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Basics of Flash Photography
Four Fundamentals we must know

4.     Flash pictures are double exposures

Any flash picture is a double exposure, once from the flash, and once from any ambient continuous light that might be present. These two effects work differently, with different rules. The ambient may be an insignificant level indoors, or it may be overwhelmingly significant when using fill flash in bright sunlight. We might be able to ignore the ambient indoors, but generally must match it in daylight. There are always these two effects to be considered in any flash picture. Note again, they have different rules.

The camera system measures ambient and TTL flash independently. The camera meter indicates only the continuous ambient light it sees (is never about the flash), and camera automation sets the camera settings appropriately. Then later, the TTL flash system meters the preflash to set flash power level for the aperture setting it discovers in effect at the time. But we don't see this TTL meter - all we see is the picture result on the camera rear LCD. The flash of course affects the picture exposure, but it does not affect the exposure of the separate ambient light.

The TTL flash doesn't affect camera settings, at least not regarding exposure. But flash does have a few effects: If the camera is aware that the flash is enabled, then 1) the maximum shutter sync speed requirement is enforced, a maximum often around a 1/200 second ballpark. 2) In camera A or P mode, the minimum Flash Shutter Speed (commonly Nikon menu E2, default 1/60 second) is enforced (unless Slow Sync). And 3) camera P mode enforces a maximum aperture value (around f/4) in dim light. And 4) older Nikon DSLR models held Auto ISO low with flash (because we are using flash instead), but newest models (D300S, D7000, D5100) always set Auto ISO based on ambient (intending to balance flash to the orange incandescent ambient?), so then indoor flash always discovers that it is facing high ISO (we can always turn Auto ISO off of course). But these are merely limits, which are not about actually metering anything.

Shutter speed can keep out continuous light, or allow it in

OK, so knowing that instantaneous flash is not affected by shutter speed (see Part 2), and knowing any continuous ambient light is so affected, we can use that fact. Indoors, using incandescent modeling lights with studio lights, we can use maximum shutter sync speed (without affecting the flash exposure at all) to reduce that ambient continuous so that it does not affect our picture. One purpose could be to prevent orange incandescent light in our flash picture. Or to prevent normal room lights from affecting our carefully lighted studio setup. Studio flash surely always uses camera M mode and maximum shutter sync speed, for this reason. But in a normal room snapshot setting at low ISO, we might intentionally use a slow shutter speed to show and maximize the contribution of that continuous room light, for a warmer "ambient" look. Caution though, at high ISO, without incandescent white balance and a CTO filter on the flash, you surely get excessive orange.

Normally it is our choice, to control the continuous ambient light with shutter speed, to block it out or allow it in, independent of the flash exposure. We only need to know that any flash picture is a double exposure of the two situations. With flash, we always have two decisions to make - proper aperture so flash power level is possible, and indoors, and shutter speed about if we want the ambient included or not (outdoors in daylight, we are probably going to have to account for the sunlight.)

The flash exposure will not care about whatever shutter speed we use, but we have choices, and our choice ought to be intentional. When we use the camera's A or P exposure mode indoors, when we turn the flash on, we see the automatic shutter speed increase from "slow" (perhaps 1/4 second metered in the dim room?) back up to 1/60 second. Not for any reason other than this is the lower limit set by the camera when using flash - we do not need a slower shutter speed if we are using flash. Many models have a menu for this called Flash Shutter Speed (often Nikon menu E2, D90 E1, default 1/60 second). There are ways around this limit - camera Manual mode allows any permissible shutter speed, or Slow Sync or Rear Curtain Sync option ignores this limit and uses whatever the camera actually metered for ambient. This minimum lower shutter speed limit with flash (often a 1/60 second default for minimum shutter speed with flash) is simply because we might be able to hand hold 1/60 second in case there is some degree of ambient light. But you may prefer to keep out the continuous ambient, because incandescent is orangish in color. Or if you are using the very fast flash for the purpose to stop motion, a 1/60 second shutter may allow dim ambient to blur what the fast flash stopped. If you noticed the metered light reading BEFORE you turned on the flash, it was surely slower than 1/60 (if dim indoors - but in sunlight, you get the actual that it meters.) So indoors (with insignificant ambient), it seems much better to always use camera Manual M mode with flash, which allows us to set the shutter speed as we wish, either slow to allow the continuous ambient in, or fast at maximum sync speed to keep it out. It is our choice, but the automation will choose 1/60 second, possibly not for reasons useful to us. The Speedlight flash power used in TTL mode will depend only on the aperture and ISO values, and the TTL flash exposure is still fully automatic even in camera M mode.

