TTL Flash Tutorial in 10 Lessons (Part 1)

Here's an easy tutorial for TTL flash.  (If you want to know why to learn flash in the first place, check out these samples of what's possible.) The tutorial will cover the following topics.

Part 1: Controls
Lesson 1: Ambient vs. Flash
Lesson 2: How exposure settings affect TTL flash
Lesson 3: How to control TTL flash
Lesson 4: Balancing Flash and Ambient
Lesson 5: Flash Value Lock

Part 2: Application
Lesson 6: Hard Light vs. Soft Light; Bouncing the Flash
Lesson 7: Using the Flash Off-Camera
Lesson 8: Subject vs. Background; Key vs. Fill; Contrast
Lesson 9: Color; color temperature and gels
Lesson 10: Lighting depth of field
Lesson 11: Bonus lesson

Note: Part 2 of tutorial is here: http://betterfamilyphotos.blogspot.com/2010/07/ttl-flash-tutorial-in-10-lessons-part-2.html

Requirements:
1. Knowledge of basic exposure concepts (exposure, aperture, shutter speed, ISO, exposure compensation, histogram). Need a refresher?  Check here.
2. DSLR or bridge camera.
3. Lesson 6 and up: requires camera with hot shoe, an external flash, and a commander or TTL cord or radio TTL.

Feel free to stop wherever you feel like you've absorbed enough, in order to try out the techniques in real life shots and allow the techniques to sink in.


Lesson 1: Ambient vs. Flash


a photo where the subject is lit by flash, while the background is lit by ambient light

This experiment clarifies the distinction between ambient light and flash, and how ambient exposure and flash exposure differ.

Exercise 1
1. Do this experiment indoors.  Find a moderately deep box.  Open the box to face you but face it away from the light in the room.  You can put an object in the box as your subject.  Take a picture of the box with the flash off.  Note that in the picture, the exposure is correct outside the box, but the box interior is shaded.


ISO: 400
Exposure: 1/5 sec
Aperture: 5.6

2. Do the same but this time turn on the flash.  Note that in the picture, the exposure is correct outside the box, and the box interior is now illuminated by the flash.


ISO: 400
Exposure: 1/8 sec
Aperture: 5.6

TAKEAWAY: The recorded light in #1 is all from ambient light - this is the ambient exposure.  In the second picture, the recorded light from the flash (i.e., the light in the box interior) is the flash exposure.  Outside of the box, there is a mix of ambient and flash but it's mostly ambient light.  Ambient exposure and flash exposure combine to give the overall exposure.

If ambient light and flash are separate, how do exposure settings affect flash exposure?  That's covered in Lesson 2.

Lesson 2: How exposure settings affect TTL Flash Exposure.


a photo with an intentionally underexposed background

This exercise demonstrates how exposure settings do or don't affect flash exposure.  Take some test shots with the same setup as in Lesson 1, with a moderately deep box open toward you. 

Exercise 1: use manual exposure mode.

a. Take a baseline shot.
Use an ISO of around 400, an aperture of f/5.6 to f/8, and a shutter speed that will result in about 1/2 or 2/3 stop underexposure based on the light meter.  Take a test shot (with flash), which will serve as a baseline.

ISO: 400
Exposure: 1/8 sec
Aperture: 5.6

b. ISO
Decrease the ISO 1 stop from the baseline (ISO 200) and take a test shot with flash.  Compare to the baseline shot.  Did the ambient exposure increase, decrease, or remain the same?  Did the flash exposure increase, decrease, or remain the same?

ISO: 200
Exposure: 1/8 sec
Aperture: 5.6
 
Increase the ISO 1 stop from the baseline (ISO 800) and take a test shot with flash.  Compare to the baseline shot.  Did the ambient exposure increase, decrease, or remain the same?  Did the flash exposure increase, decrease, or remain the same?

ISO: 800
Exposure: 1/8 sec
Aperture: 5.6
 
TAKEAWAY: In TTL mode, changing the ISO does not affect the flash exposure.

b. Aperture
Widen the aperture 1 stop from the baseline (f/4) and take a test shot with flash.  Compare to the baseline shot.  Did the ambient exposure increase, decrease, or remain the same?  Did the flash exposure increase, decrease, or remain the same?

