Advice on testing optically triggered strobes and learning how to use them - Scroll down for advice on configuring particular cameras.
A strobe, which is just another name for a flash, can be a little tricky to get working. There are many different settings on cameras and sometimes on the strobes themselves than can cause issues. It is important to know how to test that your system is all working properly together. Don't wait until you get underwater, that isn't a good time to be learning about something new, we recommend becoming thoroughly comfortable with its operation before you leave your house. Most strobes come with a warning that they shouldn't be used out of the water, this is because the manufacturers design the products allowing for the benefit of the surrounding water to keep them cool. We've never experienced a problem by testing a strobe out of the water, but just to be sure, an easy way around this issue it to fill a bucket with cold water, or use your bath or sink, and put the strobe back in the water as soon as you have fired a shot.
A very easy way to check the strobe is working is to connect all you underwater gear together including the fibre optic cable, make sure you prep your O-ring well, and then take a shot towards a mirror. Look after your eyes when doing this, don't look directly at where the flash light will come from and squint a little. Look at the picture you've taken on the camera afterwards, you should see the flash is lit up in the shot. If the strobe didn't fire or if it did fire but you can't see the light in the shot then you've got something wrong, most likely the flash isn't syncing. If you are lucky enough to see it working first time, it is still worth learning more about its operation, or it might suddenly catch you out at some point. You also need to make sure that the exposure can be changed, as in the flash can be brighter or darker depending on the situation.
TTL strobe power:
There are various settings to be found on different strobes. Often you will see an Auto or TTL mode, which are the same thing. This will mean the strobe mimics the power of the camera's own flash. Generally this is the easiest mode to use but will give you the least control over the output, your pictures might come out brighter or darker than you'd like. If you have the strobe set to TTL with the above test you should find that by covering the lens of your camera when you fire a flash then the external strobe should look much brighter to your eyes than if you don't cover the lens. If it doesn't then the strobe's TTL mode isn't functioning properly for some reason.
With some camera and strobe combinations you might find the TTL mode cannot work, sometimes there is nothing you can do about this though there might just be a setting on your camera which is causing the problem. For a strobe to work in TTL mode then camera's own flash must not be set to a fixed output, it must be on an auto output. If it was on a fixed power then the external strobe would just copy this same powered flash every time, you'd get no variation in the amount of light that it puts out. You also need to watch out for a setting known as the flash curtain. You camera's strobe should be set to fire on the 1st curtain. This option is also sometimes called "slow syncro", which should be set as off. What this means is that cameras have the option to fire the flash at the beginning of the shot or at the end of the shot. This is possible because flashes fire very fast, usually much quicker than how long the camera's shutter is open and taking in the light of the shot. If you have your camera's flash firing at the end of the shot then it is quite likely that any external strobe will fire too late as there is always going to be some delay for the external device to work. Another camera option that can cause issues is 'anti red-eye flash'. This is where cameras fire a burst of flashes before the shot which encourages people's pupils to contract, this option plays havoc with external strobes and should always be turned off.
Manual strobe power:
The alternative mode to TTL found on many strobes is to control the power manually. For this mode our strobe should have a power dial. Turn it to the right and it should get brighter, or to the left for less power. The way you use it is if you take a picture of your subject and it is too bright, then turn down your power and try again. Whereas if it is too dark then turn up your power and try again. With a bit of practice this mode can be easy to use and far more useful than a TTL mode as it'll allow you to control the exposure of the shots, whereas TTL is always a little hit or miss. There are more potential issues when using the strobe manually that you need to be aware of.
Preflash - read this bit, it's the number one cause of strobe problems:
Cameras often have a feature called preflash. This is where the camera fires a flash or a burst of flashes just before the picture is taken to measure the light. It happens so quickly that to your eyes it'll look like just a single flash fired. Camera manuals rarely mention this feature even exists, but it can cause no end of problems when working with external strobes. This is the feature that stops certain strobes and cameras working together altogether. The reason being is that the preflash works differently between camera models, camera manufacturers often tweak this, and yet the strobes need to know what to expect. Some strobes have compatibility charts available online that'll tell you which camera models they are compatible with and which have problems. Sometimes you'll see they are compatible with some cameras whilst using the strobes TTL mode but not the manual mode, or the reverse.
Pretty much all strobes that have a manual power mode also have another setting which allows you to specify whether or not the camera uses a preflash. You normally only have to set this when firing manually, not in TTL mode. On Sea and Sea strobes it is part of the mode dial, manual with preflash, manual without preflash and then TTL. On Inon strobes it is controlled with a magnet, which works just like a switch. If this option isn't set correctly then either the external strobe will fire too quickly, before the shot has even started during the camera's preflash sequence, or it may not fire at all as it is expecting more than one single flash. So you must set this correctly for your camera. To make things slightly more tricky, some cameras vary whether or not they use a preflash depending on their own settings. Usually you will find that SLR and mirrorless cameras do not use a preflash at all, meanwhile most compacts use a preflash. One big exception to this on compact cameras are where they have an option to specify their own flash power manually. If this option is turned on, or the mode you are in forces it to be turned on, then the camera will not use a preflash. The Canon Powershot cameras pretty much all work like this. In the M mode and sometimes the other advanced modes, if you look at the flash settings it'll say whether the flash power is auto or has a flash power output specified. If it has a flash output power specified then you must set the strobe to expect no preflash. If you are firing your strobe manually, then there is an advantage to specify a fixed flash output on the cameras flash if it is available as an option. If you set it at a low output you'll save a little battery power on the camera and speed things up a little. However don't set this and forget about it or all your pictures even taken on land will have the same low powered flash output, at least in that mode of the camera.