To say the obvious: There is no camera intelligence that can treat one area of the picture different than any other area. It may choose one area to meter with preference, but the camera settings can only use ONE aperture setting and ONE shutter speed setting. The flash can only use ONE power level. These single settings affect everything in the picture (there is no magic.) ISO, aperture and shutter speed affect the continuous ambient light exposure. ISO and aperture and flash power level affects the flash exposure. TTL automation tries to determine some of those settings, but the one big thing important to realize is that our manual Flash Compensation also affects the flash (power level and exposure), as we see fit. If TTL automation does not give the result you want, then you can moan, or you can simply fix it. :) Flash compensation is a main trick for getting great flash exposures. Flash compensation should be considered (if not used) for every TTL flash picture, and certainly in any new situation.

Understanding these few fundamentals will greatly simply and explain about everything you see happening with flash.

Camera exposure mode for flash

Fill Flash in Sunlight - This will need some flash compensation, discussed in next section: Balanced Flash Mode.

We must meter the continuous ambient daylight there (it being extremely significant and cannot be ignored.) Maximum shutter sync speed for flash makes this be a little harder. For example, using fill flash outdoors in the sunshine is pretty important, but sometimes fairly difficult. The daylight exposure is Sunny 16, so for ISO 200, we expect 1/200 second at f/16 in direct sunshine. The fill flash must match that (more or less, we likely want the fill to be a stop or so less - a fill is less level than the main light.) But f/16 at say 10 feet requires Guide Number of (16x10) = 160 at ISO 200 (divided by 1.414 to convert to ISO 100 is equivalent GN 113 in the ISO 100 chart.) This is possibly close to our flashes capability, at least at wide zooms at more distance. And 1/200 second is very near our shutter's maximum sync speed - so with flash, we cannot go faster - we are unable to convert to Sunny 16 equivalent of say 1/800 second at f/8 (to gain flash power or wider aperture.) We are limited to the 1/200 second maximum sync speed, and Sunny 16 says f/16 at 1/200 in bright sun.

This is a narrow window, few choices. Camera P mode and TTL BL flash mode understand all of this, so they are the point&shoot settings for fill flash in sunlight. For this reason, wedding photographers joke to call P mode: Professional Mode. :) But in contrast, indoors in dim light where we simply need flash, P mode simplicity gives up most of our options. There is nothing better than camera A or M mode, and flash TTL mode. However, outdoors, camera A mode easily allows us to blindly set f/4, and then if in bright sun, will fuss with error warning HI, simply because the shutter speed will not go to 1/3200 second with flash.

Shady conditions or overcast days are not so extreme, but Sunny 16 says flash with bright sun is going to need more like f/16 at maximum sync speed.

So then we might try Auto FP High Speed Sync mode for a faster shutter speed to do the 1/800 at f/8, or even 1/6400 at f/2.8. This does work, but the FP flash power drops drastically, and range falls to less than half. The FP flash itself is modified to generate a continuous series of smaller flash pulses to mimic continuous light, so the focal plane shutter can work. Auto FP mode (on many camera models with focal plane shutters) will allow any shutter speed faster than maximum shutter sync speed with flash, but at much reduced power level from the flash, which greatly limits the distance range of the flash (see Part 2B, HSS Auto FP.) The rear LCD on the SB-700, SB-800, SB-900 and SB-910 flashes will show that maximum distance range currently in effect.

Fill flash in bright sun is a difficult problem, so we may need to seek some shade for our picture. A neutral density filter, or a slower ISO setting, can help with a wider aperture regarding depth of field. Both of these affect flash and sun equally. However, if extra flash power is available, we of course turn the flash power back up. We cannot turn the sun back up, so turning up the flash creates a ratio change of flash to sun (with ISO or ND filter.) The easier solution for flash power is either a closer flash distance, or a more powerful flash. A large studio light used outdoors can overpower the sun at reasonable distances, but cannot overcome the maximum shutter sync speed issue.

Balanced flash mode

The rules are the same for all continuous light, and planning goes much better when we realize what is happening. Nikon's TTL BL flash mode is mainly for balancing the fill flash to the daylight. TTL BL mode tries to match reduced flash to a bright ambient (outdoors), whereas the TTL mode ignores ambient and makes flash match aperture (for indoors.) But note that the best fill flash fills subtly, adequately but without even looking like flash was used, so for this fill flash in TTL mode, we normally want to specify about -1 EV to -1.7 EV (stops) flash compensation, so the flash does not overwhelm. TTL BL mode helps us by doing this fill reduction automatically.

