The TTL Flash System
What follows was written in spring, 2004. While the theory of TTL remains the same, in 2008 we're in the iTTL world of the D3. For a more up to date explanation of using iTTL, you'll want to head to the D3 site.
Flash is still the most difficult aspect of wildlife photography to master both technically and aesthetically. It doesn't matter what camera you have, flash still takes plain old personal skill. Though this commentary can apply to any form of photography, I will as always focus in on wildlife photography (there I go with the bad puns). There a lot of theory being presented here, an exercise and basic concepts to aid in your mastering the flash. Remember this one very imprtant bit of advice. You learn flash only by doing flash. Reading what I have to say here will assist you in better understanding it, but you can only master it by doing it. So, on with the show! (Those shooting with the F5 should head directly to F5 Flash )
It doesn't matter if you're using Nikon or any other brand, flash basics are flash basics. The basics of manual flash and manual flash exposure must be understood even with today's modern TTL technology! With the introduction of the N8008 and SB-24, it was said that those not knowing flash photography would become good and those knowing flash photography, would become great with its technology. True, the breakthrough technology of that system improved the mathematical percentage of technically correct exposed photographs. But the quality and creativity of flash photographs taken by the average photographer have not made the same quantum leap. This stems from the fact that basic flash exposure and technique has never been very well understood. This includes understanding all the advantages the Nikon flash system makes available to the photographer!
Before diving into this topic, one statement needs to be made about understanding flash photography. It takes time to master flash! Because it's something we can't see, understanding flash is a very, very difficult process. Don't expect to comprehend all the theory and put flash into practice after first reading through this or any other explanation of flash. Give yourself and your equipment time to let it sink in. This book will have you up and running with flash, but it will still require some time before you're using flash as a natural, unconscious photographic tool.
Understanding flash begins with ambient light. Flash is needed when ambient light is either too low, contrasty or nonexistent to properly expose the subject. Low light can come from shooting in the shade of a tree or late evening. Contrast seems to haunt our every exposure as we fight the battle between highlights and shadow. And no light for photography can be encountered in our own homes when shooting a birthday party. In any case, understanding these deficiencies is the first step in properly using flash.
The next step is understanding how we properly expose film for ambient light. Ambient light is the light that comes from the sun (99% of the time, artificial lighting such as from household lamps could be considered the other 1%) that bounces everywhere in and out of doors. To properly expose for ambient light, we have available to us shutter speeds and apertures. We must use the correct combination of shutter speed and aperture to properly expose film for the existing light based on the film's ISO (film's ISO to light index). A proper exposure in general terms is when detail can be seen in the highlights and shadows of a photograph. (If detail is not seen in either the highlight or shadows, then the range of exposure is beyond the ISO of the film.) The shutter speed and aperture each have a separate yet combined roll to play in proper exposure.
The shutter speed determines how long the film sees the light. Shutter speeds are described in terms of time. Current Nikon cameras have shutter speeds with durations of 1/8000 of a second to a full 30 seconds (this doesn't include manual speeds/times fired using the bulb function). No matter how the shutter speed is set, whether manually dialed in or automatically derived via the camera's computer, the shutter speed determines the length of time the film is exposed to the ambient light. The shutter speed also determines how long the film sees the subject. If the subject is not moving, the length of time the shutter is open and the film sees the subject does not matter. But if the subject is moving, long shutter speeds can result in out-of-focus subjects because movement is seen by the film.
The aperture regulates the amount of light that strikes the film. The aperture is a large hole normally at the rear of the lens through which light must travel to reach the film. It has blades that can be closed down, making the hole smaller, or opened up, making the hole larger. The larger the hole, the more light that can strike the film. The smaller the hole, the less amount of light that can strike the film. The size of this hole is described most commonly in f/stops (though not technically correct, but who cares as long as we all know what we're talking about). The larger the hole, the smaller the f/stop number is. The smaller the hole, the larger the f/stop number is. For example, a large hole would have an f/stop of f/1.4 and a small hole an f/stop of f/32. The size of this hole also determines the depth-of-field in the photograph, that is, the depth of focus in front of and behind the subject.
