Use camera's flash exposure compensation instead of external strobe to save battery?

[DazedAndConfuzed]

Hi,

I have the Sea&Sea YS-01 using its optical DS-TTL and was wondering if I wanted to adjust exposure of the strobe, would it be better to use the camera's flash exposure compensation instead of external strobe?

I know on manual external strobes, it is best to set internal flash to minimal strength or slave mode if it exist).


From my understanding, when adjusting (lets say -3) from the camera, the pre-flash from the camera would be a standard strength, then the final firing of the internal would be 12.5% of standard strength, thus using less of the camera's batteries.

But if I adjust the external strobe's strength -3 stops, the pre-flash from the camera would be the same, and the camera thinks it will fire for the picture at regular strength, but the strobe would be firing at 12.5% of standard strength.


Also, if I adjust the camera's flash to -3 and the external strobe to +3 (I don't think it is possible on the YS-01), then I would get proper exposure but with much less use of the camera's internal flash. This would be useful because the camera's internal flash recharges much slower than the external one, and any less use of the camera's flash is always better.

 

[Ardy]

I am not sure, but I don't think TTL works like this. If you can set your strobe to the minimum and it will still output its settings then the external flash will read these and fire accordingly. My camera Oly 520 will only output its settings on Full auto flash. The first thing you need to find out is how your camera outputs TTL then you can work from there.

 

[DazedAndConfuzed]

From my undestanding, TTL works like (timing is example)

If I have a internal flash with GN 10 and external strobe with GN 20 (2 stops stronger):

Taking picture with just internal flash:
internal flash sends pre-flash 1/40,000 sec. CCD measures return light as 5 stop under and when taking picture, sends main flash at 1/1,250 sec

Taking picture with external strobe:
internal flash send pre-flash at 1/40,000 sec, external strobe simulates its duration with 1/40,000 sec pre-flash. The camera's CCD measures a return light as 3 stops under, thus when taking picture, send main flash at 1/5,000, which the external strobe also sends out.


So if I use the camera's flash exposure compensation to make it -2 stops dimmer.
internal flash only:
internal pre-flash: 1/40,000 sec = 5 stops under, thus when taking picture, set main flash at 1/5,000 sec for -2 stop darker picture

external strobe:
iternal pre-flash: 1/40,000 sec from main flash and external strobe, camera measures it as 3 stops under, thus when taking picture, send flash at 1/20,000 sec (emulated by external strobe at 1/20,000 sec) for a -2 stop darker picture.


If I use external strobe's flash exposoure compensation to make it 2 stops dimmer:
external strobe:
iternal pre-flash: 1/40,000 sec from main flash and external strobe, camera measures it as 3 stops under, thus when taking picture, send flash at 1/5,000 sec to properly light the image, but external strobe's -2 setting will send shorten that to 1/20,000 sec, resulting in a -2 stop darker picture.


So from above examples, internal flash would use alot less of its power, when used with external strobe, and when using the camera's flash exposure compensation, even more. This is preferred because it is much easier to change the strobe's batteries than the camera's and want to minimize opening the camera housing during a trip that has tens of dives.

 

[bvanant]

Larry:
Stop worrying about the theoretical possibilities and just do a simple test in your kitchen. Then you will know.
Bill

 

[tadawson]

I would think that if this is true TTL, there should be no need to use the internal flash at all - use the external for everything, and your camera battery will have the maximum possible life.

I don't have your gear, but this is the way TTL worked for years on film cameras, and there is really no reason for digital to be different.

True TTL is a wire connecting the strobe to the camera . . . anything else isn't really TTL but more of a "smart slave" kind of setup.

- Tim

 

[bvanant]

His camera has no way to connect a sync cord to the camera in the housing. DS-TTL is an emulation protocol that works quite well in most instances since it emulates the on-board flash quite well. It is true that on film cameras using Nikon protocols you could get TTL from some strobes but for Canon cameras that has never been true and once Canon and Nikon moved to ettl and ittl even that connection became a bit more complicated. While it is true that there is no a-priori reason that digital should be different than film the real problem is that Canon and Nikon dramatically changed their flash protocols and didn't publish the details of the protocols.

Bill

 

[DazedAndConfuzed]

I would think that if this is true TTL, there should be no need to use the internal flash at all - use the external for everything, and your camera battery will have the maximum possible life.

I don't have your gear, but this is the way TTL worked for years on film cameras, and there is really no reason for digital to be different.

True TTL is a wire connecting the strobe to the camera . . . anything else isn't really TTL but more of a "smart slave" kind of setup.

- Tim

It is still TTL because the metering is done by the camera's imaging sensor behind the lens and the light provided by the TTL metering is by the external strobe.


This works exactly like the electrical wire connection, except the light exposure duration is send b a xenon tube through an optical fiber.

If it is to be considered a slave, then the main flash would also be lighting the subject.

 

[smoore]

In the first post I thought the question was whether it made sense to reduce the flash compensation in the camera and increase the flash compensation in the external flash to save the camera's battery. For manual flash reducing the camera's flash to a minimal output makes sense as the external flash will fire for the duration dialed into the external flash regardless of the strength of the "trigger" from the camera's flash.

Lwang may be right that this approach will work and save camera battery power but for me it would be too complex. Keeping track of flash compensation this way when I am already narced would be a problem for me.

I usually only shoot about 40-60 shots in a dive and do 2 dives before changing batteries. For someone like me who only does 3 dives a day it is simpler to change batteries between the 2nd and 3rd dives.

 

[ScubaMarc]

why can't some manufacturer create a very small hot shoe LED light with minimal battery. This will by pass the camera strobe battery life issue.

I know there are diy option out there, but too thick for my g12 case.

 

[bvanant]

The DIY version that I built is not much bigger than the standard hot-shoe adapter for TTL. The bigger issue depends on what housing and strobe do you have to get the LED light to the strobe.

Bill