Film Question

[WGregVet]

cdiver2,

Chris is the expert here, I'm strictly amature! The only camera I'm familiar with UW is the Nikonos. Looking at the other Nikonos thread I'll guess you were using a Nikonos V for these shots.
There are two potential sources of light for an UW exposure, ambient light and light from the strobe. The chart on the strobe applies to exposures using light from the strobe only at maximum output (as would happen in manual mode). The F8 at 3 ft you mentioned (for whatever ASA film you were using) states that the strobe can deliver adequate exposure at that fstop and distance for an average reflective subject (as Chris mentioned more reflective subjects such as a tarpon will reflect more light than a nonreflective object like a dark coral, so you have to make mental adjustments for that and bracket the shot). If you are using TTL the the chart gives you the maximun distance of 3 ft at F8 but the TTL can reduce the strobe exposure if an object is brighter or closer. TTL reads the amount of light the film is exposed to and stops strobe output when it determines the film is properly exposed.
Ambient light is measured by the camera's built in light meter (separate meter in the Nikonos V) and is controlled by the shutter speed and fstop. Adjusting the shutter speed will only affect ambient light exposure, not strobe exposure.
Since TTL is measuring the amount of light the flim is exposed to during an exposure high ambient light can reduce strobe output to keep from over exposing a picture. Looking at the photo you posted it doesn't look like that was the case. If your strobe fired on those shots you may have been too far away for the fstop you were using. If the strobe didn't go off I'd be suspicious of a poor connection between the camera and strobe. I think Chris has written a method of testing TTL before.
If you were wanting to expose the pictures with ambient light only then ignore the chart on the strobe and use the camera's light meter to set fstop and shutter speed.
Hope this helps and isn't too confusing.

 

[Chris Bangs]

New camera and housing, rebel G 20mm lens. After thinking about your last post it may have been the metering. Camera in the vertical position so all three metering areas would have been in the blue. Duh

20 mm lens = 15mm nikonos - WIDE ANGLE!!

So forget TTL! Try manual strobe control. I am not familiar with the YS 120, but I would guess that at f8, something like 1/4 power or 1/2 may do it. do some test shooting and make your own strobe exposure chart then put it on a slate, or like I used to duct Tape it to the strobe body

 

[Chris Bangs]

cdiver2,

Great post! I shoot but can't write, more so at 5 am!

good idea, test the TTL!

so after that wonderful post I will run you through TTL testing

to do so, first open the aperture to its widest setting point the strobes at the lens. fire the camera. the output should be very short and the strobe should recycle instantly. make sure you point the camera at something it can focus on!

second, set the aperture at its smallest setting ( F 22 will do ) point the strobes away from the image area, like behind the camera. You should get a full dump and the underexposure warning will bee seen or heard. that will test to see if TTL is working.

If it passes the test then check your settings on the camera

If the test fails! then you have to trouble shoot it. I actually use a nikonos V to do this first. that way I can test everything ( cord and strobes ) at one time on a different camera. if it still does fails the test I know that it is most likely the cord or possibly the primary strobe ( if using a double cord ), I will then use a single cord I know is good, on each strobe separately.

if you strobe /cords pass the nikonos test then you may have a bad sync socket or possibly the shoe connector inside the housing. since you have a cannon there is another issue, there is a electronic gismo that is part of the internal sync connector that converts the Canon TTL protocol to Nikons ( which Sea & Sea uses ). this may also be an issue.

 

[spyder]

Chris,

I have a Nikonos V and just bought a 20mm lens (been shooting a 35mm) to get a little closer to my subjects. I'm using an Ikelite MV substrobe which doesn't have manual power reduction capability and I was hoping to use TTL to get around it. From your answer above can TTL be used for close shots or should I just crank the aperature shut? What are the drawbacks to using TTL with a 20mm? I actually have two MV's but haven't bought the extra arm. Are the dual cables good or should I get a slave sensor to use the second one? I live in Indianapolis and am dropping the strobes off tomorrow at Ikelite for service and can pick up what I need at that time.

 

[Chris Bangs]

The 20 mm is a wide angle lens,TTL can work with it IF the subject fills most of the frame. The TTL system works by reading the light as it is reflected off the film. the trouble is that the metering area is rather wide. therefore if there is a lot of water in the background the TTL system will think that this area needs more light since nothing is being reflected onto the film. you than get a full power flash and the subject most likely will be over exposed. unless of course the subject by chance only required a full power flash for proper exposure.

cords VS slave sensors. cords tend to be more reliable but more fragile. Slaves usually work fine as long as you understand that the sensor has to see the output from the main flash.