7.5 Flash Settings and Exposure

Although flash can be a godsend in poorly-lit environments, it does introduce an additional level of complexity, particularly with respect to the setting of exposure parameters.  If you’re just starting out with your first SLR camera, it might be best for you to use auto-exposure and auto-flash settings until you feel you’re ready to deal with the complexity of fully manual flash.  You can put your camera in Av mode (aperture prioritysee section 6.3), put the flash in TTL mode (or E-TTL), and then begin shooting, taking note of how the flash affects the exposure.  Try modifying the EC (exposure compensation) and FEC (flash exposure compensation) settings and seeing how these affect the foreground/background lighting of the scene, as well as the color and detail.  Though I now shoot almost exclusively in manual mode (for both the camera and the flash), I’ve had much luck in the past shooting in Av/TTL with an EC setting of -2/3 and an FEC setting of (again) -2/3.  If the image proves too bright or too dark, you can turn these settings (EC and FEC) down or up by a few clicks in tandem to correct the overall exposure.  Note that modifying just one of these two settings in isolation will typically change the flash ratio—the relative amount of light from flash versus ambient light.
    In order to retain total control over your exposures when using flash, you need to understand how flash interacts with the exposure parameters that you learned about in Chapter 6—i.e., the aperture, the shutter speed, and the ISO setting.  The table below summarizes the effect of each parameter.

shutter speed time
flash power time

Table 7.5.1 : Parameters and the dimensions they affect.  Because
shutter speed and flash
power operate in the same dimension,
they can’t (normally) be treated independently, and hence don’t
obey reciprocity.

As noted previously, the ISO setting affects the amplification of the voltage coming out of the imaging sensor, the aperture’s diaphram (or iris) restricts the cross-sectional area through which light is admitted, and the shutter speed obviously dictates how long the imaging sensor is exposed to the incoming light.  These three parameters thus address three different dimensions of the exposure problem: voltage, area, and time.  The flash’s
power setting—which as we mentioned earlier is in most cases actually a measure of flash duration—also affects the time dimension, which is where the complexity arises.  Since both the shutter speed and the flash duration operate in the time domain, they can’t be treated as independent variables (normally).
    You may recall from Chapter 6 the concept of reciprocity: the fact that a change in one parameter can be compensated by an equal but opposite change in another parameter (where
equal in this case means an equal number of stops of light—recall that a stop of light is equivalent to a doubling or halving of the shutter speed in non-flash photography).  Thus, if we decreased the aperture by two stops (from, say, f/4 to f/8), we could compensate by increasing the ISO by two stops (e.g., from 100 to 400). 
   Unfortunately, flash causes reciprocity to fail in the case of shutter speed.  As mentioned ealier, a doubling of the shutter speed from 1/50 to 1/100 sec will halve the amount of ambient light which is collected by the sensor, but won’t change the amount of flash light that reaches it, because the flash has a duration much smaller than 1/100 sec (typically around 1/1000 sec at full power and around 1/35000 sec at minimum power).  Thus, below the maximum sync speed (which is typically between 1/200 and 1/300 sec) of your camera, changes to the shutter speed will only affect the flash ratio, not the absolute quantity of flash light collected. 
    Above the maximum sync speed, two things can happen.  First, if you haven’t set your flash to high speed sync (HSS) mode, what will typically happen (on most camera models) is that the shutter speed will be slowed to the maximum sync speed, and depending on what exposure mode you’re in, you may experience over-exposure.  Alternatively, if you’re already in HSS mode (or if your particular camera or flash unit automatically switches to HSS when needed) then above the max sync speed you should again (in theory) see reciprocity between the three non-flash exposure parameters (aperture, shutter speed, and ISO), because in HSS mode the flash simulates continuous light by pulsing many times at closely-spaced intervals.  However, if your flash unit automatically adjusts its output to compensate for changes in shutter speed, then once again reciprocity may be violated. 
    Unless flash is being used as the sole light source, perfect total-light reciprocity between flash
power and the non-flash exposure parameters will generally fail, even in HSS mode.  When there is a non-negligible amount of ambient light, flash only adds to the total amount of light, so when a change is made to one of the non-flash parameters, compensating via changes to flash power will generally require unequal compensatory changes (in terms of numbers of stops of light, or clicks on the control dial).  Thus, turning flash power up by some number of clicks and turning some other parameter down by the same number of clicks generally won’t preserve the overall exposure levelIn practice there’s typically not much room for compensatory changes to the flash setting anyway, since for small birds at a distance you’ll generally have the flash turned up to the highest safe setting (i.e., the highest setting that will avoid a meltdown—see section 7.10).  Remember also that flash illumination falls off nonlinearly as distance increases, so changes to flash power can affect different parts of the scene differently.
    To summarize, changes to the shutter speed while below the MSS (maximum sync speed) mainly affect the flash ratio, while above the MSS, with a fixed flash
power setting, changes to shutter speed may or may not affect the flash and ambient light in equal proportions (i.e., no change in flash ratio), depending on equipment specification (i.e., whether your flash unit rations flash output over the exposure interval to maintain a constant output when the shutter speed changes). 

