Photographers Place

Flash Success by David Magahy

Are we there yet... every parent must know and dread this mantra.

30 years ago, when I thought a Mantra was a rather sleek Opel, my dread with flash photography was collecting my prints and wondering if they came out.  Even that phrase has a whole new connotation today.

I was only a curious amateur in those days with my own cheapie darkroom and a Pentax ME Super with a Thyristor flash gun, pretty snazzy gadget in those days. Perhaps, in hindsight, the definition for a professional photographer was an elevated fellow whose flash photographs came out.

Today we have E-TTL II with the ability to bounce flash of the clouds ( or so it appears when media photographers get in a huddle... note to self, must find out what sort of batteries they use,  must be a lot better than mine).  Then they also seem to be able to fire at 10 frames a second from 50 metres away, with the perfect flash exposure every time... definitely need to find out what batteries they are using. How times have changed.

Now that you know where I am coming from (maybe you also suffer from flashypecia, from the Latin: hair fallout owing to underexposed negatives (image files, for those who aren’t sure what a negative is), lets consider some of the industry terminology in the table below that can be used a reference as we proceed.

Term	Meaning
Hard Light	Light which will cause a hard shadow behind the subject and is not very flattering for portraits. From a small point light source such as small flash units, the sun (Although it’s large, it is a very long way away and appears like a small point source)
Soft Light	The opposite really. Soft shadows caused by a large light source... such as sunlight dispersed on a cloudy day or a large softbox or reflector used in the studio or outdoors. Includes light bounced of a wall or ceiling, even if originating from a hard light source.
Ambient Light	The light that exists prior to the use of the artificial light from the flash. This can include room lighting, floodlights in a stadium or illuminating a city landmark building, street lighting
X-Sync	The range of shutter speeds, up to a maximum that will synchronise properly with the timing of the flash illumination. Usually around 1/250sec on dSLR’s.  A few go to 1/300th and many are somewhere between 1/90th (circa 1980’s vintage... my Pentax I think) to 1/200th today. Medium formats usually support up to 1/500th.
TTL, E-TTL, E-TTL II	A system which allows the camera to measure the amount of light generated by the flash through the lens of the camera and onto the sensor (from here on that also refers to film). Explained later.
Pre-flash	A brief beam of light emitted by some flash systems to pre-measure the amount of light required to light the main subject. This is linked to the focussing point. Which makes sense... how else does the camera know what the main subject is? More on this later
Flash Exposure Lock (FEL)	Some higher end cameras have a button which when pressed, initiates a pre-flash and locks in the exposure at the focussed point. Great where the background is highly reflective or opposite in tone to the majority of the image to be captured.
High Speed Sync	Excellent diagram later shows how some Canon camera/flash combinations can exceed the normal x-sync limit and still give perfect exposures, even up to 1/8000th of a second. I’m a  Canon user so if you use Nikon, Sony, Pentax or other make, then you might need to refer to your user manual for equivalent terminology or capability)
1st and 2nd (Rear) Curtain Flash	1st curtain is normal flash sequence, rear curtain produces nice arty farty shots with long exposures. The technical details follow later.
Dedicated Flash	An external flash unit made by the camera manufacturer optimised for the particular camera model or range of models
Pop-up Flash	The flash that is built into some dSLRs.  The pop up action is either automatically activated in low light or is manually activated by pressing a nearby button or by pulling the flash into its extended position
Built-in flash	The small flash unit in many compact cameras and even mobile phones.
Flash output power	Usually known as a Guide Number which is calculated by the formula “f-stop x distance”, usually in metres in this country. For comparison purposes, manufacturers use 100 ISO as  a fixed film speed, because if you increase the ISO speed by one f-stop, then the calculation above obviously changes. See appendix 1 for a more in depth explanation.
Inverse Square Law	This is like Murphy’s Law which states that you never have enough f...ing light when you most need it. Seriously, it is a Law of Physics which states that light “drops” off by a factor which is inversely proportional to the distance from the light source. In laymans terms that means the if you double the distance from the flash to the subject e.g 1metre out to 2 metres, the amount of light falling on the subject is 1/4 of what it would have been.  This is pretty dramatic and we will observe and test this phenomenon in the club on the night.
High Speed Flash	A function of some Canon flash units which allow the use of shutter speeds above the normal x-sync top speed. A diagram on page 4 shows how this physically works.

