Underexposed photo even when using strobe

[atabaksh]

Hi friends, I have got a question and hope you will help me. I have got a canon g7x with one sea and sea YS-03 ttl strobe. This new strobe just has off and TTL option. G7x is a TTL friendly camera. I usually set the camera flash on ON mode with Auto (ttl) submode and I do lots of out of the water test shots also I use the camera manual mode. Most of the times I get nice exposed forced flashed images but sometimes when I do macro shots I get underexposed images although the flash is fired. I have no idea what is going on except the fact that every time I shot the flash fires 2 times but sometimes it fires onetime which results in underexposed image. I have attached a photo

Shutter speed 1/40 , f 2.5, iso 640.
Any help would be appreciated

 

[ScubaMarc]

Is your pre-flash in sync on the camera and strobe? are they firing at the same time. You can not really see this with the eye. Try this. Take a picture of a mirror with the camera and strobe in with the settings you want. The picture should have the strobe firing in the mirror. If the strobe is off, there is your issue. The preflash settings are wrong somewhere.

If this is not the issue, check the histogram.

 

[KeithG]

Forced Auto? Do you mean Auto mode?

you likely want to be using "on" mode to cause the onboard flash to always fire. in auto mode the camera may decide to not fire the flash.

 

[atabaksh]

Forced Auto? Do you mean Auto mode?

you likely want to be using "on" mode to cause the onboard flash to always fire. in auto mode the camera may decide to not fire the flash.

By saying forced auto I mean the flash is on the forced fire mode and it is set on Auto( ttl) .

 

[atabaksh]

Is your pre-flash in sync on the camera and strobe? are they firing at the same time. You can not really see this with the eye. Try this. Take a picture of a mirror with the camera and strobe in with the settings you want. The picture should have the strobe firing in the mirror. If the strobe is off, there is your issue. The preflash settings are wrong somewhere.

If this is not the issue, check the histogram.

Great thanks for your answer. I did what you say, I shot in front of the mirror in Av mode. In most of the images the strobe was fired but in a few it was not fired in the image ( it was fired but it was not in sync). The flash fires twice always but in the underexposed images it fires once, like there is not a preflash. So what should I do know?

 

[KeithG]

By saying forced auto I mean the flash is on the forced fire mode and it is set on Auto( ttl) .

your use of the word forced is confusing. canon does not have a "forced" flash mode.

the g7x has the following flash modes: auto, on, slow syncro and off.

auto flash mode does not mean ttl. auto flash mode means "let the camera decide if a flash is required". so in auto mode the flash may not fire if the camera decides there is enough light.

if you want to "force" the flash to fire, you need to put the flash in on mode.

 

[atabaksh]

your use of the word forced is confusing. canon does not have a "forced" flash mode.

the g7x has the following flash modes: auto, on, slow syncro and off.

auto flash mode does not mean ttl. auto flash mode means "let the camera decide if a flash is required". so in auto mode the flash may not fire if the camera decides there is enough light.

if you want to "force" the flash to fire, you need to put the flash in on mode.

I see, yes my word is a little bit confusing. When we choose On mode ( or forced flash) , we have 2 sub options : Auto (TTL) and manual mode. By saying auto I mean the auto option in the ON mode.I edited my post.

 

[Jax]

Great thanks for your answer. I did what you say, I shot in front of the mirror in Av mode. In most of the images the strobe was fired but in a few it was not fired in the image ( it was fired but it was not in sync). The flash fires twice always but in the underexposed images it fires once, like there is not a preflash. So what should I do know?

Just a guess - since the strobe is not always firing, is it possible your optic cable is loose / bent / broken so the pre-flash is not firing the strobe?

 

[atabaksh]

Just a guess - since the strobe is not always firing, is it possible your optic cable is loose / bent / broken so the pre-flash is not firing the strobe?

The strobe always is firing. When it fires 2 times everything is ok , but sometimes it fires once then the problem happens. how can I check the preflash option in the camera?

 

[KeithG]

do you have red eye turned off?

normal humans can not detect the preflash from the main flash (at least none i know). if you are detecting 2 flashes, then the first one is likely the red eye flash?