Shutter speed does Not affect flash exposure. The previous page discussed that flash is faster than shutter speed, therefore shutter speed only affects the continuous ambient light, room light or daylight, but not flash exposure. Which makes shutter speed be an optional control for flash pictures, affecting how ambient is mixed in, or not (More at Part 4).
Power differences
Why continuous bulbs are woefully underpowered for photography, at least for low ISO, stopped down aperture and fast shutters.
Sunlight is intensely strong, but continuous lamps indoors are relatively weak. Flash is typically strong, and stronger at close range. It may be a struggle, but flash can match the sun for fill in bright sunlight (if close enough, within several feet). Moving picture video does require continuous light, but still pictures can use flash.
Exposure of continuous light depends on open shutter duration. Fast shutter speeds drastically reduce the exposure. Time is a factor. And stopped down aperture and low ISO limit the available light.
Watt Seconds
There is a distinction between power and energy. Watts of power is a rate of energy consumption (named in honor of James Watt, 1736-1819, Scottish inventor of the steam engine). Continuous lights consume watts of power all of the time they are on. The total energy is the accumulation used, which depends on how long they are on.
Watts is volts × amps of current.
Watt Seconds is simply watts × seconds.
Continuous lights are measured as using watts of power, but flash units are measured as watt seconds of energy.
Our North American electric bill pays for kilowatt hours (KWH) of energy, which is larger units for the same idea, kilowatts instead of watts, and hours instead of seconds. One watt is the power of one volt at one amp of current. One watt is the same as one joule per second, and one watt second is one joule of energy.
So watts is a rate of use of power, and a 100 watt light bulb continually consumes 100 watts (power), regardless if it is turned on for one second, or all day. Our electric bill pays for the total energy consumed. Power is a rate of use, but its accumulation over time is the energy consumed. Energy = power rate x time duration used. So, the units of electrical energy are:
Watts x seconds = watt seconds of energy (one watt second is same as one joule of energy).
There is an exception that can modify actual watt seconds. An inductive load (due to transformers, electric motors, fluorescent light ballasts, etc) cause the AC current and voltage to be out of phase, causing power inefficiency. So actual watt seconds is (volts × current amps × power factor). Out of phase power units are often measured as using VA (volt x amps), which we pay for, but which is not actual watts if out of phase due to power factor. CFL bulbs are often about power factor 1.5 or 1.6. Purely resistive loads are power factor 1. A capacitive load causes an opposite phase change, so capacitors can be added (often seen on neighborhood power poles) to neutralize the power factor.
Because flash delivers a more instantaneous energy, the concept of any continuous "rate" (such as watts) is not meaningful. Studio flash units are rated in watt seconds, which is the total accumulation of the input electrical energy in that brief flash. But it is not a perfect process, there is also an efficiency of creating light involved. All things vary, types and sizes vary, but a fluorescent bulb might be around 3 or 4 times more efficient making light (more lumens per watt) than an incandescent bulb (which tends to make heat instead). And flash units have about the same efficiency as fluorescent (both are ionized gas instead of a heated glowing filament). But the huge difference is that the flash capacitor accumulates stored charge during a few previous seconds, which can then all be instantaneously released (very bright). Then it has to recycle (recharge). The continuous light just keeps on burning steadily.
This input electrical energy aspect is computed in watt seconds, like this:
The "power" of a flash (actually it is energy) is rated in watt seconds (not watts). Studio flash are rated as watt seconds, which is watts times seconds. Speedlights use Guide Number, but a regular size fully powered speedlight will typically be near about 75 watt seconds. Smaller units a bit less, and studio lights are usually much larger. For flash, watt seconds is computed as 1/2 CV², where C is flash capacitor capacitance, in farads, and V is the charge voltage of the capacitor. Watt seconds is the electrical input energy (not the light output energy). A Nikon SB-800 flash uses a 1400 µfd capacitor charged to 325 volts, which computes 73.9 watt seconds of energy. And if comparable reflectors are used (same angular coverage), it also compares that way to a studio flash. However, the speedlight flash duration is very short, for example (crude estimations here), speed light half power level is typically about 1/1000 second duration (and about 1/10,000 second at 1/16 power level). So then 37 watts in 1/1000 second would be 37,000 watts for 1/1000 second = 37 watt seconds. The speedlight can offer both power and speed.
