Best positioning for strobe to reduce backscatter

[craracer]

Is there a method, aside from trial-and-error, to determine what the strobe cone dimensions are for a given strobe?

I use a Inon S2000 and a Inon Z240.

 

[vladimir]

Is there a method, aside from trial-and-error, to determine what the strobe cone dimensions are for a given strobe?

I use a Inon S2000 and a Inon Z240.

It should be explicitly stated in the strobe's specs. I went to B&H and looked up a random strobe (they don't carry Inon, apparently):

Product Highlights

• Sea Arm VII Compact w/ Addtl Arm
• 2 Fiber Optic Cables w/ Strobe Mask Sets
• DS-TTL Auto Control
• 10 Level Manual Control Dial
• Beam Angle of 100° x 100°
• Guide Number 20 (ISO 100/m on Land)
• LED Target Light
• 1.9 Sec Recycle Time w/ NiMH Batteries
• Rated up to 250'

 

[craracer]

Z240: 100 deg & 110 with diffuser.
S-2000: 105°(horizontal) x 95°(vertical) [without a diffuser]
110° x 110°circular beam [with supplied -0.5 white diffuser]

So, ignorant question to follow: How do I measure this so I can get a mental picture of what the cone of light looks like?

 

[vladimir]

So, ignorant question to follow: How do I measure this so I can get a mental picture of what the cone of light looks like?

I'd get a 75-cent protractor and some string. I've never actually done it, but it seems like a worthwhile exercise.

 

[craracer]

I guess I'm off to the Dollar Store

 

[bvanant]

The problem with using the published specs for cone angle of the strobes is that they are undefined as to what power level you are measuring. Is it 50%, 1/e etc. The only way to get a feel for it is to test it. You can do it in your kitchen. Set up a scene put your housing together and point the strobes parallel to the port for the first test. Take the pic and adjust the power of the strobes to get a well exposed pic. Then gradually move the strobes outward from the port and repeat. I think that about 20degrees is about right for the Inon strobes with diffusers and that is my starting point. Underwater the light paths are the same but the transmission distance is of course much less.

Bill

 

[craracer]

I started thinking about using the protractor, and then realized the same thing that Bill talks about. In addition, determining where the zero point is on a strobe that has three "bulbs", one of which is at a 90 deg angle to the others, is difficult for me to figure out.

I think the above-water trial-and-error method is going to give me a good starting point. It's just time consuming, but better above water than when I have some amazing critter in front of me.

 

[DazedAndConfuzed]

The corner of a piece of a paper is 90 deg. So the cone would be slightly wider than the 2 sides of a piece of paper.

 

[Thalassamania]

45 degrees up and 45 degrees out with your subject just at the far limit of your depth of field.

 

[DoNotDstrb]

Strobe power setting has nothing to do with beam angle. The beam angle will be whatever it is at any power setting since it is a function of tube positioning and any diffuser that may be in front of the tubes. My 110a with 3 tubes arranged in a circle, gives a pretty circular cone of light. I suspect the I-240 with its "T" arrangement, would produce a more rectangular or square cone....

I tested this with a large white wall. I set up the rig on a tripod, shot some images of the wall with one strobe, then two. I used a fisheye lens to get MORE coverage than the strobes could produce at about 15'. I set my exposure to blow out the areas where the strobe hit making it very obvious. I looked at the images, marked the spots on the wall with blue painter tape, re-shot some more images for verification, then measured the angles. For my 110a, the angle came out to about 110 with diffuser -- pretty much as advertised. I then set about playing around with various positions and now apply my observations to what I do underwater.

It was a bit time consuming, but IMHO worth the time over trial and error underwater.