FLASH BASICS

Camera Shooting Mode	Main Light	Flash Behaviour
FULL AUTO	FLASH	Shutter speed limited to x-sync* range, camera chooses
P (Program Mode)	FLASH	Shutter speed limited to x-sync range and usually fast enough to hand hold with a standard lens fitted (50mm). Image Stabiliser technology has moved the goal posts on this one.
Av, (Aperture Priority) A	AMBIENT LIGHT	Fill flash at up to x-sync or above using High Speed Sync where supported.
Tv (Shutter Priority) S	AMBIENT LIGHT	Fill flash at up to x-sync or above using High Speed Sync where supported
Night Mode (Picture Modes)	AMBIENT LIGHT	Fill flash at up to x-sync or below favouring Low Speed sync for long exposures
M (Manual)	AMBIENT LIGHT	user sets both Aperture and Shutter speed, flash then adjusts illumination for the selected foreground subject (fill)

*x sync - see previous table

When does the metering occur?

For normal ambient light, the metering is performed well in advance of the shutter opening. Canon cameras, for example, activate the internal light meter when you press the shutter release button down halfway. But the subject-illuminating flash pulse, however, occurs after you press the shutter release all the way. This means that the flash pulse appears after the mirror has flipped up (blocking the ambient light meter) and the shutter has opened.

1.          you can measure the flash pulse as it is being emitted  (OLD METHOD)

2.          you can send out a low-power test pulse (pre-flash) of known brightness first and base your light calculations on that data before the shutter opens.

These two flash metering methods are used by Canon’s automated flash metering systems. TTL and A-TTL flash use the former and E-TTL the latter. Flash units capable of E-TTL also support the older FP mode flash.

Now forget most of that. Since 1995 Canon has been adopting E-TTL (evaluative through the lens) metering and since 2004 a newer updated version E-TTL II.

E-TTL fires a low-power pre-flash of known brightness from the main bulb to determine correct flash exposure. It measures the reflectance of the scene with the pre-flash, then calculates proper flash output to achieve a mid-toned subject, based on that data.

A. The E-TTL pre-flash occurs immediately before the shutter opens and not when the shutter release is pressed halfway. The E-TTL pre-flash is actually used to determine flash exposure and isn’t fired during the ambient (existing) metering stage. Some users may be surprised to learn that E-TTL actually fires a pre-fire flash before the main flash at all. Using regular settings the process happens so quickly that the pre-flash is difficult to notice, though you might catch glimpse of it before the mirror blackout - an exception being second-curtain (or rear curtain) sync.

B. The pre-flash light is analysed by the same evaluative metering system that the camera uses to meter ambient light. This means it meters through the lens and is harder to fool than external sensors, isn’t confused by bounced light and does not read anything off the surface of the sensor. For what it’s worth, unlike the TTL flash meter, the E-TTL metering sensor cannot be seen by the overly curious - it’s hidden away up in the pentaprism (or roof mirror in low-end EOS cameras) housing.

Limitations of E-TTL.

One drawback of E-TTL is that the pre-flash can cause people who blink quickly to be photographed mid-blink. The pre-flash normally occurs an extremely brief period of time before the main flash, but when using second-curtain sync with slow shutter there’s enough time for rapid blinkers to react to the pre-flash.

Another problem can be created sometimes for nature photographers who photograph skittish birds.

And again the use of pre-flash can trigger studio slave flash units which work by detecting the light from the triggering camera - analogue optical slaves. This results in flash exposure going very wrong, since the optical slave is triggered too soon. The pre-flash can also confuse handheld flash meters, making manual flash metering very difficult.

It all seemed like a good idea at the time!!

E-TTL II.

Introduced in 2004 with the EOS 1D mark II digital camera and the EOS EOS 30V film camera, E-TTL II is an improved version of regular E-TTL which includes two key innovations.

Improved flash metering algorithms.