These are our only two means of regulating the light that strikes the film. In flash work, it's important to remember that shutter speed affects how long the light strikes the film and that aperture affects how much light strikes the film.
Understanding the above is relatively simple, but now is when it becomes more difficult! In the old days (less than a decade ago), flash exposure calculation was the cause of divorces. We all have those infamous holiday photographs when dad's flash either made us white as ghosts or mere shadows against a wall. This was and still is, because of the lack of basic understanding of the two exposure regulators of the flash exposure. We have flash-to-subject distance and aperture as our only means of regulating manual flash exposure. We'll start with flash-to-subject distance as it's normally the hardest concept for folks to understand.
Flash-to-subject distance is just what it says, the physical distance between the flash and the subject. Even with TTL technology, this concept must be thoroughly understood! Back in the old days, this distance was generally determined by first focusing on the subject, and then reading the distance off the distance scale on the lens. Other methods such as a measured length of string, pacing off steps and even tape measures were used to determine this all important distance. This unit of measure was then put into a formula to calculate the needed aperture for proper exposure.
Generally, flash units would have a sliding scale on them for making this calculation easier. The table required the film's ISO and flash-to-subject distance. With this, determining the needed aperture for correct exposure was made easy. Ha! And if you were really unlucky, your flash had no such table and a hand written, mathematical formula had to be used. This often required a piece of scratch paper and pencil, and lots of time. It also required knowing the flashes guide number and having excellent mathematical skill. But most importantly, it made flash photography extremely user unfriendly! It is also what made using flash so unpopular and dreaded, a sentiment carried by some still today! Since modern TTL flash technology, knowing or reading all these formulas is useless, lets cut to the chase and leap ahead to the more currently related flash problems.
The reason that flash exposure is determined by flash-to-subject distance is because of the power of the light emitted by the flash itself. The amount of light the flash bursts off is referred to as Guide Number, GN for short. The higher the number, the brighter the light emitted by the flash. The brighter the light, the further it can travel from the flash. But as the light travels from the flash towards the subject, the power or intensity of the light falls off. In fact, the light falls off at a tremendous rate called the inverse square law. This law says light from flash falls off at the rate inverse to the square of the distance the light has traveled. In plain language, flash units have really little power.
To illustrate this point, let's look at the rate of light fall off for the SB-24/ 25/26 (which are all the same). Using film with an ISO of 125, a subject at 60 feet can be properly exposed at f/2.8. At f/4 it drops to 40 feet, f/5.6 to 30 feet, f/8 to 20 feet, f/11 to 15 feet, f/16 to 10 feet and at f/22 to a mere 7 feet. That's an incredible drop off in power in the space of a few feet. But it's important and a fact of life we must work with. (This assumes the subject is an 18% gray and does not take into account whether the subject is white or black because either case can radically change the needed f/stop.)
You should also note from this point the relationship between flash-to-subject distance and aperture. Aperture is our other means of regulating the exposure. Using the information in the above example, if the subject is 30 feet away the proper exposure is f/5.6. But if our subject is white and at that same distance, we would probably want to open the aperture up to f/4 to record the subject as white. And if the subject is dark, we would want to probably close up one stop to f/8. During this whole exercise, the flash-to-subject distance never changed. We changed exposure strictly by changing the f/stop.
The limitation with this is, what if we're shooting a group of people at a distance of 30 feet? We should be using an f/stop of f/5.6, but that's not enough depth-of-field to have the whole group in focus. And what if the group is a bunch of actors dressed in black costumes? The nightmares are just beginning for someone trying to accomplish this with manual flash. Yet thousands of situations like this were successfully taken with "old" flash technology because those photographers understood the basics of manual flash. But what does this have to do with modern TTL technology? Hold on, we're getting there!
Nikon first introduced TTL (Through The Lens) flash technology in 1980. The basics of TTL operation is rather simple and applies to all camera systems. With TTL flash exposure, when the shutter is tripped, the light from the flash fires off, racing to hit the subject. This light hits the subject then bounces back to the camera. The light traveling the speed of light, travels through the lens (and any filters which may be attached) and strikes the film plane. The light then bounces down off the film to a sensor which reads the light as it builds up exposure on the film. Once the sensor and the connected computer software determines the light has massed enough for the correct exposure, the camera's computer turns off the flash (just like a light switch in a house goes on-off). Keep in mind, this is all happening at the speed of light! Why is this technology so important to flash photography today?
