Guide numbers confusion

I had a sunpak 383 that I used with my panasonic fz30. It had a guide number of 37. I've now bought a 580ex to go with my new canon 30d, which has a guide number of 58.

Now obbviously the 580ex is a much much more capable/featured flash than the sunpak, but all of those benefits aside I was also quite happy that I was buying a much "brighter" flash. In my mind those guide numbers and the inverse square rule suggests that the 580ex should be (58 * 58) / (37 * 37) = 245% as powerful as the sunpak, so 2.5 times as "bright".

Now I have it and it doesn't seem that bright... looking into it the guide number of 58 is at a zoomed 105mm. At 35mm coverage (what the sunpak is fixed at) it delivers a guide number of - you guess it - 37.

How very misleading, b'strds! To be fair, Canon do actually say "at 105mm" in their literature, but I didn't realise the significance of that (being fairly new to these things and never having seen a zooming flash before)

Sucks

--
Eff Zed 30

You have purchased the most powerful flash Canon makes. Hundreds of thousands of people use them successfully. Stop fretting over things you can't really control and just learn to get the most out of what you have. Start by reading these...

http://super.nova.org/DPR/580ex/
http://super.nova.org/DPR/Design/
http://super.nova.org/DPR/DualFlash/
http://super.nova.org/DPR/FillFlash/
http://super.nova.org/DPR/DIY01/

CG

You also have to make sure the GN is at the same sensitivity (ISO), and that you aren't comparing GN in ft with GN in meters.

Sometimes you luck out, and can find a table that lists the GN vs. "focal length" for a given flash. I eventually found one for the Nikon SB-600 and 800, but I had to find it on the Nikon UK site. To the best of my knowledge (i.e. I couldn't find it) it's not on the Nikon US or Nikon Japan sites.

It was enough for me to recognize that either flash would be a downgrade from my Sunpak 555. I'd rather have the power than the i-TTL capability, and I think I can pick up 2 more 555s for $40 more than the price of one SB-800. That would give me not only a lot more flash power, but a lot more versatility in lighting.

Now, if Sunpak would only produce a module for at least minimal Nikon i-TTL compatibility (sans wireless capability?)

All the best,
--

• Arved

'Take only pictures. Leave only footprints.' - Photographer's creed

well yeah, it had some features I wanted, the extra pwoer that I thought I was getting over my 338 was merely a bonus ;-)

--
Eff Zed 30

Thanks for the links by the way, reading the first one is very good, it's stuff I kinda knew already but it makes it much clearer in my mind - I'll put my hand up to being a bit new to all of this

--
Eff Zed 30

So you are sayiing the 580ex should be a little more than twice as bright. That is only a little over one stop at the same distance; really not much difference.

Brian A.

The Sunpak 383 and Canon EX580 have the same power at 35mm, almost.

Canon 580EX Speedlite is 118.1

Sunpak 383 Super Flash is 120

Regards.

RE> I didn't realise the significance of that

So quit blaming Canon for your ignorance.

It's your own falut you did not know what you were doing, and how in the world is Canon supposed to know that you, with your knowledge level, was going to buy the flash?

Canon works very hard to provide online courses,
detailed brochures, and so on.

just for being misleading in his topic line.
at least he did not post "how dare Canon sell this flash?"
--
Member of The Pet Rock Owners and Breeders Association
Boarding and Training at Reasonable Rates
Photons by the bag.
-----.....------

if I mock you, it may be well deserved.

n/t

Hi Seb,

It's an easy mistake to make, as the inverse square law takes a little while to get yor head around.

The multiple is 1.4.

For example, if you were to have two of your Sunpak 383s firing together, the light output would be 1.4 times the output of one 383, not double. If you had three 383s, it would equate to 1.7 times the single output. You would actually need FOUR 383s to achieve twice the output of your single flash.

Hope this helps, don't be put off by the rude jerks, there are some very sad and lonely individuals out there.
Good Luck,NY

Hi Northy,

NorthyYorky said:

It's an easy mistake to make, as the inverse square law takes a
little while to get yor head around.

Click to expand...

...but has nothing to do with the output power of a source. Refers to distance .

NorthyYorky said:

The multiple is 1.4.
For example, if you were to have two of your Sunpak 383s firing
together, the light output would be 1.4 times the output of one
383, not double. If you had three 383s, it would equate to 1.7
times the single output. You would actually need FOUR 383s to
achieve twice the output of your single flash.

Click to expand...

The multiple is 2. Power is additive.

NorthyYorky said:

Hope this helps, don't be put off by the rude jerks, there are some
very sad and lonely individuals out there.

Click to expand...

you bet....

--
cheers, Peter

Germany

No, two flashes side by side produce twice as much light as one flash.

The inverse square law refers to distance between light source and subject. The get half the light on the subject, you have to move the light source 1.4 times further away from the subject.
--



No, that's not my middle finger....

DougH said:

No, two flashes side by side produce twice as much light as one flash.

Click to expand...

Peter Berressem said:

The multiple is 2. Power is additive.

Click to expand...

This extracted from The Professional Guide to Photo Data, Richard Platt, 1995.

"The intensity of light falling on a subject is inversely proportional to the square of the distance between subject and lamp. In other words, if you double the distance, you reduce brightness to a quarter; treble the distance and brightness falls to 1/9."

"Increase the distance by the square root of two (or about 1.4) and the brightness is halved. If you double the distance of an object from a light source, the light illuminates four times the area and is only a quarter as bright."

"If you use two identical flash units, their combined guide number is 1.4 x the individual GN. For three units, multiply by 1.7, and for four, by 2."

Can you please clarify, I'm far from an expert but your statements seem to contradict the various published sources that I have checked.
Many thanks,
NY

You are confusing guide numbers, which include distance in their formula, with power output. The GN is given as a distance. Guide numbers are usually stated for a given ISO at a theoretical f/1 and give a distance. For example the Canon 580ex has a GN of 58 metres at ISO 100, f/1 and 105 mm focal length. The GN of a flash is the theoretical maximum range of the unit at full power for a given focal length (or angle).

Max range = GN / f-stop

Two equally powered flashes have twice the power of one unit. At the same distance they will give you twice the light, which is one stop of aperture, or could be moved 1.4 times further away for the same exposure; or if they were continuous lighting, twice the shutter speed.

Brian A.

Hugowolf wrote;

Hugowolf said:

You are confusing guide numbers, which include distance in their formula, with power output.

Click to expand...

Brian, you are absolutely right.

Thank you so much for taking the time and patiently explaining - the penny has now dropped!
Good luck,
NY