E-TTL II examines all evaluative metering zones both before and after the E-TTL pre-flash goes off. Those areas with relatively small changes in brightness are then weighted for flash metering. This is done to avoid the common E-TTL problem of highly reflective materials causing specular highlights in a flash-illuminated image and throwing off the flash metering. Normally E- TTL II uses evaluative algorithms for its flash metering, but the EOS 1D mark II has a new custom function (CF 14-1) that lets you use centre-weighted averaging rather than evaluative metering for flash metering if you prefer.

Distance data incorporated into some calculations.

E-TTL II can use distance data when it’s available. Many EF lenses contain rotary encoders that can detect the current focus distance. For example, if your camera is focussed on an object 4 metres away then the lens will send this approximate focus distance data to the camera body.

Under certain conditions the distance data is factored into the calculations for determining proper flash output. This is particularly useful if you use the focus and recompose method without setting FEL - the new system can help minimize flash metering errors under these conditions. Canon describe the new system as essentially metering flash data across a flat plane rather than a point.

Up until now distance data hasn’t really been used much by Canon cameras. Some PIC (icon modes) apparently incorporate distance data into their exposure calculations, but that’s really been about it. E-TTL II is the first really useful application of this information that Canon have implemented, and is obviously very similar to the fashion in which Nikon have long relied on distance data for their flash metering system.

Exceptions to using the distance data successfully

Three conditions are bounce flash, macro flash and wireless E-TTL flash. All these give inaccurate distance data and therefore cannot be relied upon for metering accuracy.

High Speed Sync (or focal plane...FP) flash mode.

The mode is called high-speed sync since it lets you synchronize flash exposure with high shutter speeds, not that it lets you take high-speed photographs. Confused yet?

So you want to use your flash to freeze the wings of a humming bird in flight? So, despite it’s name, high speed sync mode will not work for you. When a scene is illuminated primarily by a really brief flash of light then you aren’t going to get much motion blur - it’s almost as if you used an incredibly high shutter speed in the thousandths of a second. However when you use FP mode flash, the flash unit pulses the light output over a longer period of time in order to simulate a longer-duration burst of light. Since the flash burst is no longer particularly brief you can’t freeze motion as easily, even with high shutter speeds.

This mode is useful for shooting with fill flash outdoors using wide apertures. Normally you can’t shoot outdoors and use fill flash unless you stop down the lens and use a low ISO setting because you are stuck with 1/200th of a second shutter speed. It’s physics so live with it. NO out of focus backgrounds for you today.

Ladies and gentlemen, introducing, in the blue corner, the champion from Blurred Background, Bokeh...High Speed Sync.  Problem sorted. f2.8 and 1/8000th second here we come. Almost too good to be true. The drawback is a vastly reduced flash range...get in there to 3 metres and you’ll be good to go. These numbers will vary with flash model (and the those special media batteries of course... do Bunnings have them? Note to international readers... insert your own homewares store chain).

Fill flash

This is the most talked about flash mode in photography books and magazines. Can’t find it in the manual from Canon. This is one of the world’s great mysteries. Is it a conspiracy to confuse us all?

Flash photography takes on two very basic forms. In regular flash photography, the flash is the primary light source for the photo. As said earlier, flash metering is done for the foreground subject, and the background is metered by the camera’s regular exposure metering system.

This can, however, lead to the background being underexposed and dark if the ambient light conditions are low. This is how most people think of flash - as a way of taking photos in dark places.

However, flash can also be used in bright locations or in daylight to lighten shadows, reduce the harsh contrast of full sunlight or brighten up dull images without being the primary light source for the photo. This is called “fill flash” or “fill-in flash,” the latter term is more commonly used in the UK.

It’s often a source of surprise for non- photographers, who wonder why pro photographers use flash units outdoors on sunny days in brightly lit settings. In such situations the fill flash is being used as a sort of portable reflector - shining a little extra light in certain areas.

A typical example might be a person who’s wearing a hat outdoors on a sunny day. Hat brims often cast dark shadows over the subject’s face, and a little amount of flash can lighten up this shadow nicely.

A backlit subject is another common use for fill-flash - you could simply crank up the exposure compensation to expose the subject correctly but then the background will be burnt out completely.

