Watt/sec to guide # conversion

[fishinfol145]

Hi all

Most manufacturers us a Guide # as their power reference. However, Ikelite uses Watt/sec as their measure.

Is there any way to get a direct comparison?

Thx

 

[tursiops]

Hi all

Most manufacturers us a Guide # as their power reference. However, Ikelite uses Watt/sec as their measure.

Is there any way to get a direct comparison?

Thx

I'm looking at Ikelite specs. I see guide numbers, as well as W/s.
The DS51, for example, is 50 W/s and GN 28 (feet) underwater.
I see no way to make a useful comparison since the W/s is total output, and the GN depends on the beam coverage.

 

[bvanant]

No easy way and no two companies even measure GN the same way so comparison is tricky. Kind of like lumens in torches, much pure fiction there.
Bill

 

[Backscatter::Jim]

Watt seconds is a measure of electrical energy. GN is a measure of light. While one could say a higher watt second rating would generally mean a brighter flash and higher GN, you can't make a direct comparison as there are so many other factors such a reflector design, beam angle, and other inefficiencies in the transfer of electrical energy that make such direct translations impossible.

GN is still the best way judge a strobe but it doesn't tell the whole story. A super wide and even diffuse beam of something like a Seacam 160D will have a lower GN than something like a Inon Z330 but the Z330 has a much brighter spot in the center but falls off rather quickly as it goes to the edge. The Seacam has much more energy (watt seconds) going into but has the lower GN due to the beam being spread more evenly.

Lumens works kind of the same way. It is a measure of total light, regardless of beam angle or exposure value. This is why you can have a small torch with a lumen rating of something like 500 lumens with a very tight spot beam have a higher EV value than something with a very wide and even beam that's measured at 10,000 lumens.

For either flash or LED lights it takes a lot more energy to spread a beam nice and wide than it does to make a narrow spot.

 

[tursiops]

Lumens works kind of the same way. It is a measure of total light, regardless of beam angle or exposure value. This is why you can have a small torch with a lumen rating of something like 500 lumens with a very tight spot beam have a higher EV value than something with a very wide and even beam that's measured at 10,000 lumens.

this is a confusing way of stating this; 1000 lumens spread out over a large area is not as bright (what I think you are calling EV value) as 1000 lumens concentrated into a small spot. GN is just a way of quantifying the brightness, no? Are you using "EV value" and GN interchangeably?

 

[Backscatter::Jim]

1000 lumens spread out over a large area is not as bright (what I think you are calling EV value) as 1000 lumens concentrated into a small spot.

You are correct here.

GN is just a way of quantifying the brightness, no? Are you using "EV value" and GN interchangeably?

They are not exactly interchangeable.

A guide number (GN) consists of a shutter speed and ISO to come up with the GN. Since flash is faster than shutter speed (in general of course) the shutter speed doesn't really matter when making that measurement with a light meter in flash mode. This is why GN is specified at a certain distance and ISO rating. If you just quote a guide number of ƒ22 for a flash it is meaningless unless you also state the distance and ISO.

Exposure Value (EV) is another type of measurement that is a better way of measuring a constant light source like a video light. It works the same way as GN with a light meter and is measured the same way, but this time shutter speed matters. For a constant light you would need to state the shutter speed AND ISO for a GN as the constant light is subject to shutter speed. ISO 100 at 1/125 is the same exposure as ISO 200 at 1/250. So to keep things simple and compare apples to apples, it can be expressed in EV which just gives 1 number and accounts for the differences of moving a shutter or ISO around to other equivalent values.

Here's a great site where you can take any ISO, shutter speed, and aperture combination and see what the EV is:

https://www.scantips.com/lights/evchart.html

It also has a chart of equivalent exposures for different shutter speeds, ISO, and apertures.

Sorry for the long explanation but that's why flash is measured in GN, and video lights are measured in EV. Or at least should be instead of watt seconds and lumens.