Arms length

[AussieByron]

Think about two beams of light with a given angle
If you spread the arm wide and point the strobe forward the two beam meet in the center and illuminate forward with the same angle of one of the strobe
If your strobes are closer and you point them outward so that they meet in the same point as above the cone of light you are producing is much narrower and reduces by the amount you are turning them away. Part of the frame on the sides is likely to be lit by one strobe only whilst you want both rays of light to meet to eliminate shadows
So having strobes pointed forward and changing the arm length is physically the best way to have the largest area illuminated with the least backscatter in front of the lens
Not sure this is clear to you but if you get a piece of paper and Pencil and draw it is quite visual

I have to disagree with you here Interceptor. Having strobes out wide on extra long arms isnt the best way of illuminating large areas. What you will end up is alot of shadow when your subject comes close to you and alot of missed shot from trying to reposition strobe arm lengths especially if your using Ultra wide Fisheye lenses.

The cone of light from a strobe is still going to say 100-110degree in coverage. So having strobes close to the housing and behind it with those strobes angled slightly away from the dome/lens allows you greater coverage, no problems with shadow from a subject if it gets close (unless its touching the dome itself) and backscatter is reduced as the strobes are angled away from the lens. You dont have to worry about moving arms in and out if the subject gets closer or moves away.

The whole idea of wide angle photography is to get get really close to subject and have alot of coverage from strobes. Thats why we use fisheye lens with 180 degree coverage so we can fit that large subject like a Shark or Manta into the whole frame whilst being close enough to light up the whole subject with your strobes successfully.

Regards Mark

 

[Interceptor121]

There are two parameters one is the size of the object and the other is the distance
Long arms have nothing to do with size of object but with the distance you can cover
The longer the arms greater the distance from the port you can cover without back scatter
If you're shooting subject at 1 foot distance you only need two 5" segments
However if your lens can't fit your subject at one foot your arm length needed starts growing so at two feet you need around 28" of arm segments if you have a one foot tray
At one meter you need 50" arm
Obviously this is not practical so you find a compromise that works for you
With a classic 8+5 you can perfectly cover close focus wide angle and then angling the strobes as you describe try to cover subjects further away buy results will be sub optimal
Ultimately it depends on the field of view of your lens and how close you can get
With a static subject is quite easy to get really close with moving targets not that easy that is where the long arm are useful
Having said that I use the 5+8 as bill does and on macro I go to 5+5

 

[AussieByron]

Interceptor you get more angle coverage by having the strobes mounted behind the dome and angled away from the dome than you get with them mounted out wide and pointing straight ahead. The light coming from the strobe is going to go the same distance if its mounted wide or close and angled away. Angling the strobes away from the lens also removes most of the backscatter. Having the strobes back and angles away is excellent for moving targets as you dont have to worry about moving arms in and out if the subjects are coming and going.

So at the end of the day you shoot the same as Bill and myself with a 5 & 8 inch arms. ........?

 

[James R]

I have two each: 5, 7, & 9. The default is the 5 & 7, with the 9 being just in case I need a bit longer or whatever.

 

[Interceptor121]

I can't be bothered having separate configuration and mostly I shoot cfwa so 5+8 is fine the rest I shoot in ambient light
However I can see that when I have to angle the lights I get more of the area in front of the subject illuminated and backscatter on the sides The subjects end up having More shadows and become relatively flat as more light comes frontal
At the end it depends on the shots you take, if I was sure I was having mantas in each dive i would probably get longer arms
The reality is that mostly lights are for close range work and there aren't many large subjects around worth the extra length

 

[James R]

Come on man, changing an arm out takes all of about 10 seconds

The only time I can see using the 9" arms are for big wreck dives or large subjects like mantas or something, so mostly they just are along for the ride on trips.

 

[AussieByron]

Shooting on a regular basis at home which has alot of large subjects (Manta, Sharks, Loggerhead Turtles, Big schools of fish) I can tell you from experience that strobes in close and behind the dome angled away from the dome works very well with all wide angle shots that require strobe use. It provides great coverage to large subjects from almost touching the dome to well past the 1-2 foot which Interceptor states. Regarding backscatter the strobes behind and angled out allows you to shoot in very poor vis due to the strobes angled away from the dome so the reflection of the strobe light (backscatter) is angled away from the lens.

Actually the major reason why I use 8 & 5 inch arms is becuase I use ULCS double Buoyancy arms which makes my setup weight spot on when using the heavy Ikelite DS161 strobes.

Back to the OP another thing to consider is what strobes your going to use with your Nikon D7100. If your using heavy strobes like Ikelites ds125/160 you might have to think about double bouyancy arms from ULCS or something like stiks to take the weight out of your setup so its neutral underwater. Other wise you end up with a sore arms after a few dives and even to the point you consider you have a DCI hit (especially so if your on a liveaboard doing alot of dives per day).

Honstly for wreck dives the strobes are turned off and ambient light used as strobes simiply can not light up large areas such as big wrecks.

Regards Mark

 

[Onewolf]

For the last ten years I have used 5+8 and it's been great. I am going to try out 8+8 because I am adding video lights between the two arms segments.

 

[AussieByron]

Onewolf you can try 3 way clamps in the middle of the arms layout. It works ok but I prefer to remove strobes completely when I want to concentrate on Video. It also depends on what video lights your using also (weight and lumen output).

Regards Mark

 

[Onewolf]

Mark, I have ordered a pair of AOI RGBlue lights, the Nauticam multi clamp with shackle, and the Nauticam YS light mount for the multi clamp. I'm hoping to be able to leave the video lights mounted full time, but if they get in the way I can easily move them to the strobe (YS-110a) locations on the ends of the arms because both the strobes and video light use a YS mount.