You may want to add a little sparkle of light into the subject’s eyes - the “catchlight.” Sometimes wildlife photographers use flash units at great distances from their quarry for the same reason - they aren’t using the flash to illuminate the animal but to provide a lively catchlight to the eyes.

In all these cases you are, from the point of view of the camera, using two light sources at the same time. There’s ambient lighting, which is all the available light around you - reflected light from the sun or artificial light sources. And there’s the light from the flash unit, which is supplementing this existing light.

As always, ambient light levels hitting the sensor are governed by the lens aperture and shutter speed and flash levels are governed by flash metering. This needs to be re-read until the penny drops.

By adjusting the output of the flash unit you’re essentially adjusting the ratio between flash-illuminated and ambient light-illuminated scene.

In fact you could argue that the two cases I present above -

flash as primary light source and ambient light as primary light source

are an artificial distinction and that all flash photography is fill photography in a sense; just that in the first case the ambient lighting is so low as to be insignificant, whereas in the second case it’s the reverse. This is true enough, but I think the distinction is useful to make, particularly in terms of the way full auto and P modes work versus Tv, (S) Av (A)and M modes. Unlike certain other camera systems (particularly Nikon), Canon EOS cameras always default to fill-flash mode when the camera is in Tv, Av and M modes. They also perform fill flash in P mode if ambient light levels are high enough. There’s no separate switch or pushbutton to engage fill flash. Could this be the secret to the mystery of the manual omissions?

Slow shutter sync. or dragging the shutter

There are two basic ways in which a camera can take a flash photo when light levels are low.

The camera can either use a short shutter speed to minimize camera motion blur and have the flash blast out enough light to illuminate the foreground objects whilst leaving the background dark, or the camera can extend the shutter time to allow more of the background to show up and flash-illuminate the foreground subject.

This latter technique is called slow sync, slow shutter sync or “dragging” the shutter. It’s only possible in Tv, (S) Av (A)and M modes - you can’t use it in P mode or most of the PIC (icon) modes. The one exception is the night scene PIC mode on many EOS cameras, which uses slow shutter exposure with first-curtain flash.

A typical example is a tourist snapshot of someone standing in front of a famous landmark at night. If you keep the shutter speed fast then you’ll have a nice flash- illuminated photo of your friend against a pitch black backdrop, unless the landmark is extremely brightly lit or unless you’re using very high ISO setting. New cameras (2010) are now offering usable ISOs over 100,000

However, by slowing down the shutter speed you can take a photo of the person standing against a properly exposed background. The drawback is obvious, of course. By slowing the shutter speed you’re going to need a tripod to avoid blur induced by camera movement, especially with long shutter speeds like 1/15 second or slower. Some form of  image stabilisation is going to help here.

Sometimes slow shutter sync is used to provide a dynamic motion effect in flash photos. A photo taken with flash and a slow shutter speed can provide an interesting mix of flash-illuminated subject and ambient-light-illuminated motion blur. The effect is difficult to predict, but can be very striking and exciting when it works. My arty-farty cyclist was one of at least 20 attempts, taken at 1/4sec with panning on a tripod and rear (or 2nd) curtain sync. The Velodrome has floodlighting which is patchy. I started the exposure as the rider entered a bright patch.

Program (P) mode

Jokingly we refer to P meaning for Professional. If in doubt, under pressure, it is “safe” most of the time.

The overriding principle of Program (P) mode in flash photography is that the camera tries to set a high shutter speed so that you can hold your camera by hand and not rely on a tripod. If that means the background is dark, so be it.

Program mode operates in one of two modes, depending on the ambient (existing) light levels.

1) If ambient light levels are fairly bright then P mode assumes you want to fill-flash your foreground subject. It meters for ambient light and uses flash, usually at a low-power setting, to fill in the foreground.

2) If ambient light levels are NOT bright then P mode assumes that you want to illuminate the foreground subject with the flash. It sets a shutter speed between 1/60 sec and the fastest X-sync speed (see above) your camera can attain. The aperture is determined by the camera’s built-in program. Because the camera tries to keep the shutter speed at a reasonable speed for handholding the camera you will end up with dark or black backgrounds if you take a flash photo in P mode when ambient light levels are not bright.