Manual camera controls:
If you want precise control over the exposure of your shots then you can put the camera in its M mode. This can be rewarding but if your head is already bursting then you probably don't need to read this next paragraph. In this mode you'll need to control the aperture and shutter speed of the camera. You don't need to keep changing these underwater to control the exposure, just changing the flash power will do this, however you might need to get the right settings the first time and occasionally change them for artistic purposes. So the aperture setting on the camera will change how sensitive it is to both the natural light and the flash light, the lower the aperture number the more open the aperture in the lens will be. Don't set your aperture to such a high number that the flash struggles to put out enough power or to such a low number that the flash reaches the point that it can't dim itself enough and produces over exposed pictures when up close. You should be aware that the aperture also affects the depth of field in the shot, which is how much is in focus. The shutter speed of the camera won't affect how the flash light looks in the shot at all (unless it is really fast). What the shutter speed will do is affect how much natural light gets into the shot. The faster the shutter speed, the darker the background will look. So if your shutter speed is very fast a picture will look like it was taken on a night dive even in bright conditions, if it is very long then you'll get a nice blue or green background. There is no one correct aperture or shutter speed to use, experimentation is good.
All this might sound rather complicated but it isn't complicated to use a strobe underwater once you've got all the settings and your technique worked out. Remember, try all this out at home with your flash in a bucket of water, figuring this stuff out when you're underwater is extra hard. All you need to do is find a combination of settings that work between the strobe and camera on land, test it thoroughly, then as long as you always use those same options underwater then it should also work. So if you want to use your strobe with its power controlled manually but otherwise want to keep things simple, stick your camera to the P mode, set the strobe to the manual mode, then test taking pictures in the mirror. If you see the light in the shot then try changing the power up and down to check you can see the differences to the shots. If the light isn't showing up in the shot, or the strobe isn't firing at all, set the camera's preflash setting to its other option, then try again.
Canon Powershot G7 X II:
This camera has an unusual preflash that doesn't work with any external strobes that we've tested it with. Luckily it does have a way of disabling the preflash. As long as your strobe has a power dial and is designed to work with cameras that don't have a preflash, then it should be possible to use it with the G7 X II, but it won't be able to work with any strobe's TTL (auto) mode. We tested it with the Sea and Sea YS-01, YS-D2 and Inon S-2000. It is not possible to use the Sea and Sea YS-03 with this camera.
In the Av, Tv, or M modes on the camera, if you go into your flash settings you'll find that you can set the flash output to low, medium or high. Once set to one of these then the camera will no longer produce any kind of automated flash output and instead uses a fixed amount of power, every flash will be the same. It will now no longer produce a preflash too. You might as well set the output to low in order to save battery power and speed up the flash operation of the camera. Now you need to set the strobe to control its power manually and not to expect a preflash. To do this on the Sea and Sea strobes you set the mode dial to the position that shows a picture of a single flash. Whilst on the Inon D-2000 or S-2000 you will need to add the magnet to indicate to the strobe to not expect a preflash, or on the Inon Z-240 and Z-330 you will need to push in the button that controls the preflash setting, then on all four strobes you need to set the mode dial to M (manual). So now that your strobe is set up to control the power I'd recommend that you use the camera in either aperture priority or full manual, this way your aperture won't change itself automatically which can make controlling the flash power a pain. With your system set up this way, try taking a picture of your subject, if the shot is too bright, turn down the dial on your strobe, or if it is too dark, try turning up the power. With a bit of practice you'll find that controlling your strobes this way isn't difficult and comes with lots of advantages over using strobes set to TTL(auto). These advantages include the camera being able to fire repetitive flashes far more quickly due to the flash output being set to low and your control over the shot will be hugely more precise. Usually with a strobe set to TTL, if you fire several shots in a row you'll find that every picture looks a little different, whereas with a camera set to M and a strobe set to M, you should find that every picture has pretty much the same exposure, assuming the ambient light doesn't change drastically in the meantime.
Canon Powershot G9 X II:
The TTL(auto) mode found on most strobes are not usable with this camera. The manual modes that expect a preflash found on many strobes can often work with this camera but for the best results we highly recommend you set your camera to not produce a preflash, set your strobe to not expect a preflash and to control the power manually from the strobe. Follow the instructions for the G7 X II camera above which has exactly the same flash options.