ISO: 400
Exposure: 1/8 sec
Aperture: 4.0
 
Narrow the aperture 1 stop from the baseline (f/8) and take a test shot with flash.  Compare to the baseline shot.  Did the ambient exposure increase, decrease, or remain the same?  Did the flash exposure increase, decrease, or remain the same?

ISO: 400
Exposure: 1/8 sec
Aperture: 8.0
 
TAKEAWAY: In TTL mode, changing the aperture does not affect the flash exposure.

c. shutter speed
Increase the shutter speed 1 stop from the baseline and take a test shot with flash.  Compare to the baseline shot.  Did the ambient exposure increase, decrease, or remain the same?  Did the flash exposure increase, decrease, or remain the same?

ISO: 400
Exposure: 1/4 sec
Aperture: 5.6
 
Decrease the shutter speed 1 stop from the baseline and take a test shot with flash.  Compare to the baseline shot.  Did the ambient exposure increase, decrease, or remain the same?  Did the flash exposure increase, decrease, or remain the same?

ISO: 400
Exposure: 1/15 sec
Aperture: 5.6
 
TAKEAWAY: changing the shutter speed does not affect the flash exposure.

Exercise 2:
a. Do the foregoing experiment in P, A, or S mode.


- ISO (you can use either P, A, or S mode for this exercise)

baseline (P mode)

ISO: 400
Exposure: 1/8 sec
Aperture: 4.0

P mode, ISO decreased 1 stop:

ISO: 200
Exposure: 1/5 sec
Aperture: 3.5

P mode, ISO increased 1 stop:

ISO: 800
Exposure: 1/13 sec
Aperture: 5.0


- Aperture (use A or Program Shift for this exercise)


Baseline (Aperture Priority) f/5.6

ISO: 400
Exposure: 1/4 sec
Aperture: 5.6

Aperture: f/4

ISO: 400
Exposure: 1/8 sec
Aperture: 4.0

Aperture: f/8

ISO: 400
Exposure: 1/2 sec
Aperture: 8.0


- Shutter (use Shutter Priority or Program Shift for this exercise)


Baseline (Shutter Priority): 1/4

ISO: 400
Exposure: 1/4 sec
Aperture: 5.6
  
Shutter: 1/2

ISO: 400
Exposure: 1/2 sec
Aperture: 8.0

Shutter: 1/8

ISO: 400
Exposure: 1/8 sec
Aperture: 4.0

TAKEAWAY: in P, A, or S mode, changing ISO, aperture, or shutter speed won't affect ambient exposure and won't affect TTL flash exposure either.

b. exposure compensation.

Exercise 3
Increase the exposure compensation 1 stop from the baseline and take a test shot with flash.  Compare to the baseline shot.  Did the ambient exposure increase, decrease, or remain the same?  Did the flash exposure increase, decrease, or remain the same?

Baseline (Program mode)

ISO: 400
Exposure: 1/8 sec
Aperture: 4.0

Program with +1 Exposure Compensation

ISO: 400
Exposure: 1/4 sec
Aperture: 4.0

Decrease the exposure compensation 1 stop from the baseline and take a test shot with flash.  Compare to the baseline shot.  Did the ambient exposure increase, decrease, or remain the same?  Did the flash exposure increase, decrease, or remain the same?

Program with -1 Exposure Compensation


ISO: 400
Exposure: 1/15 sec
Aperture: 4.0

TAKEAWAY: In TTL mode, changing the exposure compensation may or may not affect your flash exposure depending on your camera system.  With Canon, exposure compensation does not affect flash exposure.  With Nikon, exposure compensation does affect flash exposure.

Lesson 3: How to control TTL Flash Exposure


a photo where flash exposure was adjusted (+0.7 FEC)


a. Flash Exposure Compensation
So far, we've noted that changing ISO, aperture, shutter speed, and with some systems, exposure compensation, have no effect on TTL flash exposure.  To adjust flash exposure, you instead use flash exposure compensation (FEC), in the same way that you use exposure compensation to adjust exposure in P, A, or S mode. Adjusting FEC is done either through a dedicated button or through menus, depending on your camera.  On the Nikon D80 and D300 for example, FEC can be adjusted by holding down the flash button and turning the sub-command dial, or it can be done through the menus.