On most EOS cameras, P mode is not shiftable when flash (internal or shoe-mounted Speedlite) is used.

Tv (shutter priority) mode

In this mode the camera lets you manually select the shutter speed. It then automatically chooses a corresponding aperture setting to expose the background correctly ( at least as the camera engineers deem corrrectly)

Flash duration (flash output) is determined by the flash metering system. In other words, the camera always works in fill flash mode when it’s in Tv mode - it always tries to expose the background adequately, unlike P mode.

If the maximum aperture value of your lens starts flashing in the viewfinder it means the background of the scene you’re shooting is too dimly lit. If you want to try and expose the background correctly then you should decrease the shutter speed to compensate. Otherwise the camera will just try and expose the foreground with flash and the background will come out dark.

Naturally at slower shutter speeds you’ll need to use a tripod to avoid blurring caused by camera shake. As always, the camera will prevent you from exceeding its built in X-sync speed unless FP mode(High Speed Sync) is available to you and engaged. If the minimum aperture value of your lens starts flashing then your scene is too brightly lit. You must then either engage FP mode if it’s available or perhaps put a neutral density filter on the camera or use a lower ISO setting. (100 instead of 200 for example)

Or turn off flash altogether and simply use a reflector of some type to bounce ambient light onto the subject. If all else fails. find some shade.

Av (aperture priority) mode flash.

Av mode lets you set the depth of field by specifying the lens aperture. (I choose this mode more than 90% of the time. I believe that what is happening behind the main subject makes or breaks the image. Do I need to include it or blur it out... either partially or fully. I prefer not to use Photoshop to do it. It rarely looks right and often looks blaringly wrong). The camera then chooses a shutter speed ranging from 30 seconds to the camera’s X-sync speed, in order to expose the background correctly. If that means the shutter speed is some really low value so that you need to use a tripod to avoid camera-shake blur, so be it. In dark conditions, therefore, Av mode works in slow sync mode.

Flash duration (flash output) is determined by the flash metering system. Like Tv mode the camera always works in fill flash mode when in Av mode.  As always, the camera will prevent you from exceeding its built in X-sync speed unless FP mode is available to you and engaged.

If the shutter speed value of 30” flashes in the viewfinder then there isn’t enough light to expose the background correctly and you’ll need a larger aperture (smaller f number like 4.0 instead of 8) or higher ISO. (example: 800 instead of 200) If the camera’s X-sync flashes in the viewfinder then you’ll need to decrease the lens aperture, engage FP (high speed flash) mode if it’s available or use a lower ISO.

Manual (M) exposure mode

In manual exposure mode you specify both the aperture and shutter speed, and your exposure settings will determine how the background (ambient lighting) is exposed. The subject, however, can still be illuminated by the automatic flash metering system since the flash can automatically calculate flash output levels for you.

This is a marked contrast to the olden days, when photographers would carry around little flash exposure tables with them in order to work out manual flash settings. We are talking about Manual on your camera dial or LCD Not to be confused with the Manual Mode on your flash.

Set your camera to M for manual exposure mode. Set the aperture and shutter speed to expose the background correctly. Press the shutter button down halfway if your flash has a rear-panel LCD (liquid crystal display).

The flash coupling range will appear in the flash unit’s LCD. This range is the distance that can safely be covered by the flash. If your lens has a distance scale you can check the current focussing distance to ensure that the distance to your subject falls within this range. Otherwise you’ll have to estimate.

If the “flash ready” lightning bolt symbol appears in the viewfinder you can press the shutter all the way to take the photo. The flash’s E-TTL system will determine the flash exposure level of the subject. Many wrongly exposed flash images are because the subject was inside or outside the range shown on the flash.

Also, LCD-equipped flash units will not calculate the flash coupling range if you’re using bounce flash, and the coupling range will not necessarily be correct if you have a diffuser (Stofen or the like) on the flash head.

E-TTL flash metering patterns:

The camera uses its evaluative metering system to meter the flash output, based upon the pre-flash. When in autofocus mode most earlier EOS bodies which do not use E-TTL II bias flash metering toward the currently selected AF point, but always in an evaluative mode pattern - they don’t use spot or partial metering patterns.

