Guide number of a strobe

Hi.

 

I know guide number is calculated by multiplying distance and the aperture number at ISO100

 

Say example Inon s2000 id rated GN 20 on land. Say visibility is good and it brings down the GN to GN 10.

 

Meaning to say if the subject is 1m away, I set my strobe to max (GN 10 underwater), i will get my subject correctly exposed or illuminated.

 

What about shutter speed? Does it influence the exposure as well?

 

At what shutter speed does the rating on the Guide Number of a strobe is based on??

 

Thanks in advance.

 

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The strobe's guide number is for the flash exposure only and is mainly useful as a guide to compare strobes. This assumes the various manufactures use the same method to determine guide number. Underwater guide numbers are generally not the same as air as your surmise. However for underwater strobes the angle of coverage is more important than guide number - the wider the better since we tend to use the edges of the light from the flash.

 

Shutter speed will also affect the overall exposure with longer exposures increasing ambient light. For underwater photography I tend to increase ISO rather than decrease shutter speed to increase ambient light exposure since I want to stay close to the synch speed whenever possible.

Hi.

 

I know guide number is calculated by multiplying distance and the aperture number at ISO100

 

Say example Inon s2000 id rated GN 20 on land. Say visibility is good and it brings down the GN to GN 10.

 

Meaning to say if the subject is 1m away, I set my strobe to max (GN 10 underwater), i will get my subject correctly exposed or illuminated.

 

What about shutter speed? Does it influence the exposure as well?

 

At what shutter speed does the rating on the Guide Number of a strobe is based on??

 

Thanks in advance.

 

Sent from my SM-N9005 using Tapatalk

The GN doesn't care about shutter speed, because the duration of the flash is very short (like 1/10000 s) compared to the shutter speed, so all the light from the strobe gets to the image sensor, no matter how long the shutter is open. Shutter speed affects ambient light, not strobe light. And, of course, the shutter speed has to be within the allowable range to let the strobes sync. On my OM10, that means no faster than 1/250 s.

The GN doesn't care about shutter speed, because the duration of the flash is very short (like 1/10000 s) compared to the shutter speed, so all the light from the strobe gets to the image sensor, no matter how long the shutter is open. Shutter speed affects ambient light, not strobe light. And, of course, the shutter speed has to be within the allowable range to let the strobes sync. On my OM10, that means no faster than 1/250 s.

Haha... never thought of that. Thanks bud.

 

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The guide number of a strobe works like this.

GN = Distance x F-stop

For a strobe with a GN of 20 (meters ISO 100) on land, I would estimate a GN of 10 (or less) underwater. If I want to shoot at f:11 then I can be around 0.9 meters or a bit less than 3 feet away. That is why (even with strobes) the rules about get closer and closer are what they are.

Bill

Can I pile on here?

 

What happens to the GN when you add a 2nd strobe?

 

For example: I have 2 strobes that are both GN 24. Obviously, the law of inverse squares tells me that I do not now have a GN of 48. I.e. I cannot leave my aperture the same, double the distance to my subject, and get the same exposure.

 

So, with 2 x GN 24, what's my new effective GN? Is it mulitplied by the square root of 2? So, now a GN of 34? Or, what I could get at 3m with 1 strobe, I can now get at 4.2m?

Yes. Square root of 2. But actually less than that unless both strobes are illuminating the same area. Imagine being 12 ft

away with one strobe, so using f/2.0. With two strobes you have twice the light, so can close down to f/2.8. At 12 feet, that implies a GN of 34.

Yes. Square root of 2. But actually less than that unless both strobes are illuminating the same area. Imagine being 12 ft

away with one strobe, so using f/2.0. With two strobes you have twice the light, so can close down to f/2.8. At 12 feet, that implies a GN of 34.

Agreed, but I would emphasize the "same area" issue. Your most likely outcome is similar amount of light over a wider area.

 

Why? Because they are mounted at different locations and pointing in different directions at different things. Despite your best efforts to align them their beams will not 100% coincide.

 

My strobes have target lights and it is very obvious during night dives that the 2 target lights hardly ever intersect (I use the target lights as my night dive light). They are always off - up, down, too close, too far, ... When they do intersect it is in a very small sweet spot. 2 feet closer or 2 feet further away means the light has much less overlap.

 

If you really want more light on the same subject, you will need to buy a more powerful strobe OR co-locate the 2 strobes together so that they face in the same direction. I made a small arm widget that allows me to mount 2 YS style strobes on the same arm. But I found that dragging 4 strobes around is too heavy and bulky...

Thanks for the info and tips, guys.

 

I have now obtained 2 Inon Z240 strobes. But, per the advice from Martin Edge in his book, I'm only diving with one for now. Until I really feel comfortable with it and the camera, etc..

 

I took one into the local quarry yesterday. Viz was crap but the results were still even worse than I expected. I was shooting brim as I could come across them. I was pretty disappointed at how close I had to get to them for the strobe to really do anything. I mean, I know I would have to get close, but dang! I had to get REALLY close!