutter speed and aperture is required. I learned the hard way, then bought a polaroid back for the Hasselblad.

For the OP: Guide Numbers are easiest to use in practical application of the inverse square law: double the distance requires 4X the quantity of light (or 2 full F-stops).10 feet @ F-16, 14 feet @ F-11, 20 feet @ F-8 are all GN 160. Establishing a manual GN via a test roll is the best way to go. If you know a GN for a 100 ISO exposure (i.e.: 160), you can therefore extrapolate a GN for almost any ISO speed. In this example, switching to a 200 speed-rated exposure yields a GN of 220 and a 400 speed exposure value yields a GN of 320, or 10 feet @ F-32!

As far as flash duration, shoe-mounted strobes tend to have a shorter flash duration, whereas "potato masher" strobes like the Metz 60 CT-4 operate at around 1/400th sec. on a full-power manual dump. Check your flashs' manual for it to determine whether or not any exposure compensation is required if your shutter speed is shorter than the flash duration of the strobe in use, i.e.: 1/500 sec. in daylight with between-the-lens shutters.

Hope this helps the OP.

Dave

Presumably, this must mean you are also incorrect.

 

[Sarcophilus Harrisii]

I should add that I don't consider your answer as being any more right or wrong than the other members who endeavoured to help, including myself, as I went to some effort to discuss the relationship between flash fill and other light sources based on the combination of shutter and aperture employed for a given exposure value. Did you miss that part?
Regards
Brett

 

[21-135 Apo All Good]

I should add that I don't consider your answer as being any more right or wrong than the other members who endeavoured to help, including myself, as I went to some effort to discuss the relationship between flash fill and other light sources based on the combination of shutter and aperture employed for a given exposure value. Did you miss that part?
Regards
Brett

Yes I guess I did. After re-reading your post, I must say that you did tell the OP essentially the same info on balancing the ambient background exposure.

His main question however was about Guide Numbers and his confusion. I provided the needed information (with examples at various ASA/ISO ratings) in this area, and he disappeared?😉