Underwater Photography

Through the Lens Master Class: When (and When Not) to Use TTL

Introduction

I have been shooting underwater since the start of the digital era, when we didn’t have an option to shoot TTL; it was manual all the way. So, I had to learn to use manual flash exposure. And even though TTL is now a feature I have in my photographic equipment arsenal, I choose to shoot in full manual mode for the majority of my underwater photography. It gives me the exposure control that I want, as well as letting me get a bit more creative with my lighting when I wish.

One situation where TTL comes in handy: blackwater dives.

Once you learn the basics of explore control, it is actually quite easy to shoot with your strobe in Manual Mode. But there are some circumstances that I choose to shoot TTL, which I’ll cover later, but first of all let me explain what TTL is and how it works.

TTL is an acronym for ‘Through The Lens’ which is an automated flash metering method. To keep things simple, your strobe emits a pre-flash and the emitted light is reflected back from your subject (or whatever is within strobe range) that then travels ‘through-the-lens’ to a sensor in the camera for metering. From this, the camera ascertains the correct flash duration (i.e how much power) for an accurate exposure to be achieved when the main flash is fired immediately afterwards. This all happens within milliseconds–all you will see if you look closely is multiple flashes being fired from the strobe. The end result should be a perfectly exposed image.

Why not always shoot in TTL?

Given the above, why would you not always shoot in TTL mode?

Well, for TTL to give an accurate flash exposure it needs a certain amount of the frame filled with subject matter that reflects back the strobe’s light. This is generally the case with many close-up and macro shots, but, for wide-angle, we commonly have a good proportion of the water column in our composition that doesn’t reflect back any light.

The lack of light returning back to the camera can sometimes ‘trick’ the TTL metering system into thinking that more strobe power is needed, therefore causing overexposure to the foreground subjects. With any possibility of inconstancy in exposures, I personally prefer to not take the risk of ruining the perfect shot with blown-out highlights and will therefore stick with my tried-and-tested manual settings for all wide-angle shooting, and the majority of my macro, for that matter. 

So why even bother with TTL?

In some shooting scenarios TTL could be advantageous.

Say you are diving in particularly cold water, finger dexterity could become limited and then fiddling about with the power dials on your strobes whilst wearing thick gloves may become somewhat awkward.

This is where TTL could nail exposure much more easily. Or, you could be shooting an extremely shy subject where any movement will startle it. Changing aperture can be done without moving your hand as the control is on the housing, so varying the depth of field can easily be done without spooking a subject, but reaching out to adjust the strobe power

TTL can be useful for shooting very shy or skittish critters.

probably will.

TTL could do this for you, and there is no chance your subject would be frightened off in the process.

Another scenario is you are shooting macro behavior where the subject is changing distance to the camera in an unpredictable manner, not giving you time to adjust settings whilst you are attempting to capture a precise moment.

Here, TTL can adjust strobe power in an instant, potentially capturing a shot that would otherwise have been missed.

Saying all that, having patience and judging when to shoot as your subject is at the correct distance for perfect exposure, is probably more important for good shots of marine life behavior than relying on TTL.

TTL for Blackwater Diving

For me, the biggest advantage of shooting TTL and really the only circumstance I regularly use it (as a warm water diver living in the tropics) comes on blackwater dives.

Many of my fellow blackwater enthusiasts swear by manual mode and obtain great results, however, where I feel an advantage is gained with TTL is that I do not need to take my eye away from the viewfinder to check exposure nearly as often (though you do still have to check and sometimes make adjustments on the strobes to dial in a plus/minus flash compensation).

A pair of pelagic octopus (Argonauta hians) reflected on a blackwater dive.

This is the precise time you can easily lose sight of a tiny subject that seems to miraculously disappear in front of your eyes!

I rarely have a dive guide to assist me in keeping an eye on the subject, so once I have lost sight of it, relocating it in a featureless environment is not always easy. Being able to continually track my subject through the VF has increased my hit rate on blackwater dives, so why not take advantage of TTL in this situation?

A caveat for this is to not have a reduction in shooting speed, and the Smart Turtle LED Flash Trigger I use ensures that my strobe recycle time is the limiting factor, rather than the cameras internal flash (more on this soon).