Lets look back at the path of the light once it leaves the flash. The correct exposure for this equation was not determined by using either flash-to-subject distance or aperture. The correct exposure was achieved by simply turning off the flash! Whatever the size of the aperture, the hole the light must pass through on its way to the film, is dealt with by the camera's computer at the film plane. Does this mean that flash-to-subject distance and aperture are not important to flash exposure? Not hardly, but it means we can control the flash rather than it controlling our photography.
With TTL technology, the camera's computer provides the correct exposure regardless of the aperture or flash-to-subject distance (as long as they're within the realm of the flash's power). TTL technology puts the control of depth-of-field back into the hands of the photographer. We can pick the aperture for the desired depth-of-field and not have it preselected by the flash-to-subject distance. We can do this because the camera's computer delivers the correct exposure for the aperture we've selected. If I'm going too fast and getting too excited for you to follow, let me explain it through an example.
Photographing nesting birds was, until TTL technology, a major pain. Large, bulky flash units had to be introduced into the home of the bird, often causing stress because of their size and presence. These large flashes were needed to light the nest sufficiently to obtain the desired depth-of-field. Because of their large size, these units had to be placed back from the nest and consequently had to have even more power because of the flash-to-subject distance. Then came the TTL flash units.
I remember this very scenario back in 1981 when I shot a nest with two SB-12 flash units and another photographer used two Norman 200b units. I stood four feet from the nest, had the two small flash units right beside the lens for the light pattern I wanted and most importantly, selected whatever f/stop I wanted for the depth-of-field I desired. I did this without any concern for the exposure as the camera's computer did it all. The other photographer had to carefully place his flash units and use a flash meter to determine the correct f/stop. And once selected, he could not change that f/stop without moving his lights and thereby changing his flash-to-subject distance.
So TTL has taken all the calculations out of basic flash exposure. This is true for those who only want basic flash photographs. But I assume you bought this book because you want to take your flash photography beyond the basics by using flash as a creative tool. For this to happen, the melding of flash and ambient light is required.
Lets understand the terms. Flash key is where the light from the flash
is the main light. Flash fill is when the light from the flash is filling
in the deficiencies in the ambient light, in other words, filling in
the shadows. The Nikon TTL flash system is more than capable of handling
both of these situations without any help from us. But getting more
than the standard canned light from the flash takes more from us. The
specifics for programming the flash units will be covered under the
appropriate sections. Here, I just want to discuss the concepts and
Flash key, like I said, is when the flash is the main light source for the photograph. This is not to say there is no ambient light, but that it's not the major light source for the exposure. The main point to remember when the flash is the key light, it's creating the light pattern for the photograph. This is a very important concept that will be discussed in greater depth later.
Exposure for flash key is normally a straight line affair (unless bouncing the flash). The TTL metering calculates the exposure based on what the light does bouncing off the subject. The light from the flash fires and shuts off, requiring the photographer only to remember to turn on the unit. When operated in this manor, the exposure for the ambient light will fall wherever it may, possibly influencing the final photograph or not. Generally, the only concern is the exposure of the flash.
Flash fill is used to fill in the shadows found in ambient light. This is a technique compacting the exposure range of the scene to fit within the latitude of the film. Most films can only hold information in the shadows and highlights when the range between the two is less than three stops. For example, if the exposure for the highlights is f/11 and the exposure for the shadows is f/5.6, the film will record detail in both. But if the exposure should be one stop more in either direction, the film will loose detail in the overexposed or underexposed area. The flash helps by pumping light into the shadows, decreasing the range and aiding the film in recording the detail.
Flash fill is described in terms of ratios. A 1:1 ratio means the light from the flash is equal to the light level of the ambient light. This means there are no shadows because the light from the flash wipes them out. A ratio of 1:2 means the flash is one stop less in exposure compared to that of the ambient light. In this case, there will be a shadow that is one stop less in exposure than the ambient light and well within the range of a film's exposure latitude. The ratios keep going, 1:3, 1:4 and soon on, but after 1:4, the fill light is doing very little for your photograph.
