ACTUAL guide number of 380EX

[colin_miller3]

I have two 380EX speedlites. After much thinking, I have not been able to come up with an accurate guess on what the actual GN of the 380EX is. I have heard elsewhere that all flashes are given a higher GN than it actually has. So does anyone know what the actual ranges of 25, 100 , 400 and 800 ASA films would be?

[matt_orth]

Colin:

 

<p>

 

Guide Number = f stop x flash-to-subject distance

 

<p>

 

The "official" GN for the 380 is 100 (in feet, 100 ASA is assumed)

 

<p>

 

To check to see what the actual GN is, use 100 speed slide film, and

take some pictures at 10 feet with the flash on full power manual (is

this an option with the 380? I'm not familiar with that flash).

According to the official GN, you would need an f stop of 10 (10 ft x

f10 = 100 GN). Bracket the pictures around f 10. (Don't use any type

of flash modifier, Sto-Fen, Lumiquest, etc; just bare flash). TAKE

NOTES.

 

<p>

 

When you get the slides back, see which setting gave the best

exposure. Then plug that fstop into the formula and you'll find the

actual GN for your flash. Of course, ceiling height, nearness of

walls, color of ceiling and walls will all affect the GN.

 

<p>

 

TTL flash is a lot easier than messing with GNs and f stops -- I've

recently moved from a Vivitar 283 on Canon FD's to an A2e with a

540EZ, and WOW is flash effortless now. But it's still important to

understand the theory so you know when you have to out-think the

camera.

[name_unknown20]

Focal length-GN (in meters, for feet multiply by 3 approximately):

24-21;28-23;35-28;50-31;70-33;105-38.

[roy_kekewich]

Good stuff above, to which I can add:

 

<p>

 

There's a guide number chart on page 44 of your 380EX's manual.

 

<p>

 

In meters, the G.N. is 31 or 102.30 feet. I multiply be 3.3 to convert

meters to feet. Keep in mind that your Guide Number changes not only

with ASA, but also with focal length. It decreases at the wide end and

increases at telephoto. Finally, when using high speed sync (FP

Flash) guide numbers plummet (see the chart).

 

<p>

 

By the way, I get excellent results with both normal and FP flash.The

lower guide numbers associated with FP flash are of no consequence in

my photography since I use high speed synch only to add a bit of fill

to knock out shadows caused by bright, harsh sunlight, as well as to

add catchlights to my subjects'eyes.

 

<p>

 

Great feature: I can shoot wide open to blur out the background to

make my portrait subjects stand out while shooting flash synched to

any shutter speed up to and including 1/4000th of a second. This is

otherwise unheard of in 35mm photography, except with EX flashes used

on specific EOS models. Good going, Canon!

[pto189]

Test