How to shoot TTL 

To use TTL you need to set the camera and strobe correctly. The norm is to use the camera’s on-board flash to trigger the strobe. Some cameras only have TTL Flash (Sony compacts for example) whereas others have an option for both Manual and TTL Flash Modes and you obviously have to ensure that TTL is set (time to check the instruction manual if you’re not sure how) and then on the strobe you need to set the Mode Dial to TTL.

The most commonly used brands of strobe are Inon, Sea & Sea and Ikelite – for Inon set S-TTL, with Sea & Sea you will need to set TTL or DS-TTL, depending on the model, and Ikelite set to TTL.

Nearly all compact and mirrorless camera systems use Fiber Optic Cables to trigger the strobe, as do many DSLR’s nowadays. A point to note is high-quality multi-core Fiber Optic Cables ensure accurate TTL exposures, so go for a known brand like Inon, Sea & Sea, Nauticam, etc., which have all been tested to perform correctly.

Ikelite strobes mainly function via electronic sync cords and some of their housings have in-built TTL Circuitry, or there are optional External TTL Convertors available for most camera brands. You can also get Optical TTL convertors to trigger their strobes via fibre optic cables. 

Flash Triggers

Some Pro-Body DSLR’s (think the Nikon D850, D500 and Canon 5D Mk4) do not have a pop-up flash to trigger the strobe and this is where an LED Flash Trigger comes onto the scene.

Nauticam have their own LED Flash Boards that can be purchased with the housing or retro-fitted if you didn’t opt for this at the time of purchase. There are also third-party manufacturers like UW Technics that make replacement flash boards and TRT-Electronics that make the Smart Turtle that will work with most camera systems and are an easy retro fit.

The Smart Turtle comes in different flavours to match your camera brands TTL protocols, i.e. Nikon, Canon, Sony and Olympus/Panasonic so choose the appropriate model when purchasing.

Even if your camera has a pop-up flash, you should consider a Flash Trigger as this will dramatically improve your shooting speed, as your camera’s flash needs to recharge before each flash is fired, whereas the LED’s on the triggers do not, and therefore recycle instantly allowing burst shooting if desired.

If you don’t need to shoot fast, we still recommend a flash trigger; pop-up flashes can be overly slow whereby you could miss shots. Also, a Flash Trigger does not use the cameras battery to fire and therefore your system’s autonomy is greatly extended.  

How Accurate is TTL?

When dialed in correctly, you should have fairly accurate exposures at most apertures, at least within a range that is easily adjusted in post processing, but you may need to dial in an Exposure Value Compensation to fine-tune.

I tend to test the exposure pre-dive by shooting a light-coloured or reflective subject at varying apertures. The shots should come out with a similar exposure, though it is common to have a slightly brighter exposures with open apertures and darker with closed f-stops.

The same subject with closed (left) and open (right) apertures.

For me, as I only use TTL for blackwater where I always shoot with more closed apertures for greater depth of field (about f/22 to f/32 given that there are rarely options for creating bokeh), it is quite easy to dial in the settings to get good exposure throughout this short range.

To adjust EV Comp, you can use the strobe’s power dial – Inon has a + / – guide above the dial, whereas Sea & Sea shows -2.0 through to +2.0 EV Adjustment. An alternative is to use the cameras Flash Compensation to adjust the flash power for the desired exposure.

This can then be pre-set as necessary and then if on a dive you require more or less power than the camera calculates for a particular subject, you can use the strobe EV Control to dial this in. Alternatively, you can do the opposite and set a +/- EV on the strobe and then adjust on a dive using the cameras Flash Compensation.

I would suggest using what is easiest and quickest to change on the fly for your particular camera/housing model.

Shooting Manual

To shoot your strobes in manual mode we need to understand what effects and controls the light. Five elements effect your strobe exposure:

  • Aperture
  • ISO
  • Strobe Power
  • Strobe to Subject Distance
  • Subject Reflectivity

Use the Shoot – Review – Adjust Procedure to dial in the correct strobe exposure. 