Exercise 1
Increase FEC 1 stop from the baseline and take a test shot with flash.  Compare to the baseline shot.  Did the ambient exposure increase, decrease, or remain the same?  Did the flash exposure increase, decrease, or remain the same?

Baseline:


With +1 FEC:


Decrease FEC 1 stop from the baseline and take a test shot with flash.  Compare to the baseline shot.  Did the ambient exposure increase, decrease, or remain the same?  Did the flash exposure increase, decrease, or remain the same?

With -1 FEC:


TAKEAWAY: In TTL mode, changing the FEC affects the flash exposure but does not affect the ambient exposure.

Review question: if your camera (e.g. Nikon) allows exposure compensation to affect your flash exposure, how would you bring back the flash exposure to normal?  Use FEC to compensate.  For example, if you decrease exposure compensation by 1 stop, your flash exposure with Nikon is also decreased by 1 stop, but you can "zero out" the effect of the exposure compensation by adding 1 stop FEC.

b. Maximum power

Exercise 2
Find a non-shiny target that is at least 10 feet away.  Choose manual exposure mode, and select the lowest ISO and smallest aperture on your camera, choose a shutter speed of 1/125 or so.  Take a shot with the flash.  Note that the target is underexposed.

ISO: 100
Exposure: 1/125 sec
Aperture: 32.0

Increase the FEC as much as you can then take a shot.  Note the result is still an underexposed image.

ISO: 100
Exposure: 1/125 sec
Aperture: 32.0
+1 FEC

TAKEAWAY: with a low ISO, small aperture, or large distance between subject and flash, or any combination of the foregoing, the flash may not have enough power to give a correct flash exposure.

d. Minimum power

Exercise 3
Choose manual exposure mode, and select the highest ISO and widest aperture on your camera, choose a shutter speed of 1/125 or so.  With the subject as close as possible to the flash, take a shot with the flash.  Note that the target is overexposed.

ISO: 6400
Exposure: 1/125 sec
Aperture: 2.8
 
Decrease the FEC as much as you can then take a shot.  Note the result is still an overexposed image.

ISO: 6400
Exposure: 1/125 sec
Aperture: 2.8
-3 FEC
 
TAKEAWAY: with a high ISO, wide aperture, or short distance between subject and flash, or any combination of the foregoing, the flash may have too much power to give a correct flash exposure.

On some external flashes, the minimum and maximum range for given exposure settings is displayed on the flash.

SUMMARY: TTL flash exposure is controlled with flash exposure compensation (FEC).  The only limits are the maximum and minimum power of the flash.  The effect of FEC may vary with flash exposure mode.

Lesson 4: Balancing Flash and Ambient Light


An illustration of flash balanced with ambient light.

Knowing how to increase or decrease the proportion of ambient and flash is critical to getting good results with flash.

a. Review: adjusting ambient and flash exposure.  Here's a recap of Lesson 2 and 3.

To increase ambient: in M mode, use a wider aperture, slower shutter, or higher ISO.  In P, A or S mode, use exposure compensation*.
To decrease ambient: in M mode, use a narrower aperture, faster shutter, or lower ISO.  In P, A or S mode, use exposure compensation*.
To increase or decrease flash: use FEC.

b. Balancing when flash and ambient do not overlap.
Balancing flash and ambient when they do not overlap is simply a matter of adjusting TTL flash exposure (with FEC) and adjusting ambient exposure (with exposure settings in M mode or with exposure compensation in P, A, or S mode).  Usually you set exposure to get the ambient that you want, then add TTL and adjust the TTL as needed.

The setup in Lesson 2 is representative of a situation where the flash and ambient do not overlap (the box interior was being lit entirely by flash, with almost no contribution from ambient), while the background was lit mostly by ambient.

Exercise 1 in balancing flash and ambient (without overlap):
Take a baseline shot.


Switch to M mode
Without changing the flash, increase the ambient.

In the shot above, I increased the ambient by increasing ISO.

Without changing the flash, decrease the ambient.

In the shot above, I decreased the ambient by decreasing ISO.

Without changing the ambient, increase the flash.

+1 FEC

Without changing the ambient, decrease the flash.

-1 FEC


Switch to P, A, or S mode
Without changing the flash, increase the ambient.