Taking flash to a higher level starts here! We must meld two light sources to really be successful in communicating photographically with flash. This requires understanding the two influencing factors of ambient light and flash.
Ambient light exposure is regulated by shutter speed and aperture. Flash exposure is regulated by flash-to-subject distance and aperture. The only variable in common for properly exposing the two light sources is aperture. This means that the aperture is the constant when melding the two light sources. The shutter speed must be changed to match up with the constant aperture (for proper flash exposure) to properly expose the ambient light. At the same time, the flash-to-subject distance must remain constant to provide the correct exposure for that aperture.
To further illustrate the point, let's throw some numbers into the discussion. Using the SB-28 GN, we know that we must use an f/stop of f/5.6 when the subject is 30 feet from the flash. Our shutter speed must then match up with f/5.6 to properly expose the ambient light. As long as the flash stays 30 feet away, our aperture at f/5.6 and the shutter speed is correctly set, we'll have beautifully exposed ambient and flash light. At least until the subject moves, the sun is blocked by a cloud or we need more depth-of-field. We are locked in to this formula when working with manual flashes if we want to properly expose ambient light and flash. But TTL gives us another option.
Because flash-to-subject distance doesn't affect our f/stop with TTL (as along as we're within the realm of the flash's power), we can maximize and manipulate aperture and shutter speed to our advantage. In the instance of flash key, we can manually set the shutter speeds to correctly expose for the ambient light. We can also set the shutter speed to underexpose the ambient light. This can be as little as one stop to make a busy background disappear. Even in bright daylight, we can make the photograph have the appearance of being shot at night by underexposing two or three stops by using the shutter speed. This whole time, the flash is providing us with the proper exposure for the subject. We can do this only because TTL provides us with the proper exposure for the aperture selected regardless of the flash-to-subject distance! Wow!!
There's a lot more to this than this simple explanation provides. But the specifics of how all this can be put into practice will be saved until actually talking about the specifics of the particular flash units. Just being aware of this and thinking about it though will help prepare you for those sections.
Many wonder why the shutter speed does not affect flash exposure. It's simple, the shutter simply isn't fast enough! The duration of the burst of light from the SB-24, 25, 26 and SB-28 ranges between 1/1000 to 1/23000 of a second. The light from the flash must be able to expose the film in one, solid burst to properly expose the film (this is not true with the FP feature on the F5, N90s/F90x). This means the shutter blades of the shutter must be completely out of the way when the light strikes the film or there will be a shadow on the film. Currently with Nikon's focal plane shutters, a top shutter speed of 1/250 is available to sync with the flash. If a shutter speed of 1/500 or faster is used, a dark ban will be on the final photograph (the shadow of a shutter curtain). So why can't the shutter control flash exposure, it's simply not fast enough to freeze that burst of light!
Can TTL Metering be Fooled?
You bet, even with the new flash matrix technology! The mechanics of how the new matrix technology works will be covered later. But be aware that the camera's meters are still looking for 18% gray even in flash exposure. White or black subjects can throw the metering off and cause bad exposures. Size of the subject and placement in the frame can likewise cause exposure problems. The drawbacks with each flash unit and camera body when it comes to exposure will be covered under the appropriate sections. Just be aware that you, the photographer, still must be in charge and that all this technology is still only a tool and not the final word on exposure perfection!
There's a lot of material in this page on the theory of flash. Being successful in using flash means no one can see that flash was used in a photograph. This requires understanding the nuances of flash and ambient light exposure. There's a lot more to learn in using the Nikon SB-24/ 25 / 26 / 28 that will aid you in this quest. There are a number of special effects that with knowledge of flash, can be programmed into the camera's and flash's computers that can take dramatic photographs without all the calculations. But to arrive at that point, understanding all the points made here is imperative!
Getting past theory and into using Nikon's TTL flash system is an easy step. Nikon's flagship flashes are the SB-24 (discontinued but owned by thousands) the SB-25 and current SB-26. These are TTL units with standard ISO shoes which fit the F4, N90 and N8008 series cameras. Using the two flash units on the respective bodies in practice is not that different, so the following operation information is generalized for all models.