  • Take a shot using your best guess (this will improve with experience) at what strobe power you need for the given Aperture and ISO you have elected to use and subject distance and reflectivity. 
  • Review the shot on the LCD to ascertain if you have over or under exposed. 
  • Then adjust the strobe power accordingly. You could also adjust Aperture or ISO but this will mean a change in ambient light exposure too (if there is any), plus it will effect the depth of field and possibly introduce digital noise if the ISO is pushed too high. 

One important factor to consider: you need to be hyper-aware of your strobe-to-subject distance, as, if this is varying from shot to shot, so will your flash exposure. So when you take a shot after making an adjustment to the settings, ensure you are at the same distance as the previous shot. This will mean that the changes you made to increase or decrease the flash exposure will be as expected.

Of course you may want to change your composition, which could mean a change in strobe to subject distance and then you would need to compensate for this by adjusting strobe power as needed.

The Inverse Square Law

To explain why strobe-to-subject distance is relevant, without getting overly technical, light underwater is governed by the Inverse Square Law. This means that if you half the strobe to subject distance, 4-times more light will illuminate the subject. Likewise, if you double the strobe to subject distance 4-times less light will illuminate the subject.

To put this into context, if you had correct exposure set with a strobe to subject distance of 20cm and then moved 10cm forward, so halving the distance, you would massively over expose the shot as 4-times more light would hit the subject than previously. If you then adjusted strobe power for correct exposure at 10cm and moved back to 20cm, so doubling the strobe to subject distance, then took a shot without making a power adjustment, the image would be way under exposed as 4-times less light would hit the subject. This is all proportional though, a 10cm change in distance if you are shooting from further away (say 1m strobe to subject distance), will not make such a big difference.

But we all know that the Golden Rule of U/W Photography is… ‘Get Close, then Get Closer’!  

Who will benefit from TTL?

If you are new to underwater strobe photography and shoot mainly mid range, close-up and macro, then TTL will ensure the majority of your shots have a decent exposure, as the camera will make changes to strobe power automatically as you change distance to your subject or make a change to aperture or ISO.

I appreciate that not everyone has the time or inclination to practice strobe exposure, especially if you only get to shoot whilst on vacation for one or two weeks a year. Sacrificing your precious time underwater to learning strobe exposure may not be on the agenda, you might just want to get nice shots from the get-go!

However, if you do have the time and want to progress your underwater imaging, I would highly recommend learning how to expose manually using your strobes.

When I teach u/w photography I use the analogy of driving an automatic vs manual gear shift car. If you learn in a manual you can easily drive an automatic, but the other way around doesn’t work! So I always teach my students how expose correctly using manual strobe power, and, once this is mastered they can revert back to TTL if they want.

For more experienced shooters it is knowing in what circumstances shooting TTL will be advantageous over shooting in Manual Mode. Some marine life behaviour specialist swear by TTL to capture the moment. Other blackwater enthusiasts and I use TTL to their advantage during these specialist dives. Cold water divers may use TTL to allow them to capture well-exposed images when numb fingers make strobe controls adjustments difficult.

It is all about knowing when to use TTL and when to stay in Manual Mode.

Conclusions

Strobe technique and knowing when (or not) to use certain settings like TTL can make the difference between getting and missing the shot!

To summarize, there are both advantages and disadvantages to shooting in TTL mode.

For new UW shooters it is nice to get more shots that are correctly exposed, though I would urge everyone to learn how to manually control strobe exposures and understand how light behaves underwater to allow progression as an underwater photographer. Relying on the camera to do the thinking for you all the time will surely hinder your progress.

Is TTL necessary?

I would say no, you can get by perfectly well without it if you learn to control manual strobe exposure.

Is it handy to have this additional tool in our arsenal?

To this I would answer yes, why would you not want an advantage in certain shooting situations?

Personally, I would say use TTL if and where it is advantageous. It is not good in all situations and I do not use it for the majority of my shooting, but having the option to switch into TTL Mode could mean bagging a shot that I would otherwise have missed.

Most new strobes on the market are TTL compatible, allowing anyone who shoots with a camera using the onboard flash to trigger their strobes to utilise this function.

Hopefully the above has explained what TTL is, how it works, and if your set-up is suitable, so you can decide if it’s for you or not.

Then it’s time to start thinking about strobe positioning, which is a whole other topic!

Alex Tyrrell
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