Strobe Arm Selection Help

I'm thinking of adding a second DS125 strobe to my setup--in anticipation of a trip to Baja this fall. I'd like to get a better arm system to go along with it. The basic Ikelite strobe arm is functional but a bit inflexable.

I was thinking about a hybrid Ikelite ultralite 1" ball arm set, with two arm segments on each side. I was thinking about using the Ultralight bouyancy arm segments to get some additional lift for the heavy ds125s. On the left strobe I was thinking about using the Ikelite quick release handle near the strobe for easy hand holding.

What length strobe arms should I get? should both arm segments be the same length? Should they be symetrical side to side? I'll probably be shooting about 50% macro and 50% wide angle. Right now I have the 18-70 kit lens for WA but plan to upgrade to the 12-24 in 1-2 years.

for the heavy ds125s

In salt water, each DS 125 is @ 4 ounces negatively buoyant.

Along with size comes displacement. For each cubic inch of water displaced by the size of the strobe, .59 ounces ( the weight of a cubic inch of salt water) of lift is achieved. When you substract the lift created by the displacement of the strobe from the actual weight on land of the strobe, the net is only - 4 ounces.

hth,

b

Thanks bobf.

I are an engineer (not the train kind) so of course I do understand this in principle. I had heard others on this board complain about the negative bouyancy of Ikelite DSLR systems with two DS125s. There was a post about a homemade Bouyancy arm system that came to mind. Without any personal experience with more than one DS125, I thougt this might be a problem.

My one DS125 "feels" much heavier than 4 ounces in the water. Maybe its the lead weighted tray that is contributing so much weight? Ultralight claims that their bouyancy arms with a 16" and 16" arm is -5 oz in salt water--which would be perfect if the DS125 is +4 oz. But I think two 16s would be a bit much. On the other hand, two 8" segments while still +4 oz, they are still 6 oz lighter in salt water than the standard two 8" segments. That is something. With two arms 8 and 12" , the rig would be nealy a pound lighter in water.

Does anyone have experience with the ikelite Drebel housing with two DS125s? I expect it to be roughly the same since the housing is the same mold and the cameras weigh about the same. How is the Bouyancy? Do the ultralight bouyancy arms help?

Thanks

One other possibility for adding buoyancy to a rig with DS 125's is to fill an Ikelite Mini C light holder (Ikelite part # 4073 ) with buoyancy material. The light holder is inexpensive, and has the required connections from arm to strobe pre-molded ( see photo ).

When filled with " Core Cell A " brand of closed cell foam ( Divinycell being another option) each #4073 holder displaces 10.53 oz of salt water. One #4073 filled with Core-Cell, as shown, weighs 2 oz. Net buoyancy for each #4073 holder therefore is 8.53 oz ( lift). The addition of the light holder will also add a few inches of length to your arm setup.

Below is a photo showing the buoyancy arms I constructed with 1/2" Core Cell, a matt knife, and Devcon Two Ton Epoxy. 2x6x1.25" Ikelite arms and #4073 light holder designated with the letter "A " have been on three salt water shore dives. Letter "B" arms and holder have been on around 25 salt water shore dives. Except for staining by the water, arm system "B" has shown no deterioration. To avoid staining, the closed cell foam can be finished. The finish chosen will add weight to the final product.

For more information, follow this link:

http://www.digitaldiver.net/yabbse/index.p...id=5613;start=0

Thanks Bobf.

Actually I had read your post and it was in the back of my mind when I saw the Ultralight bouyancy arms. Of course your solution gives MUCH more bouyancy.

So what was the impeutus for the float arms? Obviously you felt that your system felt quite heavy underwater. 4 oz negative for each strobe doesn't seem like much--even 8 oz for two. Ikelite claims that the d70 housing is neutral underwater (not sure if he means complete housing AND camera). But the whole thing might not be that heavy. Was your camera the weight problem or was it the arms?

What length are your arms? Looks like you have a similur setup to the one I'm considering.

Thanks

So what was the impeutus for the float arms?

Trim in the water, overall buoyancy control while diving.

According to my own calculations, my whole rig was @ 4 pounds negatively buoyant. Initially I made the mistake of compensating for the added gear weight by removing lead from my weight belt. Jeff (sharpdiver) correctly pointed out to me the danger in this practice. In the event of an emergency, if I ever had to ditch my camera rig, I'd potentially go shooting to the top!