The SB-24/25/26 can do all, part, or none of the flash exposure calculations for the photographer, depending on how it's programmed. These flashes can also fill, mix or overpower (with limitations) the ambient light. The LCD panel on the back of the camera is where all settings are programmed to achieve this control. It's where we'll start to get a handle on the technological side of flash work.
The SB-24/25/26 are able to sync with the camera's shutter in two ways: "NORMAL" and "REAR". In "NORMAL" sync mode (selected by simply sliding the switch to the Normal position) the flash is fired the exact moment the last shutter blade of the first set opens the shutter. In this mode, the flash records the action the instant the shutter opens because it's firing at that exact moment (remember the duration of the flash is faster than the shutter). When the F4/N90/N8008 are in Aperture priority and the flash is set to "NORMAL," the only available shutter sync speeds are 1/60-1/250 (these are stepless speeds though). The N90/N90s and F5, this speed goes down to 1/20.
If the ambient light requires a shutter speed slower than 1/20, the camera underexposes the ambient light. In this mode, the camera is unable to go to 1/19 or slower shutter speed to correctly expose the ambient light. When working in dimly lit situations which require such low shutter speeds, switch the camera to manual mode (taking it out of aperture priority) and check the ambient exposure. If a slower shutter speed is required to properly expose the ambient light, manually dial in the shutter speed and shoot (stepless shutter speeds are lost when in manual mode). Or, switch the sync on the back of the flash to "REAR".
In "REAR" sync mode, the flash fires the moment the first shutter blade starts closing the shutter after the ambient exposure is made (opposite of "NORMAL"). With the flash in this mode and the camera in Aperture priority, the camera can sync using stepless shutter speeds ranging from 30 seconds to 1/250. This is a tremendous range of exposure flexibility with TTL control!
Rear Curtain sync is a great feature, but it's not where the flash should be permanently set. There are a number of reasons for this. Mainly, the photographer needs to be in control of what the flash is doing and in this mode, it's easy to become complacent. There are many instances when a 30 second exposure wouldn't work (such as nocturnal photography). There are times when the flash should be the key light and the shutter speed needs to be regulated. There will also be times when the fastest shutter speed possible is needed. For these and many more reasons, the flash should not be permanently set to "REAR" sync.
Understand these two modes and use them when the special application works the best for a given situation. For the majority of applications, the flash should be left in the "NORMAL" position. Switch it to "REAR" in those instances where it's required, then switch it back to "NORMAL" when done. And whenever possible, take advantage of stepless shutter speeds by staying in Aperture priority.
The flash isn't always used for fill-flash. For example, for night shooting when the flash is the only light, set it on "NORMAL". Then set the camera on manual mode and set the shutter speed at 1/250. Since there is no ambient light to be properly exposing, use the fastest shutter speed possible. It speeds up the process, the motordrive goes faster, the shutter goes faster and everything else moves quicker. This gives that slight edge needed to get the photograph. It also insures the sharpest image possible when hand holding.
An obvious but none the less important switch is the Flash Mode Selector. There are four modes of flash operation with the SB-24/25: "A", "M", "Lightning Bolt" and "TTL". The "A" makes use of the built-in flash sensor (not to be confused with TTL) for exposure. The "M" throws the photographer back to the days of manual flash, calculating flash-to-subject distance for proper exposure. The "Lightning Bolt" provides repeating flash operation, a strobographic effect which is more fun than practical. The "TTL" is where the flash should be set, taking advantage of its full potential.
Be aware of the distance scale on the LCD panel whether in "REAR", "NORMAL" and "TTL" mode. When a lens with a CPU is in use, the distance scale and f/stop are displayed on the panel (this is true only when the flash head is set at the ninety degree position). The distance scale shows how close or far the subject can be from the flash with the f/stop selected (which is displayed). This is important information because beyond these distances, accurate TTL exposure cannot be made.