There is no equivalence or conversion of watt seconds of energy and Guide Number. Guide Number indicates the actual light output, the brightness of the exposure (at specified distance due to Inverse Square Law), but which depends on the reflector and the concentrated angle of coverage. Watt seconds is Not a measure of the light output, it only measures the electrical energy input to the flash tube. Then efficiency at making light is a variable affecting output. Incandescent bulbs are very low efficiency. Flash and CFL and LED bulbs are relatively higher efficiency.
Continuous light... might run all day, but the light can only be useful to camera exposure for the (typically a small fraction of a second) duration while the shutter is open. Flash is only present for a few milliseconds, normally significantly less than the shutter open time (shutter must just be open to pass the flash, which is typically called flash sync).
If a one second shutter, then 150 watts x 1 second = 150 watt seconds of electrical energy, quite bright.
If a 1/200 second shutter, then 150 watts x 1/200 second = 0.75 watt seconds, hardly even dim. Multiple bulbs and high ISO will help.
The efficiency of incandescent bulbs making light is pretty low, 2 or 3%. The rest is heat. Maybe 15 lumens per watt.
Let's say a 45 watt CFL bulb is said to be equivalent to 150 watts of incandescent (fluorescent is about 3 or 4 times more efficient than incandescent.
If a one second shutter, then 45 watts x 1 second is 45 watt seconds.
If a 1/200 second shutter, then 45 watts x 1/200 second is 0.22 watt seconds. But it does make light more efficiently, and it should up towards 2 EV brighter than the incandescent, and that much more exposure. The photos below show only 2/3 EV more, however that small reflector was quite insufficient for the much larger CFL bulb. It is an inch taller than the one shown in the reflector. Again multiple bulbs and higher ISO will help.
CFL is higher efficiency than incandescent, producing more light with less power, and less heat which becomes important in the studio. Perhaps 60 lumens per watt.
1. My first large 45 watt CFL bulb was no-name, no brand mentioned on bulb or packaging. And its power was marked falsely. It claims 45 watts. And using a $30 Kill A Watt meter (and there are cheaper ones too). It does measure 44 VA, but VA is NOT the same thing as watts. The measured VA and 1.6 power factor measures only 26 watts of effective power. So this one is falsely marked, it should be marked 26 watts. I assume fraud over ignorance — because if they know how to make CFL bulbs, they know better. It was my bad choice, I shouldn't be so cheap. 😊
This is a 45 watt no name CFL bulb from China (the only larger CFL I had). I know all CFL will necessarily have a significant power factor, but I checked three other branded 13 watt CFL bulbs, and all three brands of them marked power correctly — they all measured 13 watts and 21 VA, and the same 0.6 PF, but which is honestly marked. I assumed most are honest. But I don't know now.
2. So needing a redo here, I bought a Fovitec Studio Pro 85 watt CFL (I went cheap again, but it's a top choice at Amazon. Amazon says vendor is Fovitec, but the bulb and packaging have no names on them. It is marked 85 watts, however it measures only 51 watts, 88 VA, and 0.58 PF. Again mislabeled falsely, it obviously is not 85 watts if it only uses 51 watts. The bulb is large, but it is not 85 watts.
Very puzzling why this is allowed? USA law requires the packaging labels of light bulbs (those with the common medium screw base) to show lumens and watts. But these were a plain box, did not mention lumens, and the watts were false. So I'm thinking we should choose a choice that at least mentions lumens. If they have anything to say, they will say it. My next Amazon search for CFL will instead be for "CFL lumens". And a high CRI would also be needed for photography (so it also should be mentioned). Incandescent bulbs are inherently CRI 100 (Color Rendering Index, see Spectrum, towards bottom below).