Box 7.1: The Flash Reciprocity Rule

When using flash, reciprocity still holds for everything except (possibly) shutter speed and flash power.

  • Below the maximum sync speed, shutter speed and flash power generally do not obey total-light reciprocity relative to any of the other exposure parameters.
  • Above the maximum sync speed, shutter speed may obey total-light reciprocity with the other non-flash exposure parameters (depending on equipment specification), but flash power generally does not.

One exception to this rule is when shooting with virtually no ambient light below the max sync speed, in which case flash power should obey reciprocity with aperture and ISO.  Above the MSS, the behavior of the flash power control is equipment-specific, so you’ll need to consult your flash unit’s user manual as to whether flash “power” and resulting luminance are linearly related.
     Don’t get too hung up on understanding reciprocity.  In practice, you’ll want to take some test shots anyway to see whether you’ve got the right flash ratio, and the only way reciprocity can help you in that case is to possibly reduce the number of test shots you’ll need to take before honing in on the right settings.  Except for birds in flight, I usually set my shutter speed to the maximum sync speed, in which case aperture and ISO are fully reciprocal with each other, and the flash power setting can be used to tweak the flash ratio.  For birds in flight I enable HSS, set my flash power to 1/4 or 1/2, set my shutter speed to whatever speed is needed to freeze the birds (which may be 1/500 for slow-flapping birds or 1/1250 for faster birds), and then use aperture and ISO (which always obey reciprocity with each other) to obtain the proper exposure.  If you follow a similar methodology, you won’t have to worry about remembering when reciprocity is or isn’t affected by flash.
    The box below provides an alternative recipe for setting exposure parameters when using flash.

Box 7.2: Setting Flash Exposure Parameters

  Step 1: choose a shutter speed

  • If the max sync speed isn’t too slow for freezing the bird (or if you’re freezing the bird via flash), then set the shutter speed to the max sync speed (or just below it).
  • Otherwise, if you really need to exceed the max sync speed, then do so.

  Step 2: set the aperture

  • Try to choose the widest aperture (smallest f-number) that doesn’t sacrifice sharpness.  Most lenses need to be stopped down 1 or 2 stops; premium-quality lenses may only need stopped down by 2/3 stop or so.  For depth-of-field constraints (or to accommodate front/back focusing errors), you may also need to stop down.

  Step 3: set the flash power

  • Put the flash unit in manual mode and set it to 1/4 power.

  Step 4: set the ISO

  • Find the ISO setting that gives you the desired exposure (image brightness).  In the case of ETTR (section 6.2), that’ll mean maximizing brightness without blowing highlights.
  • If you end up turning ISO all the way down and the exposure is still too bright, then try turning down the flash power.  You could also close down the aperture, but that’ll increase DOF (which can be good or bad), and below f/16 or f/22 you might suffer from diffraction effects.
  • If you can’t turn ISO up high enough (due to noise), then turn up the flash power to 1/2 or even full power, but limit your number of shots to avoid melting your flash head.

    As mentioned previously, the flash ratio can affect the amount of illumination in the background of the scene, relative to the illumination of the subject—assuming the subject is some distance in front of the background.  The flash ratio can also affect the color (or white balance—see Chapter 11) of the image, since flash and ambient light usually have different color temperatures (see section 7.1).  In practice, as long as you’re using flash at its maximum
safe setting and with a flash extender, birds that are reasonably close should have a neutral white balance; the background’s white balance will depend on its distance from the flash, but since that’s not something you can easily change in most situations, there’s little sense worrying about it while shooting.
     One thing to be aware of while shooting is your flash’s ability to keep up with your rate of shooting.  Most flash units have a pilot light that indicates when its internal capacitor is charged and ready to go for the next shot.  Using higher flash
power settings typically means that the capacitor will take longer to recharge—or, in flash photography parlance, the flash unit will take longer to recycle.  In practice, I typically find that this means that I have to be more selective, when using higher flash power settings, with the shots that I take, being careful to wait for just the right pose before taking the next shot. 
    Finally, note that engaging HSS mode can reduce the effective flash exposure significantly, because the flash unit has to expend more energy to make the individual pulses comprising the near-continuous beam of flash light.  Thus, if you’re shooting at or just above the maximum sync speed and aren’t getting enough flash exposure, try turning off HSS; you’ll typically see a significant increase in effective flash exposure, though you may lose a bit of
stopping power due to a lower shutter speed.