The SB-24/25/26 can be set for "Automatic Balanced Fill-Flash" and "Standard TTL" flash exposure. With Automatic Balanced Fill-Flash, the camera and flash computer are in complete control of fill-flash exposure. These will vary the flash exposure to equal the ambient light to underexposing the flash by minus 1 1/3 stop. The photographer never knows the amount of fill-flash used until the exposed film comes back from the lab and then it's too late to see if the camera's meter and computer figured correctly. With Standard TTL, the photographer is in complete control of the fill-flash, the way it should be.
Photographers want to be in control of the Exposure. Accomplishing this requires programming information into the SB-24/25/26's LCD panel. This can be a little tricky, so I want to go through the steps to clarify any holes in the instruction book.
To program Standard TTL on the SB-24, press the "M" button (not to be confused with "M" mode next to the "TTL") causing the man/sun symbol to blink. On the SB-25/26, press the "M" button to make the man/sun symbol disappear. The flashes are now set to Standard TTL. The flash will now automatically expose the fill-flash to equal the exposure of the ambient light. In many instances, this is what's needed (because of the subject's absorption of light), but more often the flash is only needed to fill in the dead zones giving them more life. A little more programming is required to accomplish this.
With both units, press the "SEL" button to call up an analog scale which is where the amount of fill-flash is programmed. On the SB-24, press the up arrow to program minus compensation. On the SB-25/26, press the down arrow to program minus compensation. On both units, once compensation is dialed in, press the "SEL" to register the compensation or press nothing and after ten seconds, the compensation is automatically registered. Inside the camera's viewfinder, the compensation warning will light when flash compensation has been programmed. The camera/flash computers will deliver the programmed amount every time the flash is fired.
This brings us to the question of how much fill-flash is the correct amount? Generally, I have my flash units programmed with minus 2/3 stop compensation. This is an amount that will clean up shadows and colors while not overpowering the quantity or quality of the ambient light. Keep in mind that dark subjects suck up light and light subjects bounce light back.
If there's a doubt concerning just how the flash affects a subject, do a test. Apply the exposure compensation test to flash. Set up the teddy bears and shoot a test in various lighting conditions varying the flash compensation. Remember that we'll all want a different amount of compensation according to individual personal taste in photographic exposure. The only way to find out that amount without the loss of good images is to test.
Ever seen a photograph of wildlife you know are out during the day but the photograph looks like it was taken at night? How about those images of wildlife taken during the day but the light is coming from different directions? My favorites are the over lit flash photos or those with five catch lights in the eye.
These are all mistakes in using flash that can be avoided and corrected. The black background comes from incorrectly exposing for the ambient light. The multi directional light comes from misunderstanding ambient light. Over lit derives from a lack of control of the flash exposure and multi catch lights are simply evidence of misplaced flash units. Provided throughout this book are the techniques to avoid these pitfalls.
SB-28 The really new thing about the SB-28 is its look. Besides being smaller, trimmer, it has an entirely new back panel, LCD display. The buttons are rubber coated and recessed on the SB-28, there are no longer any sliding MODE switches or the like. Even the On/Off switch is now a button so you can no longer accidentally change a function or turn on the flash. In fact, you must depress and hold down the On/Off button for a few seconds in order for the flash to come on. And the SB-28 will automatically go into Standby mode after approximate 80 seconds.
There are five basic modes possible with the SB-28 depending on the camera in use. There are no real surprises here, the instruction book pretty well lays things out. If you want to shoot in "standard" mode, you simply press the MODE button until just the TTL message is displayed in the top, left corner of the LCD. This puts you in control of the flash fill compensation as previously described.
There might be two "drawbacks" to the SB-28 depending on how you look at it. The first is it does not have Red-Eye reduction capability with the F5. As this is a feature I have never personally used, I don't find this a drawback. The second drawback and more important to my own photography, is the smaller and dimmer AF illuminator. I don't have a way of measuring how much dimmer it is, but it's enough that I use the SB-25 for nocturnal shooting as my main flash, to take advantage of its AF illuminator.
The unique capabilities of TTL technology can be taken one step further. With a firm grasp of flash compensation, use it in conjunction with the camera's compensation. In situations where the flash is to be the key light and the background underexposed, the camera and flash computers can do it all, thus avoiding lengthy exposure computations.