3. And hurrah! Searches with mention of lumens does seem a good thing. I ordered a third that way, and that Alzo 45 watt CFL is labeled 45 watts, 2800 lumens, 5500K, and it does in fact measure 47 watts and 88 VA. It is a larger bulb than the first 45W (that doesn't), 70 mm diameter vs. 55 mm, and one inch taller. This Alzo 45 watt CFL claims 2800 lumens and equivalent to 150 watt incandescent. A 150 watt incandescent claims 2640 lumens. I cannot measure lumens, but I'm happy with the watts now. The Alzo also claims CRI 91, relatively high for CFL.
Our regular full size camera hot shoe speedlights (are typically about 75 watt seconds energy for a full size hot shoe speedlight) will run circles around continuous lights for photography. Maybe the flash charges at a 25 watt rate for 3 seconds, and then can discharge at a 75,000 watt rate in 1/1000 second (both numbers are 75 watt seconds). At any usable shutter speed (up to maximum shutter sync speed, typically about 1/200 second), exposures like f/8 ISO 100, or f/11 ISO 200, are possible for portraits with speedlights in close white umbrellas (full power level with main light's reflected fabric at four feet). Battery recycle time will be slow at full power level however (a couple of seconds before next flash), to replace the energy after each shot. The heat can cause damage if fired too fast without cooling time.
A Capability Test of a 150 watt incandescent bulb, a 45 watt CFL bulb, and a speedlight
10 inch clamp-on reflector, which seems way too small for the CFL bulb. Mixed lighting, this white balance set for indoor Incandescent, making the Daylight in the window be blue. A compromise WB would simply make Both be wrong.
Electric power was measured with a Kill-A-Watt meter (measures volts, watts, VA, and PF). The exposures were metered with the Sekonic light meter and verified with the camera pictures (white balance and crop were adjusted).
All lights were placed on the same unmoved stand at the same 8.33 foot distance (2.54 meters).
Fake flowers, camera Nikon D800, 105mm lens, ISO 100.
Flashes were full Manual power.
150 watt incandescent bulb
158 watts, 1.0 PF
1 sec, f/7.1, EV 5.67
White Bal 2900K -1 tint
Nikon SB-800 speedlight
Direct flash, full power
1/250 sec, f/11, EV 15
White Bal 5900K +1 tint
Yongnuo YN565 speedlight
Direct flash, full power
1/250 sec, f/11, EV 15
White Bal 6450K -10 tint
Alzo 45 watt CFL bulb
47 watts, 88 VA, 0.54 PF
1 sec, f/5.6, EV 5.0
White Bal 4850K +13 tint
No-name 45 watt CFL bulb
26 watts, 44 VA, 0.6 PF
1 sec, f/5, EV 4.67
White Bal 2900K -3 tint
This No-name CFL (#4) claims 45 watts, and equivalence to 225 watts incandescent, which are both false claims. This one does measure 44 VA, but only 26 watts (this false rating ignores the big difference). Instead of being equivalent, this no-name CFL metered a stop less than the 150 watt incandescent. That's about 50%, or equivalent to 75 watts incandescent (however, it is fair to say the CFL really needs a better deeper reflector than I provided). It claims 5500K Daylight, but I see it as 2900K Incandescent. The false rating of this 26 watt CFL is not equivalent.
The Alzo CFL (#3) does accurately consume its rated 45 watts, and is roughly equivalent light to the 150 watt incandescent (Alzo claims 2800 lumens, the Sylvania incandescent claims 2640 lumens). However here, it meters 2/3 stop under the incandescent, which is my fault, because this little reflector is pitifully small for the even larger Alzo bulb, and can't be a fair comparison (the smaller CFL bulb is shown in the reflector). The incandescent sits deep in the reflector, but the Alzo CFL extends out of it, within an inch of the protective wires.
When directly metering all three bulbs themselves, individually at 6 feet, side view in a bare socket (no reflector, to be an equal situation), and that view shows 150W incandescent 5.7 EV, 45W Alzo 5.6 EV, and No Name 45W CFL 5.0 EV. That would be the metered exposure of a subject located at the meters location.