Note if you are using fill flash in bright sun, the amount of Flash Compensation needed will depend on which flash mode you use.

Different opinions guess about how TTL BL works, what it actually does. No one but Nikon actually knows, and they don't say much about details. Nikon used to say more words in the manuals of cameras and flashes over the last dozen years, which does not conflict with what they say today, but all they say today is this:

Nikons manuals (SB-600 page 33, SB-700 page C-2, SB-800 page 37 , SB-900 page D2) say the modes do this:

Implications which we should realize:

Nikon surely knows how they designed this system, but their descriptions say words Subject and Background, and of course, the dumb computer can have no clue about that. The flash is effective on the foreground, likely the subject, and ambient is on the distant background, so I think the words subject and background mean Flash and Continuous ambient light (which is what it tries to balance.) Some of their camera manuals (section about internal flash) add the word ambient for TTL BL: "for natural balance between main subject and ambient background" - which means reduced flash level. The camera manuals say TTL is for "when main subject is emphasized at expense of background details" and all camera manuals say TTL mode is "recommended if Flash Compensation is used".

TTL BL mode typically is for fill flash outdoors in daylight, when there is enough ambient light to give a good regular exposure,. The flash is added and matched (reduced) to balance that background exposure. The camera meters the background for proper exposure, and the flash is balanced to that. The flash power is typically reduced a good stop, maybe more, as needed in sunlight, and maybe not at all indoors. It is just a computer, and Matrix metering has effect too, so we do not know the rules, and we are not always sure what it will do. Sure, we certainly can always compensate TTL BL flash too, it usually works, but it is two cooks salting the soup, without knowing what the other is doing. Sometimes unexpected variations can occur. The camera manuals recommend TTL if Flash Compensation is used - but then, our attention is required. The Nikon system must try to accomodate those who don't know how, and those who can do it themselves. Frankly, I like to know what to expect, and I'm rarely in TTL BL mode, or matrix metering mode (old fogey). Very many people love it however.

TTL mode is typically intended more for indoors, for places when flash is considered to be the only light source. In TTL mode, the flash intensity is not matched to anything, nothing else is considered except the flash exposure. The flash output is simply what it should be if flash is the sole light source, which is usually true indoors. It assumes no background light exists to be balanced. If there is bright ambient, it may well overexpose TTL mode. Fill flash in bright sunlight is the classic case for TTL BL mode (TTL BL flash mode and camera P mode knows how to do this, is point&shoot), but TTL mode with about -1.7 EV flash compensation works great too.

For example, imagine doing it backwards.. using TTL outdoors for fill flash in daylight (as opposed to TTL BL.) The camera will meter the camera settings for the ambient background for what it should be (if acting alone.) The TTL flash power level is metered for what it should be (if acting alone, like if it were dark.) These two exposures added together will be twice as much light as is proper or needed. If BOTH are providing sufficient light, then it should be overexposed one stop. Which is not good, but both systems are doing what they were told to do (there are definitely two different exposure systems in there.) Many people prefer to work this way though, they know what to expect (no computer meddling to surprise them), and they automatically know to start off with about -1.3EV to -1.7EV Flash Compensation to handle it.

In contrast, TTL BL mode automatically adds some amount of similar Flash Compensation (we just do not know the numbers or its rules.) TTL BL would take the sum of the two exposures into account, and would back off on the flash exposure (power level) about one stop, and this result would then be a good level to be a good proper fill flash. Fill flash has to be weaker than the main light (sun in this case.)

So, assume we do the same fill flash situation in daylight, in both modes:

TTL mode probably needs about -1 EV to -1.7 EV manual flash compensation to be right (as a fill flash.) However, TTL BL will simply automatically approximate this compensation by itself (some of it) without any manual compensation by us (we may still want to tweak either slightly.)

Indoors in a dim room (no significant ambient to match), both results should be approximately equal, but we are telling TTL BL to something which can only fail (in a dim room indoors.) More recent cameras are better about not failing.

That said, this TTL BL adjustment could be an automatic plan, done when it is needed, and it is in fact the automatic plan for the internal flash, and for the Commander driven remote wireless flash. But on the hot shoe, Nikon provides this extra option TTL for when that is not what we want done... which is generally always true of indoor flash. Note: In the general case, watching and always considering need for any possible manual flash compensation is always a good plan, regardless.