In our example, we'll assume we want the background (ambient light) underexposed by one full stop. The camera is set to Aperture priority, the metering set on Matrix. Dial in minus one stop via the exposure compensation on the body. This underexposes the ambient light by one stop; it also underexposes the flash by one stop. No problem because we'll go to the flash and program it to overexpose by one stop. Now, the flash is the key light with no compensation.
If the subject is light colored and we want the flash to expose it to come out white, dial in no compensation or up to plus one stop. If it's a dark subject, program in more than one stop plus compensation. Remember though that ambient light is underexposed and the flash is the key light, don't make a mistake and underexpose the flash thinking its fill-flash. This is an excellent technique for making a distracting background disappear.
It's often difficult to judge the direction flash illumination will travel and its effect. When is the right time to take the flash off the camera? Previsualize by converting the flash to a flashlight. Point the imaginary flashlight at the subject and visualize the effect it will have. Where will the light strike the subject? Does it create a shadow and if so, where does the shadow fall around the subject? If there is a shadow on the subject that we want to make disappear using a flash, where should the flash be placed? Imagining the flash as a flashlight will help assess how the flash will effect the photograph.
Keep in mind that we want to imitate the sun and its effect when using flash. The key word here is imitate, not replace. The sun comes from above us, always. This means that shadows should always fall below subjects and not above them. So when using flash, shadows must appear below the subject and not above. There is also just one sun so there should only be one highlight reflected in the eye. With proper technique, multiple flash can be used while maintaining just one highlight.
The SB-24/25 can be taken off camera while maintaining full TTL connections via the SC-17 cord. This permits the flash to be placed anywhere around the camera up to three feet away. This provides all of the features of TTL flash and allows complete control of the flash's light. Up to three SC-17 cords can be connected together creating a nine foot off camera cord. More than this creates too much resistance in the cord for all the TTL information to quickly communicate between flash and camera computers.
When attaching the SC-17, it's recommended that it be attached to the camera body first, then the flash. The foot on the SC-17 where the flash is attached must have a flash attached to complete the cord's circuit. Plugging an SC-18/19 cord into the foot of the SC-17 for a slaved flash without a "Master" flash attached to the foot will not work. The SC-17 has a 1/4-20 threaded socket on its base to allow attachment of the unit to a bracket or stand.
Nikon's flash system is capable of controlling five TTL flash units. These flash units are linked together via the SC-17, SC-18 or SC-19 cords and the AS-10. The SC-18 cord is five feet long, the SC-19 is ten feet long. The AS-10 is required on all slaved units to complete the flash circuits (it attaches to the flash foot). The AS-10 also provides a 1/4-20 thread enabling the flash to be attached to a stand or bracket. The SC-17 foot and AS-10 provide TTL sockets for the SC-18/19. Use the sockets on the side of the flash rather than these.
Since the SC-18 and SC-19 are only three lead cords (five leads are required for full computer communication), as many as three SC-19s or six SC-18s (a total of 30 feet of extension in either case) can be coupled together for TTL operation. These cords are connected together via the AS-10 which doesn't require a flash to complete the circuit. The SC-18 can be trimmed down (not recommended by Nikon) to accommodate any special brackets or applications thereby eliminating any excess cord from being in the way.
The SB-24/25/26 have two connections on their sides. One is a standard PC connection and the other is a TTL connection where the SC-18 and SC-19 cords are plugged in. When a TTL flash is used as a second light (slaved unit) via an SC-18 or SC-19 cord, all programmable functions on the second light are governed by the master flash connected to the camera's hot shoe (whether on camera or attached via the SC-17 cord). The flash head Zoom function does work on the slaved units and should be manually set to match the master flash.
The SB-24/25/26 power switch has two modes, "STBY" and "ON". Standby ("STBY") saves on battery power and is a great feature for wildlife photographers. In general applications the flash is used for a period of time and then turned off. When the flash is needed over a long period but only sporadically fired, "STBY" is an excellent option. In this mode, the flash turns itself off after 60 seconds and comes on immediately when the shutter release button is pressed. (All slaved units must be set to "ON" and not "STBY" because they do not receive the impulse from the shutter release when pressed.)
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