The flash (#2 and #2b) is certainly worth attention. The camera settings were EV 15 (f/11 at 1/250 second, ISO 100), which is a direct bright Sunlight exposure, but the flash exposure is independent of shutter speed, it could have even used one second too, with this same flash result. So that may be apples and oranges about flash exposure, but had it been a portrait, this 1/250 second shutter speed obviously was possible, which is sometimes rather important to us. The continuous bulbs obviously required the one second shutter. The camera settings (not exposure) used here for flash were in fact 9.3 EV greater (a huge number, 29.333 is 645 times more light) than the 150 watt incandescent bulb could provide. The effect that (low ISO 100, stopped down f/11 and fast 1/250 second) has is to keep room ambient light out (very desirable in planned portrait lighting setups). The D800 camera can sync flash at 1/250 second, however 1/200 second is the maximum number for many cameras, which is only 1/3 stop difference, and still 512 times further down than the little flash.
The incandescent bulb (#1) was used at 150 watts x 1 second = 150 watt seconds, but being about 1/4 efficiency of the flash, compares at about 150ws/4 = 37 watt seconds, or about half of this speedlight 75 watt seconds. And with the one second exposure, the incandescent aperture is in fact about 1.3 EV less light, or near half the power of a speedlight (only if at one second, which is 8 EV less than 1/250). However, shutter speed is a big deal for us too. The flash can easily use 1/250 second, and the light bulb cannot. This often matters. I'm trying to say that flash is rather a big deal, and it stops motion too, quite advantageous for portraits indoors.
But the 150 watt incandescent light at one second can compete with the speedlight if a slow shutter speed is acceptable. So for pictures of still life that does not move, the continuous light can be extremely usable. Possibly even arguably better for still life, because we can see continuous light (can see to adjust it on our subject), and the camera can meter continuous light. Neither is true of flash, so we have to learn different procedures for flash. But it is not hard at all, very many do use flash, and the ample light is well worth it. You will want flash for portraits of humans. An Equivalent Exposure of the incandescent IS0 100 one second f/7.1 would be ISO 800, 1/50 second f/2.8, which is about where we hopefully end up working with continuous lights. Without flash, it's a hard fight to win well.
ISO helps both flash and continuous equally. We can turn the flash power down, but the longer 8 foot distance for comparison was so the speedlight flash would not overwhelm the exposure. And f/11 at ISO 100 is a substantial requirement (the other lights need a couple of seconds exposure to touch it). Doubling the number of bulbs or watts would create one more stop of exposure, as would doubling the number of flashes or watt seconds (double is one stop).
This was not about lighting, it was about comparing the intensity of the types of lights. But the smaller 2 inch wide flash reflector does cast a harder shadow than the larger 10 inch incandescent reflector. And the larger CFL bulb being not as deep into this reflector allowed more stray room spill to scatter. See soft light, an umbrella can really make all the difference. For White Balance, here I merely clicked on white things in the scene, specifically the white paper numbered label here, which is cheap copy paper. White Balance is relative to Daylight WB about 5000K, a lower Temperature K is yellow, a higher Temperature K is blue. A negative Tint is green, a positive Tint is magenta. FWIW, the first picture of reflector shows blue daylight outside the windows because the white balance is for an incandescent overhead ceiling light used indoors (kinda cool though).
If shopping for studio flash, be aware that ISO is important too. You should be aware that ISO 100 with 320 watt seconds will give the same exposure combinations that ISO 200 users see with 160 watt seconds. Both cases will have to be turned down to shoot close portraits at f/8, and are likely between 1/8 and 1/4 power level. Or full power on both should allow f/8 for groups with white reflected umbrellas at ten feet. That seems plenty of power, and excessive power can be an issue in the living room, because we can only turn a big flash down so low. What needs more power is greater distances (school gym maybe, or fill flash from behind the camera is more distant too), or small apertures like f/22, or trying to overpower the sun outdoors.
See possibly useful info about the hardware needed to use your speedlights in umbrellas.