Indoors - Flash conditions indoors usually see an insignificant amount of continuous ambient room light. The light is dim (compared to daylight), which is why we need flash. Metering that low ambient room light is normally not possible with flash, because the camera normally enforces a minimum 1/60 second shutter speed when the flash is used (discussed above.) The room where we need flash is probably closer to 1/2 to 1/10 second ambient. If 1/60 worked, we may not need the flash. So... this 1/60 is NOT about the flash. The 1/60 second commonly seen is just a minimum shutter speed limit with flash, since we do not need a slower shutter with flash.

Point is, this ambient room light is normally insignificant indoors, and we cannot reasonably attempt to balance it with fill flash, not like using fill flash in sunlight. The flash is the major light indoors, pretty nearly the only light indoors. So that makes the TTL flash mode be the correct flash mode indoors, NOT TTL BL indoors. Normally nothing to balance it with indoors. Flash TTL mode is for when the flash is the major significant light source, and TTL BL flash mode for when the flash is to be balanced with significant ambient light (which is normally sunlight.)

Second point is that any automatically metered camera exposure settings are only about the room ambient light, which is normally insignificant indoors. Instead of trying to meter the insignificant ambient room light, we can instead choose an aperture more appropriate for our flash goals (like available flash power.) The flash does not care what shutter speed is, but we can select a shutter speed to control (boost or reduce in our picture) the small amount of ambient room light (our choice.)

Camera exposure modes for flash INDOORS

No matter what P, S, A, or M camera mode you select, TTL flash is always still automatic flash exposure. TTL flash automatically chooses the appropriate flash power level for whatever aperture you set, regardless of how you selected it.

Camera P mode (Program) is NOT a versatile choice for flash indoors. It might be great for fill flash outdoors in daylight, but indoors with a hot shoe flash, it will use 1/60 second at f/5, every time (Slow Sync or Rear Curtain Sync can bypass the 1/60 second lower limit.) I am saying f/5 (generic value here). It can vary a bit depending on which camera model (see list at right). In minimal lighting (indoors), this depends only on ISO, not on the flash range needed. Higher ISO setting will worsen and reduce this maximum aperture more. Here is from the Nikon D90 and D300 manuals:

This f/5 is a fair guess for average flash situations, conservative and probably always works, which is good, but which is not exhibiting great intelligence about the scenes requirements. Every flash picture you ever took, or ever will take, in P mode will be 1/60 f/5 (P mode, indoors with flash, low ISO.) If you bought the expensive f/1.4 lens, P mode will never use that wide aperture for flash indoors. If you specify ISO 800 for conditions needing more light, camera P mode probably will only use f/7 then, maybe when you most need the wider aperture (P mode seems counterproductive for indoor flash.) We really do not have to accept f/5 1/60 second for every flash picture we take indoors (see A or M mode.) But P mode works very well with fill flash outside in daylight however, and knows about maximum sync speed.

Camera S mode (Shutter) -- ??? Wrong choice indoors. It will work, the TTL flash is automatic for any aperture, but what is the aperture? (rhetoric, it surely is always wide open in S mode when indoors where we need flash.) Instantaneous flash could not care less about shutter speed, so why are we giving preference to that? Perhaps for action outdoors in daylight, but aperture is critical for flash power. Mode S has no advantage for flash indoors. Flash power level is about aperture. Indoors with no significant ambient to interfere, the flash duration is what stops action. The flash duration is faster than any possible shutter speed. Instead think camera modes A or M for indoor flash.

Camera A mode (Aperture) will use 1/60 second at the aperture you set for most cases of flash indoors. (For flash in sunshine outdoors, since the shutter speed cannot exceed maximum shutter sync speed, we probably see many warnings of HI exposure in A mode (unless we know to set f/16), for apertures needing faster shutter speeds, but which cannot do them due to max sync speed limitation - use P mode with flash outdoors to prevent that.) Back to indoors: You can choose a wide aperture to minimize flash power (like for bounce which needs a lot of power), and reduce recycle time. Or you can choose a narrow aperture for closeup subjects, for depth of field, or to not overpower the subject. Again, f/5 is good average and reasonable choice in many cases. For bounce from high ceilings, or from a rear or side wall, you probably may want to use ISO 400 too, and/or f/4 (for less risk, and for faster recycle.) But ISO 200 and f/5.6 may work on low ceilings (watch your flash Ready LED for indication of power used.) We normally always get 1/60 second shutter indoors with flash, which doesn't affect the flash, but it does affect the continuous ambient room light. There are often menus and options to allow other shutter speeds than 1/60 second (Flash Shutter Speed menu, Slow Sync, Rear Curtain Sync), but if you want other shutter speeds, then you should be using camera M mode (flash indoors), because it is so much simpler to do anything you wish there.

