TTL Flash Photography Explained

TTL flash is a lot easier to master than you think:

As a digital SLR photographer you will more than likely use your flash in one of two ways, manual flash mode or TTL flash mode. Manual mode is where you set a fixed flash power output and take the camera’s metering out of the equation. TTL mode on the other hand is where you depend entirely on camera flash metering and in this day and age it is pretty accurate once you know how to control it.

Film TTL was introduced by Nikon back in 1980 and by Canon in 1987. So TTL flash, or more accurately known as Through-The-Lens flash is relatively new in terms of technology. It has had a few different iterations during the last 3 decades but all are dependent on how your cameras flash metering system sees the subject in a scene through the lens and how it automatically adjusts the output of the flash based on what it sees. For the purposes of this article we are going to concentrate on pure evaluative TTL metering. This is called eTTL II for Canon and iTTL for Nikon. Other TTL modes such as normal film TTL and automatic TTL are a little out of date and for the purposes of this article we will be skipping them. But conceptually the different TTL iterations follow the same process; the flash metering system within the camera does the thinking. As a general recommendation, if you really want most control from your camera and flash you are best using manual exposure mode. In manual exposure mode you fix the ambient exposure leaving the camera the single job of calculating correct flash exposure. In any other exposure mode the camera will have to calculate both the ambient exposure and the flash exposure. This means more variables and more room for inconsistencies across photos. More variables are bad! Check out flash basics for a quick refresh.

These days photographers are completely spoiled. Obviously the cameras have come on in leaps and bounds in the last 3 decades but in terms of flash, its use has been completely transformed. Back in the pre-TTL days when photographers were using film cameras and fixed-power flash units in constantly changing environments, flash was well and truly a nightmare. Considerations needed to be given to the particular ISO of the film you were using, the lens aperture, the guide number of the flash unit you were using and of course the distance of your flash to your subject. It was an absolute necessity to have a light meter and a good mathematical brain. Flash units had associated exposure tables or a mathematical equation which allowed calculation of correct aperture based on distance to subject and ISO film speed as per that flash units guide number. And of course if you needed to change the position of the flash you would have to start the calculations again. If you needed to adjust the depth of field then you needed to adjust flash position. One adjustment affected the other instantaneously. Nightmare! Nowadays it’s almost a case of point and shoot. The TTL sensor in the camera does it all for you. Piece of cake!

In the case of evaluative TTL flash metering, or what we will now call just TTL, it uses what is known as a pre-flash, an almost imperceptible, low-power burst of light prior to the main flash burst to evaluate the flash power to accurately expose the subject in the frame. The subject will normally be identified by the metering system within the camera so the flash will emit a pre-flash, the cameras TTL sensor will see how the reflected light appears for the selected metering method and will then adjust the flash output depending on the light bounced back. All of this of course occurs within hundredths of a second. TTL works using reflected light, what the TTL sensor sees reflected back during the pre-flash. This is important because different surfaces and colours reflect differently.

Before we develop this point further we will discuss how you control TTL flash. As mentioned earlier, the best camera exposure mode to be in is manual. In manual you are essentially fixing your ambient exposure. The manual selection of Aperture, Shutter Speed and ISO means the camera will fix exposure of the overall scene. In any other exposure mode the camera will be calculating how it thinks you want the scene exposed and this is what we want to avoid. We want our ambient exposure fixed. Now that ambient exposure (aperture, shutter speed and ISO) is fixed the only thing that will require adjusting is flash exposure. So what adjusts flash exposure? Let’s try and change our flash exposure using combinations of aperture and ISO (shutter speed has no effect on flash exposure). I have demonstrated this below using a series of photos with different aperture and ISO combinations.

In this series of shots taken during 'Semana Santa' in Málaga there is exactly 4 f-stops difference in ambient light between the brightest and the darkest. Notice how the subject lighting looks the same for all images. The men at the front of the image, the subjects, are reasonably similar in terms of exposure (being the flash exposure). This is because the TTL algorithms within the camera are taking the aperture and ISO settings into account always and it is increasing or decreasing flash output based on the reflected light picked up on the subject. But do you see how the ambient light changes with different aperture and ISO selections. This is because across the series of photos there is a 4 f-stop difference in ambient light exposure since aperture and ISO adjustments by default will affect ambient light received by the sensor (same way as it does during normal scene exposure). So, changes in aperture and ISO will affect the ambient exposure but not the flash exposure when using flash in TTL mode. This concept is vital to understanding how TTL and flash exposure works in general. In the case of TTL flash, the lower the ISO or smaller the aperture the higher the power output requested by the cameras flash metering system and vice versa. TTL will constantly evaluate flash exposure, constantly adjust flash output and remain completely independent of your manual exposure settings. In TTL mode there is only one variable that controls flash exposure. This is the Flash Exposure Compensation setting or FEC. Once you are allowing the flash adequate time to recycle and are operating within the limits of your camera and flash absolutely nothing else will make a difference to your flash exposure when in TTL mode. In manual flash mode the situation is rather different, a bit like the olden days in fact, but thankfully with TTL mode, a photographer’s job is simplified tremendously.