I tried a couple of other techniques using pvc, but as craig correctly pointed out, this material also added significant overall weight. In a local search of shipyards for Divinycell, I found the Core Cell.

Any one aspect of a camera rig' weight may appear insignificant, but you can see what happens when you add up all the components.

It was mentioned to me by divers with much more experience than I that a slightly negative rig is ideal. It was also suggested by divers with engineering backgrounds that placing the buoyancy material at the site of each component was a logical course of action. It was also pointed out to keep the buoyant material above the ballast, or heaviest most point. Otherwise the rig will have a tendency to go belly up.

Was your camera the weight problem or was it the arms?

I think it's a combination again of every component adding a few ounces here and there, but the housing, camera, and dual arm tray are actually lighter underwater that the arms. This is because the the displacement of water factor.

What length are your arms?

Each arm consists of two six inch by 1.25 dia segments. Three clamps and a quick release mount for the controller along with the #4073 light holder complete the arms set up. Total wingspan actually adds up to @ 52" from strobe head to strobe head when including the width of the dual arm tray. 52" still requires me to aim my strobes out slightly for wide angle shots due to the coverage offered by the DS 125's. IOW, you could go even wider.

IOW, you could go even wider.

So, would 2 sets of two 8" sections each be ideal?

What do you think is the optimal arm length?

Thanks Bob for the advice.

2 8" arm segments is fairly common with strobes such as the DS-125.

Strobe arm length, Strobe Angle of Coverage, and minimizing backscatter.

I created a simple visual aid to help comprehend what angle of coverage was offered by a DS 125. It was important for me to visualize this concept in order to better understand strobe aiming strategies and to diminish the potential for illuminating backscatter.

(please disregard this paragraph if applicable.....) Backscatter is suspended particles in the water column that are illuminated by the strobe. The reflected light off the unwanted particles enters the camera lens and is captured by the digital sensor. Because these particles are also out of focus, they appear even larger, adding to the problem.

I knew that with the diffuser installed, the AOC for a DS 125 was listed at @ 100°. Using a simple plastic protractor and large piece of paper, I measured out two opposing lines at 50° each that were spaced @ 3" apart. I then cut out the form and placed it in front of one of my DS 125's. As you can see from the photo, this "theoretical" beam angle meets the center of my lens at @20" distance. If my strobe was aimed straight ahead and my intended target was further from the lens, say 36", I'd be needlessly illuminating suspended particles between the lens and subject. Thus, to reduce backscatter I aim the strobes slightly outwards.

If my strobe arms were longer, I would not have to aim the strobe heads away from the center of the composition as much

Great visual aid.........thanks Bob

Karl

WOW Bobf, thanks.

Now that you have my engineering juices flowing. What is the definition of beam angle in this case? Maybe Ike can help here. There is clearly some light falloff from the centerline. Is beam angle defined at the 1/2 power point--the point at which the light power is 3 db down from the centerline power?

In any case, there will likely be some light beyond this angle to cause backscatter but the intensity will be very low. On the other hand, at 20" the subject will be lit by the edge of the beam which will give the most brightness at the right side of the image. With the diffuser your beam angle will be greater, and the lighting spread more evenly across the frame.

So I think the 8" arms will work well. I can cock them forward a bit for macro shots, and get the beam more toward the center of the frame without sending much light into the warer in front of the lens. I'll work this out in a drawing and post it soon.

Thanks Bobf

You don't want to get ike on another coverage rampage. He screamed for years about YS60 and YS90 claims of 100 and 110 degrees for strobes that provided the same coverage and intensity as the SubStrobe DS-50. Coverage claim is "supposed to" be for intensity difference of one f/stop from center to edge, so this is what we claim. Add the Inon 220 to the list of strobes that have the same intensity and coverage as the insignificant little DS-50. DS-50 is 2/10 of one f/stop brighter at the center, Inon 220 is 3/10 of one f/stop brighter at 90 degree.

edit: I was typing my response while Ike was posting his.

What is the definition of beam angle in this case?

Good question considering there is no industry standard that I'm aware of. It's obvious that the angle of coverage spec isn't as cut and dry as my paper cut out example makes it appear.

Ike has commented in the past that he uses the -1 f/stop guideline. When the intensity of the beam varies by more than one f/stop, that's where the "line" is drawn for an Ikelite product.

