About strobes and light

Various strobes to housing connections

Strobe connections Strobe connections are an essential part of  the underwater  hardware.  Often,  when things go wrong in the electrical circuitry that fires the strobes, the connections  are  the culprit.  A broken or  loose connector,  a short out caused by leakage,  a flooded sync cord   etc.

Basically, there are three ways to fire your strobes (see cartoons at the right)

-an electrical  sync cable  that connects to your housing via a bulkhead. All  dSLR cameras and housings support this method. You will need the right sync cable for your strobe + housing combination (upper figure, 1= camera, 2= cable, 3=strobes)

-for both point & shoot and dSLR cameras, you may use an additional strobe with a cordless optical slave. The optical sensor triggers the strobe by detecting another strobe’s discharge (middle  figure, 4= optical slave plus strobe)

- the pop-up flash of the camera and a fiber optic cable to carry the light to the strobe sensor (lower figure,  5=  internal flash plus cables). Most cropped sensor dSLR cameras have pop-up flashes. Almost all compact camera housings support this. Unfortunately,  most dSLR housings do not have enough  room inside the housing for the internal flash to pop up and fire the strobes. The problem of a pop-up flash is that  it uses up more battery life, and also some compact cameras are very slow to recycle their internal flash. The lowest setting of the internal flash intensity (e.g. 1/64)  is recommended. Some housings already have an  optical  cable  insertion slot to plug in your fiber optic cables.

Notes

1. Uncropped  camera systems often do not have a built in flash. But a nice innovation  here is  a fast  optical  (LED) trigger on the hot shoe that does not drain the battery and has less heat build-up (its the same set up as shown in the lower cartoon). These triggers are rather bulky, so they need a specially designed  housing that supports these units.  However since the LED signal output  of these triggers is not as strong as the signal from a  pop-up flash,  not all strobes are compatible. LED based triggers also allow fast shooting sequences, perfect for fast moving action. The camera does not not have to wait for the popup flash to recycle, meaning faster shooting, that  is as fast as your external strobes can handle or you buffer can take.

2. On some electrically triggered  strobes its possible to attach a adapter plug on the strobes bulkhead. This transforms the optical signal received via an optical cord  into an electrical signal, sometimes even preserving the TTL option. These adapters also allow triggering your strobe by a remote light source like a pop-up flash or even a LED trigger.

3. TTLis an option that is mostly  used by macroshooters. TTL circuitry can be built in the housing or in the strobes. It will work with sync as well as with optical cables. I never use TTL.  TTL is certainly not recommended for wide-angle UW photogaphy. Using the manual settings on your strobes will allow you to create a more balanced lighting of your subject.

4. Future housings might offer fiber optic as well as electrical connectors for strobe triggering. The corresponding strobes will then also accept electrical as well as optical input.

Strobe positions The cartoons  below show some popular strobe positions seen from above. Left: macro (1:1, flat port), middle: fish-eye lense with the subject very close (10-50 cm, strobes pulled backward and close to the housing), right: wide angle with the subject further away (e.g. 1-2 meters). Here  in the horizontal plane the 9-3 and  10-2 o'clock positions will  often give satisfactory results. If you are close to a sandy floor, its better to keep your strobes a bit higher to avoid over- exposure of the sand.

 For macro shots (left position) you can turn the strobes inward to create a  focused light bundle on your subject, or more outward to avoid backscatter of particles  drifting  in front of your port.  Backscatter  will  often not be a big  problem with macro shots, since the subject of interest and small particles floating in the water will have about the same luminance.  For CFWA shots the middle position seems to be most appropriate. Here the primary subject you want to catch in your light beams can be pretty close  like at 10 cm, so in this respect its a bit like  macro shooting. But since you are using  a fish eye  lense, you have to pull the strobes to the back in line with the handles and close to the housing to avoid the strobes or 'hot spots' entering your image. This will also improve the quality of your lighting of  the nearby subject. The general principle for wide angle: put your strobes further apart and more forward for subjects at a greater distance. And slightly pivoted outward to avoid back scatter (right position).

Alex Mustard, a master of  underwater lighting, also recommends the crossed strobes for objects close to the domeIt resembles a bit the CFWA position;  the strobes are also pulled backward in line with the handles, but now on long arms furher outward and pointing towards the object. This position is recommended to enhance texture of  a large object or lighten up a small object in a darkish environment, like a cavern. 