Metering capability
Our camera's light meter easily meters continuous light, but it does not meter flash. We must have a flash meter to meter flash. The exception is TTL flash, which the camera meter can meter. Even then, the automatic exposure settings that the camera selects are only about the continuous ambient light. The camera meter reading is NOT about the TTL flash at all (flash has its own metering system, invisible to us). Camera exposure settings are done by camera automation according to only the ambient continuous light. Or we may set the camera ourself manually. Then the TTL flash system necessarily uses that preexisting aperture setting to meter the TTL preflash, and it sets flash power level accordingly for it.
The flash system does not change the camera settings, it only changes TTL power level. However, there are two exceptions, not related to exposure (the ifs and buts can get confusing). In camera S or M mode, we can manually select any shutter speed, regardless if the flash can sync or not. But in camera A or P mode, the camera automation enforces a Maximum shutter sync speed with flash, so that flash can work. And in A or P modes, the camera automation normally enforces a Minimum shutter speed with flash, so in dim places, we normally always see 1/60 second instead of a too-slow shutter like perhaps 1/4 second shutter (which the dim ambient actually meters, but Slow Sync or Rear Curtain Sync are exceptions allowing it). But if we are using flash, the dim ambient is probably unimportant to us, it's why we use flash. Shutter speed has no effect on the flash exposure, but it does affect the continuous ambient exposure. Part 4 here has more about controlling the ambient light existing in a flash picture.
No matter what aperture you use, the automatic TTL flash system will try to automatically provide the right flash power level for it, if possible — if the flash has ability to provide that much power. Therefore, regardless of aperture, we always get the same TTL flash exposure (in that one metered situation). And shutter speed has no effect on flash exposure. So about any and all camera settings will give the same TTL flash exposure. Therefore, two big things to know:
- Flash Compensation is the way that we can control or adjust what the TTL flash automation is doing.
- Shutter speed is the control to adjust ambient light level relative to the flash.
This stuff is a BIG deal, this is the tools we have to use. Aperture and ISO does affect the amount of flash power needed, so aperture and ISO have great affect on Manual flash, but TTL flash tries to deal with readjusting flash exposure automatically. Saying, we can change aperture and ISO (and shutter speed), but TTL flash will still meter the appropriate power level, so the flash exposure stays the same. Flash Compensation is how we adjust TTL flash results.
The camera systems with the fancy TTL wireless remote flash features can operate a couple of their own flash units in an automatic way. For example, Nikon's CLS Commander/Remote flash system is quite awesome, a point & shoot remote wireless multiple flash system, which instantly and automatically does the equivalent of several minutes of manual setup to equalize the two lights at the subject. It has pros and cons, here is a discussion of that. We can also specify the lighting ratio between main and fill by simply specifying it, and the Commander does it. It has many fans, but automation always gives up some control, and there are associated downsides, like the preflashing that makes our subjects blink in the picture (but there are solutions).
But for manual flash, we can tweak flash power level by trial and error; or a good solution is a handheld flash meter, like Sekonic. In a studio situation with multiple lights which work in manual flash mode, we meter each flash individually to manually set its power level so that it does in fact meter what we want it to meter. This is big plus, a huge advantage called "control". We can set them exactly like we want them. For example, maybe we meter the main light (alone) at the subject to give f/8. We meter the fill light (alone) at the subject to give our lighting ratio, maybe to be one stop less if desired, or to meter f/5.6. We meter the background light at the background to give f/8, or whatever effect we want there. We meter the hair light at the subjects head, to give maybe f/11 for black hair (more light) or f/5.6 for light hair (less light), whatever we know we want. Then we also meter the main and fill light together to get the lens aperture setting (both together will be a fraction of one stop more than the main light, depending on lighting ratio). This is full control, consistent and repeatable, which we can easily do again when we setup next time.
The incident flash meter for manual flash has another advantage: The automatic camera TTL meter necessarily uses reflected light, which is dependent on the light reflected from the bright or dark color of the subject. For example, we get different readings if the subject's dress is black or white (see metering). But the incident manual flash meter is pointed at the camera instead of at the subject. It measures the actual light intensity itself (at the subjects position), which is totally independent of the subject, and frankly, is pretty awesome. Any subject will come out about right then.
