Why are TTL compensation levels different on the FJ trigger and FJ strobe?Updated a month ago

A Guide to Using TTL Compensation

This article addresses why TTL compensation levels are different from the trigger to the unit along with the differences when moving from Manual power settings to using TTL and TTL compensation to adjust your lighting output for creating accurate exposures.    

TTL compensation numbers on a FJ trigger, or FJ host speed light used as a triggering device, are not the same as those reflected by the client strobes and speed lights.  When in Manual mode the power levels on the triggering device are consistent with the numbers shown on the client units.   In TTL mode, the strobe and trigger do not reflect each other.  In TTL mode, these numbers are completely independent of each other.  Were you to set your strobe to -1 and your trigger to +1, the effective exposure compensation would be "0"  (+1-1=0). 

Using Manual Mode  

In Manual mode (M), the photographer adjusts the power setting of the Westcott’s FJ strobes from level 9 to level 1. This power level will not change unless the photographer changes it, often making manual mode the best option for performance and consistency.  Once you determine your desired camera settings you can adjust the power level on the FJ gear in full stops or tenths up or down to the output that provides the amount of light required to expose the image to your preferred exposure.   If the maximum (9) power level does not provide an adequate amount of light, or the minimum (1) power level adds too much light, you may need to adjust your camera ISO or aperture. Shutter speed will not affect flash exposure when within normal sync speeds (around/under 1/200). When setting the shutter above normal sync speeds, High-Speed Sync (HSS) must be utilized to avoid “half frames” (when part of your photo is dark). When in HSS, flash exposure will be influenced by your shutter speed, similar to shutter speed adjustments using consistent light.  Power level changes you make on the trigger or host FJ speed light show up immediately on the screen of the client (off-camera strobe or speed light). 

Why use TTL Mode 

TTL automates the lighting output process to allow photographers to concentrate on composition.  TTL stands for Through-the-Lens.  This means the metering (measured amount of light) for exposure is determined inside the camera, not from an external meter reading.   

How does the camera determine a proper TTL exposure with flash? 

TTL mode (TTL), takes data recorded by the camera’s sensor from a small pre-flash fired from the Strobe(s) immediately before the flash for the actual exposure.   Working together, the camera and FJ trigger or host speed light, calculate and apply the final power level required for the exposure at your chosen aperture (f-stop) and ISO. Note that Auto ISO is not supported for most camera systems with off camera lighting.   TTL exposure levels are mostly determined based on a scene where the tones are even or divided in even amounts between dark and light areas.  Some camera systems will add algorithms (rules that commonly affect calculations) such as distance and contrast levels to provide a more accurate exposure calculation when the range of tones does not provide an even distribution. Note that because TTL uses multiple firings for each image, recycle time and frame capture rates can be reduced. This is true for all flashes that offer TTL regardless of make or model. Westcott recommends using manual mode when fast capture rates are needed. 

When and why is TTL flash compensation required? 

TTL compensation is a feature of the FJ triggers and strobes.  This feature provides photographers an avenue to adjust the resulting TTL exposure predetermined by the camera.  Most often compensation is used to address the tones in the composition captured being either darker or lighter than a medium (18%) gray level.  For instance, a bride in white on a light background would likely be underexposed using TTL without compensation adjustments as the white and lighter areas would be rendered as medium gray.  Similarly, the white spots on a mostly black dog being photographed on a black background would likely be overexposed as the TTL calculation tells the strobe to add more light to make the overall scene come out as medium gray.  If a photographer’s camera provides consistent miscalculations of these scenes, the photographer can dial-in the estimated compensation amount based on the tones in the subject matter.  Compensation can also be used to manipulate the exposure based on the photographer’s creative preferences or the tendencies of different camera models and lenses toward over or underexposure of TTL images.     

Which FJ device should you use to adjust compensation settings in TTL mode?  

Best practice is typically to keep your strobe/client at a zero-compensation level and make plus or minus adjustments via the trigger or host.  There are however times when this is not the case.  If you tend to prefer exposures to the right (slightly overexposed from medium levels) or have a camera or lens that consistently underexposes in TTL, photographers may want to set their client strobe to a plus compensation amount.   This will calibrate the gear to the photographer’s preferences, while still allowing 3 stops of compensation via the trigger based on the tones in the scene.  Some camera models not listed as compatible may be able to provide quality TTL exposure features using this method. 

The variations in TTL compensation for both trigger and strobe can be displayed on the larger screen of the FJ400. 

Here’s how: 

1. Long-press the top (menu) button to get to menu 6 (settings menu). 
2. Hit the select button and turn the TTL-M option to "ON." 
3. Press the menu button to go back to menu 1. 
4. When you’re in TTL mode, you’ll see three numbers on the display screen of the FJ400: 
   1. The large number shows the strobe compensation setting. 
   2. The first set of smaller numbers displays the trigger compensation setting. 
   3. The last number indicates the Manual equivalent power level of the last TTL exposure for group "A".  This assumes the strobe was fired via a trigger or host FJ speed light using the camera's shutter release button. 

Additional note to consider when using TTL

The power levels available for a TTL exposure are the same as those available in Manual power mode.  Therefore, recording an image at a narrow aperture (f22) at a low ISO (ISO 50 or ISO 100) may result in underexposed images as the amount of power needed to achieve good exposure exceeds what the flash can produce.  This is true even when positive compensation is applied if the strobe or speed light is already operating in TTL at a level equal to Power level 9 (maximum).  Conversely, use of TTL with a wide aperture (F2) at a high ISO (3200 or 6400) may result in overexposed images. This is true even when negative compensation is applied if the strobe or speed light is already operating in TTL at a level equal to Power level 1 (minimum). Adding neutral density or diffusion material to the strobe or speedlight may provide a solution for reducing output further.