# Flashpoint 620m guide numbers



## RichieT (Dec 26, 2013)

I just got the flashpoint 620m for Christmas (how did they know) and the instructions are a little sparse. My question is with guide numbers. My SB 800 has a similar full power GN, 184 vs 192. I'm curious if the GN's would also be similar as you reduce the power since there is the chart with the SB 800. I do have a light meter which is what I'm planning on using for most of my exposures. I also understand somewhat the inverse square rule. I'm curious, since the SB 800 is a smaller light source and the flash duration is different, if these factors will change the difference in GN's between the two flashes as output is decreased. Thanks for your help.


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## dxqcanada (Dec 26, 2013)

Distance = GN/aperture

So, your 620m would have the same light exposure on the subject by placing it a little further back than the SB800.

ex. full power
620m ... @ f8 = 24m 
SB800 ... @ f8 = 23m

Not sure what the GN is when either flash is set for less than full.


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## KmH (Dec 26, 2013)

A GN of 184 for the SB-800 only applies when the flash head is zoomed to it's maximum of 105 mm, full power is set, and ISO 100 is used.
At full power the GN gets smaller as flash head focal length is decreased. At a zoom head setting of 24 mm the GN has been reduced by about 1/2 and is shown as a GN of 98.
At a zoom head setting of 105 mm the beam of light the SB-800 projects is a lot narrower than the beam projected at 24 mm.

From the SB-800 user's manual specifications page:



> Guide number (at 35mm zoom-head position, 20°C/68°F)
> 38/125 (*ISO 100*, m/ft), 53/174 (*ISO 200*, m/ft)



Note that the zoom head setting and camera ISO setting affect the GN such that the SB-800 has many guide numbers when used at full power.
The temperature of the xenon flash tube in the SB-800 flash head will also slightly alter the GN, which is why Nikon notes the temperature their specification was derived at.

Adorama stated GN of 192 does not specify what camera ISO setting the GN of 192 applies to. The 620M doesn't have a zoom head so that is a variable it doesn't have compared to the SB-800.

Basically, you cannot compare the rating numbers for the 2 units.
From practical use you will likely discover the 620M at full power is substantially more powerful than an SB-800 at full power when both are the same distance from the point of measurement or subject.


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## Derrel (Dec 26, 2013)

As KmH was touching upon, the Nikon speedlight's highest GN is for the flash zoomed to the tightest, most-constricted beam spread. The GN of 184 for the SB-800 is for a NARROW, concentrated flash beam; the Guide Number for the Flashpoint monolight is for the unit with its supplied "Standard Reflector", which covers a wider angle than the speedlight. Here's a good review of the 620M and some accessories.Flashpoint II AC/DC Monolights: Battery Powered For The Photographer On The Go | Shutterbug

As KmH said, you will find the monolight is "*substantially more powerful*" than an SB 800. Especially when you fit a softbox or an umbrella to the monolight. I normally think of a full-power speedlight flash pop as being about 50 to 60 Watt-seconds in output as compared against my studio flash units. Flash duration really is not a factor in Guide Number calculations when shooting at "normal synch speeds", and not doing oddball off-list things like say, shooting the monolight at realllllly fast shutter speeds using a PC cord or sophisticated trigger and delay systems, where you're shooting on the long, diminished output "tail-end" of the flash output, the way some outdoors+ bright sunlight shooters do to "overpower the sun". For regular, everyday studio flash work, the Guide Number is what it is.

The thing about a speedlight is that it has a SMALL flashtube; smaller than a pencil in diameter, and under two inches in length, and it has a tiny,tiny reflector, and then uses a Fresnel lens system to concentrate and in effect "amplify" the light that bounces off of its tiny,tiny reflector, which has the surface area of say, HALF of a Wrigley's chewing gum wrapper foil. A monolight, OTOH, has a larger, longer, circular flash tube, and a MASSIVELY larger parabolic reflector, and six to twenty five times MORE stored energy which is discharged and fired into the gas in thew flashtube. The resulting excitation of the gas is bigger, generates more light, and also the light is sent on down the highway off of a BIG, efficient reflector.

Think about the SB-800 as a revolver chambered for .22 Magnum... versus the monolight as being a .357 magnum. That's about the difference.


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## RichieT (Dec 26, 2013)

Thanks for the replies. I'm aware that the 184 GN for the SB800 was at 105mm zoom and full power and that each combination of ISO, zoom, and power level will produce different GN's. It was also an assumption that the 192 GN on the 620 was at 100 ISO (The information for the lightstand and umbrella was more complete than what I got for the light.) I guess my question was more theoretical about the properties of light and the effect of duration and size of the light source on the drop off of light between the 2 units. Derrel, you pretty much answered the questions, thanks. Duration isn't much of a factor. I've been using the Graslon diffuser on the SB 800, with all it's bells and whistles and smoke and mirrors, to give a bigger light source which helped some but is still a relatively small light source. Could be  useful for fill now. I know there are rules and guidelines but experimenting and seeing what works for me is where the real learning comes in. Unfortunately, had too many people with kids over yesterday to set anything up securely, so experimenting has to wait. Thanks again for your replies


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