Shutter Speed Sync differences
We can use any shutter speed with continuous light, like sunlight. Faster shutter speed does limit the amount of light seen, but we simply open the aperture for any other equivalent exposure.
But flash is different. Shutter speed may not affect flash exposure, but our camera has a maximum shutter sync speed for flash, in the ballpark of about 1/200 second for the focal plane shutters used on most DSLR. The shutter must be fully open when the flash fires, to expose the entire area of the photo frame in that instant. At faster shutter speeds, the focal plane shutter is never fully open all at once, it is only a narrow open slit moving across the frame. This means faster shutter speeds cannot be used for flash, or else we would get a dark unexposed band in our picture, where the total frame area was not open. The fastest shutter speed when the shutter is in fact 100% open all at one time to allow flash to pass through is the definition of the "Maximum shutter sync speed" (found in camera specs). It is a hard limit for speedlights, but see Auto FP HSS flash mode which some flash and cameras offer.
Spectrum differences of CFL
Even if the lamp is called "Daylight", no fluorescent has a continuous color spectrum, which can make matching White Balance be a special problem; an exact match with every color is never possible. Fluorescent lamps have a CRI rating (Color Rendering Index), which is how well fluorescent is able to render colors. See the spectrum test at top of this CRI link, shown by single slit diffraction — discrete lines vs. continuous spectrum. CRI 90 is possible today for best tri-phosphor types (but not common). Basic fluorescent bulbs may be CRI 50. When choosing new bulbs, a high rating (CRI 80+) has better color — not perfect, but better, often nearly acceptable, but a lower CRI is poor for color photography (and also poor in the closet where your wife selects her clothing — she wants high CRI too, CRI 80+). Perhaps most colors look OK, but the colors not in its spectrum won't look right. But fluorescent will not be CRI 100%. And LED is likely worse.
In contrast, incandescent lamps do have a continuous spectrum and they are the definition of theoretical maximum CRI 100. Incandescent may be orange, but it can easily be matched, the entire spectrum is present (from tungsten filaments). Direct sunshine is continuous spectrum too, and flash virtually so, but fluorescent and LED lights simply are not continuous spectrum (they add phosphorus coatings which help it).
Flicker of older Fluorescent lighting fixtures with magnetic ballast This is speaking of older fluorescent light fixtures specifically using magnetic ballast. It is NOT speaking of CFL bulbs, nor of newer lighting fixtures using electronic ballast (these don't flicker).
Fluorescent lights are continuously on, except they are powered with AC. Using the old magnetic ballast, they flicker with the AC line frequency — twice per cycle of 60 Hz in North America (120 times a second), 50 Hz in many other places (100 times a second). The 60 Hz AC voltage swings plus and minus, passing through zero 120 times a second. This is a big problem for shutter speed duration, which can randomly sample partial cycles and see varying color from shot to shot (often this problem randomly sees a strong brown cast). I am speaking 60 Hz, but the solution is in setting the shutter speed to match the AC line frequency, either to 1/60 second, or double at 1/120 second (in North America, or 1/50 or 1/100 second in 50 Hz locations) helps to see only complete cycles for more stable color. Slower even fractions of 1/60 second work OK (like 1/30, 1/20, 1/15, 1/10 second, capturing a few complete cycles), but faster shutters, like 1/80 or 1/90 or 1/400 second, would be a bad match, randomly capturing incomplete cycles of flicker.
However, modern electronic ballast (available maybe 20 years) has changed this situation greatly today, which converts fluorescent operation to about 20,000 Hz for no flicker (in newer light systems). CFL bulbs, and desk lamps too, use the electronic ballast, inside the bulb base. And many new ceiling light installations too, but the old magnetic ballast is still available to buy. I recently replaced an ordinary two 40-watt tube fixture, and the 25 year old ballast was magnetic, and the new ballast was electronic (and much smaller).