There have been discussions about running further objective tests to verify posted angles of coverage of various strobes. It has been mentioned also that these tests could be conducted underwater to take into consideration the effects of water and the shape of the strobe lens on the angle of coverage. Another variable, the color temperature of the strobe, may also influence the apparent results.

One f/stop difference between center and edge was industry standard for above water strobes that were so famous for their acccurate ratings that the name "Guide Number" was chosen.

Craig was going to generate accurate measurements of the various strobes, but heater problem has left his pool too cold. (He won't be happy when he finds the insignificant little DS-50 with its battery door he hates is equal to his Inon 220)

Ike Wrote:

One f/stop difference between center and edge was industry standard for above water strobes that were so famous for their acccurate ratings that the name "Guide Number" was chosen.

Ok, so if each progressive fstop is equal to a doubling of the light intensity, then this definition fits with the standard definition of 1/2 power beamwidth (or -3dB beamwidth) that we use in RF communications system design. So I would agree wholeheartedly with Ike, this is the logical way to spec a strobe--even comming from a completely different field.

bobf wrote:

It's obvious that the angle of coverage spec isn't as cut and dry as my paper cut out example makes it appear.

Your paper cutout is great for visualization--it really got me thinking. But the reason I bring up the definition of coverage angle is that given the 1/2 power beamwidth, you will get significant light spillage beyond this area that could light the particles in the water. Maybe moving the arms in toward your subject would help reduce this? Also, with two strobes the total intensity on the subject directly in front of the lens (in your example) should be the same as directly in front of the strobe (1/2 + 1/2 = 1 where the two light cones meet). So you should get even lighting across the frame.

Again, it looks like two 8" segments should do the trick for wide angle subjects about 3' away, which should be my normal working distance anyway. I understand that my maximum distance will be about 6' with any strobe?

Thanks

If your strobes have significant light falloff at the edges (1 stop) and you position them as in Bob's excellent training aid, then you will have uneven lighting in your photo.

But the solution for wideangle is not to pull the strobes in - it's to get strobes that throw even lighting.

This is why I feel that the power numbers in Jeff's strobe comparison article are not fair to the Ikelite DS125. This strobe throws a nice wide even beam - it's not "hot" in the middle. So measuring its power by taking a single-point reading in the middle of the beam does not accurately depict its performance. I haven't done a quantitative test to measure this as I don't have both strobes - however, I HAVE used both and can say qualitatively that the DS125 puts out more power per discharge than the Inon. It doesn't mean one is "better" than the other though - it just means that comparing wideangle strobes with a single point measurement is meaningless.

Another solution for strobe placement is to leave the strobes out and pointed straight ahead, but actually pull them BACK so that they overlap further.

Bob, can you pull the strobe back, adjust the foam, and post another photo please?

Cheers

James

James,

Yes, but it will have to wait until I return from a business trip. I leave today, in a few hours for Maryland. Feel free though if you'd like to expand on the concept in one of your essays. I think your understanding of proper strobe placement would benefit others including me.

You don't want to get ike on another coverage rampage. He screamed for years about YS60 and YS90 claims of 100 and 110 degrees for strobes that provided the same coverage and intensity as the SubStrobe DS-50. Coverage claim is "supposed to" be for intensity difference of one f/stop from center to edge, so this is what we claim. Add the Inon 220 to the list of strobes that have the same intensity and coverage as the insignificant little DS-50. DS-50 is 2/10 of one f/stop brighter at the center, Inon 220 is 3/10 of one f/stop brighter at 90 degree.

Ike,

Sorry can you clarify your last point. The 'standard' that has been used for calculating strobe angle is one full f/stop difference from centre to outer edge of the beam, that is understood. You compare the DS50 & Inon220 saying they are 2/10 and 3/10 of one f/stop brighter repectively, but brighter than what? Are you referring to the YS60 & YS90?

Gazzer: DS50 is slightly brighter than the Inon 220 in the center, 220 is lightly brighter than the DS50 at ninety degrees. In other words, DS5-50, Inon 220, YS60 and YS90 are all identical for all practical purposes. Inon 180 is half the power of their 220 which would be one f/stop less.

(He won't be happy when he finds the insignificant little DS-50 with its battery door he hates is equal to his Inon 220)

I remember that thread...

I'll try my best to get the battery door thing right!

Cheers,

Rob

(Proud new owner of insignificant little DS-50)