When in manual focus mode (the lens switched to MF instead of AF) it appears that at least some EOS bodies switch to centre-weighted averaging. Note, however, that this biasing of E-TTL metering to the active point is potentially problematic, since it means that the flash metering is done in almost a spot-metering fashion. Many user complaints regarding flash metering problems in E-TTL mode appear to be linked to this issue. If the camera happens to be over a dark object, for example, flash metering can be considerably overexposed, and vice-versa.

Flash Exposure Lock (FEL)

The standard answer to this problem is to use FEL(flash exposure lock and not all cameras have this...check your manual) and meter off something mid-toned, but this is clearly not a solution for rapid-shooting situations such as weddings and sports.

FEL allows you to:

a)    focus on your subject (by half-depressing the shutter button) and determine the ambient exposure then

b)        lock the correct flash exposure by pressing and releasing the “*” button or FEL button on EOS 1 series cameras (the camera holds the setting for 16 seconds), then

c)        recompose the image with the subject off-centre and take the photo.

Another approach is to set the camera lens to manual focus, since the body apparently switches to centre- weighted average metering in that mode, but that’s obviously not a useful answer much of the time either.

Early renditions of the Canon EOS Digital SLR.s were a bit dodgy with E-TTL. Since the release of the 20D and subsequent models, up to 2004 saw better results.

In 2004, E-TTL II was released and addresses this problem by altering flash metering considerably. It examines each evaluative metering zone before and after the E-TTL pre-flash. It then calculates the weighting for each zone independently, biasing against those zones with high reflectivity in the pre-flash. This means that E-TTL II does not have a flash metering pattern as such, since it’s calculated dynamically.

By employing multiple metering zones to measure both ambient light and pre-flash, then comparing the two and taking metering distances into account, E-TTL II automatically adjusts the flash level to achieve natural reproduction with ideal exposure of both the background and subject. Sounds like great marketing hype. (BS in other words). In various shooting situations, even when the background is highly reflective, E-TTL II can use distance information from the lens to eliminate underexposure and achieve optimal automatic flash exposure control. So says Canon but I beg to differ in my experience.

When an EX Series Speedlite is mounted on an EOS DIGITAL camera, it automatically informs the camera of the color temperature of its illumination. This enables the camera to calculate white balance with extreme accuracy.* White balance must be set to “automatic” or “flash use” mode

The angle of view of an EOS DIGITAL camera is different depending on the size of its image sensor

• APS-C, (300, 350 and 4/500D, 10D, 20D, 30, 40 and 50D and 7 and the older D30/D60),

• APS-H ( 1D, 1DMkII, III, and IV series), or

• 35mm full-size (1Ds, 1DsMkII and III and 5D, 5D Mk II),

regardless of the focal length of the lens being used.

This factor is taken into account by EX Series Speedlites, which automatically adjust the flash zoom mechanism depending on your camera’s image sensor size to deliver an ideal flash angle for the effective angle of view. Since only necessary areas of the scene are illuminated, Speedlites are especially efficient at conserving energy. More marketing BS... now go ahead, buy two, they are small!!

Sorry to finish on a cynical note but it’s now 9 years since I bought my first Canon D30. IMHO scenes and your creative ideas vary too much to be calculated precisely every time. But going right back to the opening paragraphs. It sure is easier than it was 30 years ago.

Bibliography

Guy, N K,  Flash Photography with Canon EOS Cameras - Part I, II and III.

http://photonotes.org/articles/eos-flash/

Canon, Flash Work, (http://web.canon.jp/Imaging/flashwork/index.html)

 

By way of an every rule has an exception cliche, I will mention that the makers of the new Pocket Wizard radio transmitters have exploded all the foregoing urban myths: they've somehow managed to rewrite the time honoured principles already mentioned before such as increasing the sync speed on all Canon cameras up to 1/500th sec in some cases, increasing the power output of the high speed sync flashes and reducing the recycling time of the flash with a theoretical flash performance up to 12 frames per second. If that whets your appetite then go to the pocket wizard website or wait for my follow up article with my own experiences of these devices.