You can determine if your fluorescents flicker, this way: For this, do NOT use Auto WB (Auto WB changes what was real). Shutter speeds of 1/60 or 1/120 second should match the 60 Hz cycles, but your test is seeking flicker, so any intentional faster shutter would be good, or rather bad — it captures incomplete cycles of the flicker. 1/250 second at 60 Hz captures 120/250 = 48% of a cycle, incomplete cycle for old magnetic ballast, incomplete color, which can vary randomly from shot to shot. Or 1/200 second for 50 Hz lights captures 50% of a cycle. But in newer electronic ballast, 1/200 second captures many complete rapid cycles, for complete color. You can aim the camera directly at the lamp for this test, for brighter light to achieve faster shutter. Take several, 5 or 6 all the same, without Auto WB to mess it up. If color results vary dramatically (a few are randomly brown), that's flicker. If all of several tries are the same color, then no random flicker. If all are the same, but color is off, that's just White Balance, not flicker.
But regarding White Balance... Even if the lamp is called "Daylight", no fluorescent has a continuous See the preceding Spectrum section. When choosing new bulbs, a high rating (CRI 80+) has better color — not perfect, but better, often nearly acceptable. Direct sunshine and incandescent bulbs are continuous too, but fluorescent and LED lights simply are not continuous spectrum.
Incandescent bulbs heat a filament which does not flicker at AC frequencies. However, Sodium vapor and Mercury vapor lamps have similar flicker problems, and much lower CRI ratings than fluorescent bulbs. Saying, you will probably see this flicker effect shooting under football stadium lights too.
Differences Stopping Motion
Continuous light is continuous, which cannot stop motion at all. Flash is a nearly instantaneous burst, illuminating the subject for only a brief instant, faster than the shutter speed, so the flash can freeze the subject movement. This assumes the flash intensity is significantly stronger than the ambient continuous light (indoors for example). If both sources are significantly present, the continuous ambient light can continue showing blur that the flash alone could have stopped. The ambient needs to be underexposed a couple of stops.
A speedlight is called a speedlight because the way it reduces power level is to cut its flash duration short, which reduces power, and also effectively stops motion (when the only light lasts only a very short duration. A speedlight at maximum full power level might be 1/350 second duration (t.1), pretty fast itself. But 1/4 power level will be like 1/2500 second duration, and lowest 1/128 power level could be like 1/40,000 second duration (Nikon specs are in the spec chart in rear of flash manual). Lowest power level might only allow a range of about one foot at ISO 400 f/16, but still very effective for water drop splashes, bursting water balloons, etc .This is how high speed flash is done, and it is amazingly effective.
Indoor vs. Daylight Shutter Speed differences
Indoors, the continuous ambient light is usually dim and insignificant (the reason we need flash). Shutter sync speed will be limited to ballpark of 1/200 second, but this is rarely any concern indoors, since the flash duration is faster, especially speedlights. We may set the Manual shutter speed slow to emphasize the ambient a little (camera A mode likely uses 1/60 second minimum with flash indoors), or we may set shutter speed fast (maximum sync speed) to keep any ambient out of the picture (to keep white balance more pure, etc). Generally shutter speed is little issue indoors, it only affects the ambient, which is dim and insignificant.
For fill flash in bright sunlight, the continuous ambient is very significant, we must deal with it. To expose it properly, the Sunny 16 Rule says for ISO 200, typical exposure in bright sun is 1/200 at f/16. If using flash, we cannot use equivalent exposure of 1/400 f/11 or wider, because the shutter cannot sync the flash faster than sync speed. This means with flash fill in bright sun, 1/200 at f/16 is about all that can work (due to sync speed requirements). Camera mode P knows this, but camera mode A will allow you to set f/4, and then fuss at you about it being unusable (HI warning). Mode P is a good thing for flash in bright sun.
But in bright sun, it would seem great to have a way to increase the sync speed so we can open the aperture. This is simply impossible with the focal plane shutters, but many DSLR systems do have a way (sort of a kludge) to allow this, called High Speed Sync flash (Auto FP). However, it does reduce the effective power of the flash to about 20% (HSS maximum distance range will be less than half of speedlight range).
Maximum Shutter Sync Speed, continued on next page.
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