+1 EC, -1 FEC (net: 0 FEC)


Without changing the flash, decrease the ambient.
 
-1 EC, +1 FEC (net: 0 FEC)

Without changing the ambient, increase the flash.

+1 FEC

Without changing the ambient, decrease the flash.

-1 FEC

d. Balancing when flash and ambient overlap.
When flash and ambient overlap, the difference is that you need to avoid overexposure.  Other than that, it's similar to balancing flash and ambient when they don't overlap.

Exercise 2:
Pick a subject that is illuminated by ambient light.  Take a baseline shot without flash (make sure the exposure is correct).

Manual exposure mode
ISO: 400
Exposure: 1/20 sec
Aperture: 2.8

Add flash. Is the overall exposure correct?

Manual exposure mode
ISO: 400
Exposure: 1/20 sec
Aperture: 2.8
Ambient: correctly exposed. Flash added.  Result: overexposure

Adjust so that ambient exposure is underexposed by 1 stop.  Add flash. Is the overall exposure correct?

Manual exposure mode
ISO: 400
Exposure: 1/40 sec
Aperture: 2.8
Ambient: underexposed 1 stop. Flash added.  Result: correct exposure


I tried the same exercise with program mode. 
Baseline:

ISO: 800
Exposure: 1/30 sec
Aperture: 2.8
 
Flash added:

ISO: 800
Exposure: 1/15 sec
Aperture: 5.0


ISO: 800
Exposure: 1/30 sec
Aperture: 5.0


TAKEAWAY: When flash and ambient overlap, underexpose ambient slightly so that adding flash does not result in overexposure.

e. Minimizing ambient light: sync speed


Exercise 3
Use the open box setup.  Take a baseline shot with flash.

ISO: 400
Exposure: 1/4 sec
Aperture: 5.6
  
Find your camera's sync speed (listed in the manual or find it on the internet).  Select your camera's sync speed.  Take a shot with flash.  Compare to the baseline shot.  Did the ambient exposure increase, decrease, or remain the same?  Did the flash exposure increase, decrease, or remain the same?

ISO: 400
Exposure: 1/250 sec
Aperture: 5.6
Result: ambient exposure decreased, flash exposure remained the same.

f. Flash shutter speed

On some cameras, you can specify the slowest shutter speed that will be used when flash fires.  The typical default is 1/60.  Here's the logic: if the ambient light is very low (let's say total darkness), you normally have to use a slow shutter speed to get a correctly exposed subject.  But with flash, you don't have to -- the subject can be correctly exposed from the flash alone.  Because you don't HAVE TO use a slow shutter speed, the camera is asking you what's the slowest shutter speed you'll let it use (in P or A mode) when you're using flash.  That's what this setting is for.

Although you don't have to use a slow shutter speed when using flash, it's a good idea to leave this setting at a moderately slow speed such as 1/30 in order to allow sufficient ambient light in the picture to preserve the atmosphere.


Lesson 5: Flash Value Lock


a photo where FV lock was used to get correct flash exposure of an off-center subject

In some systems, TTL flash exposure is measured primarily from the center of the frame.  If the subject is off-center, the flash exposure can be inaccurate.  The solution is to use FV lock.


Exercise 1
Pick two objects.  Place one object in the center of the frame, and farther from the camera.  Place the other object at the edge of the frame and closer to the camera.
Suppose the farther object (at the center) is your subject.  Focus on it and take a shot.  Note the exposure of the far object and the exposure of the near object.


Suppose the near object (at the edge) is your subject.  Without moving your camera, focus on it and take a shot.  Note the exposure of the far object and the exposure of the near object.


Suppose the near object (at the edge) is still your subject.  Aim the camera toward the near object and press FV lock.  Then go back to your original composition with the near object at the edge of the frame.  Take a shot.  Note the exposure of the far object and the exposure of the near object.


TAKEAWAY: When the subject is off-center, FV lock helps to make sure the flash exposure is accurate.

Having completed lesson 5, you now have the basic foundation for controlling TTL flash.  Lessons 6 and above are about applying what you've learned to use flash effectively: http://betterfamilyphotos.blogspot.com/2010/07/ttl-flash-tutorial-in-10-lessons-part-2.html