How many strobe arms should you use? With a large object further away, two  arms at each side of the housing will give you a greater reach and spread of light. For example, the short arm (e.g. 5 inch) near the housing and the longer (e.g. 7 inch) near the strobe. Some people even use three arms which when spread out look like gigantic spiders.   Strobes will then need to be set  at full power.  But if you are shooting with a fish eye lense  your target will often be very close to the lens, and the strobes should be  much closer to the housing and pulled backward. In this case just one arm at each side of the housing might be a good solution and easier to set up, with the strobes set at less power.

Balancing light  When you are shooting wide angle, you enter the domain of mixed lighting: the quality of your pictures depends on the balance between  a strobe-lit foreground and an ambient light-lit background. The balance becomes even more critical  for CFWA shots taken with a fish eye lense and a very close foreground subject. Its important to realise that the effect of your strobe on an  UW subject depends on three factors: its power setting, the aperture of the camera and the distance to the subject.  Shutter speed plays no role here. Since the foreground subject will be very close to your camera and strobe, its exposure will be strongly affected by the light of the strobe as well as the aperture of the camera. Alternatively, exposure of the background depends on both the aperture and shutter speed of the camera.  But since it lies at a greater distance from the strobe, it will be more affected by ambient light than by the light of the flash.  Choosing a longer shutter speed then seems a good option to control exposure of the background. The take home message: strobes control the foreground, shutter speed the background.

 A useful strategy  in CFWA shooting is to start with the strobes set at half power. Since both strobes are at the same distance from the foreground subject (see middlepicture above) they can be set at the same power. Then select the aperture to get the correct  lighting of the foreground, and as a final step adjust the shutter speed to bring in that nice blue background.  

Notes:

-a slower shutter speed in CFWA  will usually not affect exposure of the flash-lit foreground.  But with a fast moving foreground subject like a shark at a greater distance (say 50-100cm) its a slightly different story. Here it can create a double image of the foreground subject, showing up as a thin line of shadow in front of the sharks nose (see for an example  this  fast moving Carribean reef shark: https://www.flickr.com/photos/fttpwwwflickrcomalbert/4499541799/in/album-72157618226419179/). The blur occurs because with a  slower shutter speed ambient light  will affect the image in the remaining the time frame, after the subject is 'frozen' by the flash at the start of the exposure. Suggesting that it is moving backward.  By selecting the  'rear curtain' option in your camera (and perhaps even a longer exposure for more dramatic effects), the blur will show up behind the shark, creating a  more natural  suggestion of  forward movement (see also *, page 132). A simple rule of the thumb: Front curtain sync, Blur in Front. Back (rear)curtain sync, blur  Back (Behind) of subject,

 When the object is off-centre  be aware that you need to reduce the power of the strobe closest to it. Uneven lighting when shooting CFWA pictures may also turn up with vertical shots and the cameral  slightly tilted upwards. Here the closer lower strobe needs reduced power to prevent unnatural 'uplighting'.

Snoots Snoots are extensions of a strobe, most often used in macro photography to create a sharp and narrow bundle of light on a small object such as the eye of a blenny (see the macro section for references). In a  simple snoot the width of the light bundle depends on its length and the front opening (see two pictures below). 

Snoots can be very helpful in murky water, to lighten up a small subject at  a short distance. But they can  also be of use  in wide-angle shooting  to lighten up more tightly a foreground subject like a fish or a anemone,  while still preserving the background blue. Or to lighten up a nice anemone of soft coral in a cavern without affecting the natural lighting of its interior. An example is shown below  in  two topside pictures. The left is taken with ambient light. In the right picture (taken with the same settings)  the lighting of the oranges on the foreground with a self made snoot did not affect the exposure of the background.  Here the narrowness or edges of the light bundle are less critical . Snoots used in combination with macro is getting a more and more technical issue.  Some examples: what tripod, how to focus your snoot, using one normal strobe and a second strobe with with a double snoot on its front etc. 

 

                                       

Sources:

*Underwater photography masterclass. Alex Mustard. 2016. Ammonite Press. 

http://www.uwphotographyguide.com/underwater-photography-strobe-positioning

*www.nauticam.com/product.asp?id=318

http://www.divephotoguide.com/underwater-photography-techniques/article/underwater-strobes-connections/