So now that we have determined the main control for TTL flash is FEC let us see what happens when we change FEC and how it changes the tone of the flash exposed subject. As mentioned earlier, TTL flash is based on reflected light and different surfaces and colours will reflect light differently. In the case of TTL flash as your main source of light, the camera will meter flash exposure and adjust flash output to give a grey or mid-toned subject. Like ambient exposure, flash exposure (when flash is the main source of light) will be metered to be middle of the road, not pure white and not black. So FEC needs to be borne in mind to ensure correct subject exposure depending on your subject colour. How does this work then? Well, it is simple enough really. Just like ambient exposure, once you know your camera will calculate flash exposure to be mid toned then for brighter subjects such as beautiful brides in wedding dresses it will reduce flash output as it sees the subject brighter than mid-toned and consequentially will request less flash for what the camera thinks is a correct exposure. So with FEC at 0 (no compensation) the dress will appear somewhat off-white or a bright grey. To counteract this, the flash output needs to be increased to brighten the dress, making it whiter. This is done by increasing the FEC. This will tell the camera to look for more flash power from the flash unit. More flash power means more light. More light means brighter wedding dress. Brighter wedding dress means a better exposure and a happier bride. Success! How do you know how much FEC to dial in? This depends on camera model and manufacturer but as a rule for a bright subject you are best dialling in about 2/3 of a stop extra (0.7EV) in terms of FEC. You can then do a quick check on the LCD for blinking highlights to determine if you have overdone it. The converse applies for men in black suits. The cameras TTL metering system will try to overexpose the black to be grey as it is always aiming for the mid-tones. So in this case you need to dial down the FEC, reducing the flash output which should keep the black as black.

In the case where TTL flash is being used as supplemental or fill light, then the opposite applies. In the case of fill light, you will have first set the ambient exposure based on the ambient light. If you find that after taking a shot without flash that a bright-toned or white subject appears somewhat darker than you want then you need to add some fill flash to bring up the light tones. However in this case you would dial the FEC down for a subtle blast of flash to bring up those whites. Adding full power TTL flash to the ambient light will more than likely blow out the whites. So, in summary, where flash is the main light, then flash exposure needs to be considered similar to standard exposure, i.e. up the FEC for bright subjects and lower it for darker toned subjects. Where flash is used as a fill light then all you need is a touch of flash so you would dial the FEC down by a couple of stops.

And that is really all there is to getting to grips with TTL flash. It is simply a case of fixing your ambient exposure using manual exposure mode on the camera then using the FEC dial to alter the power emitted by your flash. For bright subjects give it more juice (if flash is your main source of light), for darker subjects, then back off on the FEC. This is the general approach that should be used. Exact values are too difficult to give because there are so many variables at play (ambient light levels, backlighting if present, subject tones, distance of subject to camera, subject reflectivity and of course the camera flash metering algorithms themselves). But the concepts will hold true and once you start with these concepts in mind then it will not be long before you arrive at the correct camera settings for good exposure for all scene types.

You do need to get familiar with your camera as brands and camera models vary as to how they expose automatically. The best way to do this is to take a bright subject and see what FEC settings give a correct and proper white exposure for where flash is the main source of light and where it is only a fill light. The same can then be repeated for darker subjects. Once you have determined the FEC values for bright and dark subjects you can then put them into practice without even having to think about it.

TTL flash is a boon for photographers. It makes dynamic shooting situations much more tolerable as the camera will do the calculating leaving you, the photographer, doing what you like most, taking photos. When in a fixed studio environment manual flash will still provide more consistent exposures but when outside photographing in a constantly changing environment then TTL is your best friend. There is no need to be afraid of it. Once you determine what FEC values work best for your camera/flash combinations then that is the equipment aspects under control. And if you are shooting in RAW then you will have a lot of leeway available to you in post processing so capturing picture perfect exposure is nowhere near as critical as it was back in those nasty film days!

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