Camera M mode (Manual) - The TTL flash mode is still automatic. It is only the camera settings that are manual. Any automatic camera meter reading was determined from the ambient light, which is not about the flash. That automatic metering (like A or P mode) is helpful to match ambient outdoors in daylight, but the light is insignificant indoors now, which is why we need flash. So indoors, camera Manual mode lets us set things up for the flash picture double exposure, any way we want it. Said another way, we could simply set M mode to f/5 1/60 and duplicate what P mode will do (flash indoors, ISO 200). Or, we can change things a little to suite better purposes.

Aperture controls the flash power needed (camera M mode or any other mode), and therefore the flash recycle time. We set aperture for the flash power level (judged by recycle time.) Trying to be very clear: Aperture and ISO certainly affect the flash exposure, so we do see the exposure of manual flash mode at a fixed power level change with aperture or ISO. We have to manually set that manual flash power level to be correct for the aperture or ISO. But automatic TTL flash mode will simply automatically react to aperture or ISO with same automatic "correct" TTL exposure, by using a different flash power level (automatically.) So we may not see it change, may not realize it changed, but flash power level does change big time with aperture or ISO. But flash power is relatively limited, not infinite, so we must pay attention. Watch your Ready LED.

In camera M mode, we can set any shutter speed up to the cameras maximum shutter sync speed, or we can set it as slow as we want it to capture more of the dim room ambient light. The flash exposure does not care what shutter speed is, and will not change. The flash duration is fast, faster than maximum shutter sync speed, so therefore, actual shutter speed controls only the ambient room light, fast shutter to keep it out, or slow shutter to allow it in - our choice. Studio portraits always want maximum shutter sync speed to keep the ambient out (for example, the modeling lights.) Family snapshots may want shutter slower, for the "warming" look of the incandescent room lights. Again, it is our choice. We do not have to accept 1/60 second. We can think about it instead. But as first beginning step in using camera M mode for indoor flash, we can always use 1/60 second and f/5 as a starting point, a home base. This is the same values P mode would have used, and it will do the same thing in camera Manual mode, and it usually should give a decent picture (remember, the TTL flash is always automatic flash exposure.) But with camera Manual mode, there are other choices too. We can more optimally change aperture or shutter speed, for the specific purposes mentioned. A little experimentation will be very helpful.

The automatic flash in TTL mode gives automatic flash exposure in EVERY camera mode (including camera Manual mode.) This is true for direct flash, bounce flash, flash in umbrellas, etc. Not to say it is always perfect, it's not. We often have to add a bit of flash compensation to "expose to the right" on the histogram, and then it is perfect (How lightmeters work may be of interest.) In automatic TTL flash mode ("Through The Lens" metering for flash exposure), the first thing that happens after the shutter button is a preflash. Some of that preflash reflects back into the camera lens, where is it metered (measured), and the flash power level is set to give the correct exposure for the lens aperture in effect at the moment. What varies in automatic TTL flash operation is the flash power level (the camera exposure settings remain constant, whatever we set, or whatever the ambient light meters.) We can choose the lens aperture ourselves to determine the ballpark flash power level that will be required, but we always get correct TTL flash exposure, regardless of which aperture we choose (within the range of what the flash power can reasonably deliver.) This means however, that A (Aperture) and M (Manual) modes are very important and useful for flash indoors, since the aperture setting controls the flash power (and recycle time.)

Note: Flash compensation changes TTL flash power level, and it only applies to TTL flash, either on hot shoe or TTL remotes. Manual PC cord flash is affected by the flashes power level setting, but the camera cannot affect that. The camera has an Exposure Compensation which affects both the metered ambient and the TTL flash, and it has Flash Compensation with only affects the TTL flash. These two compensations add together for the TTL flash.

A homework assignment, to be sure you "get it"

I could show you some test pictures about how your flash pictures can control the continuous ambient light with shutter speed, but you really really need to do it and see it and believe it, to make this info be actually available to you. Please? You surely can spare five minutes for the simplest exercise?

Indoors at night (or at least in room with no bright window sunshine to confuse things at first), turn on a regular table lamp, hopefully placed within a couple of feet from a wall, to illuminate that wall. You are simply going to take a picture of that wall and that lamp, from say 8 to 10 feet away. Use any lens and zoom that shows several feet of wall width, including the lighted table lamp in the picture.

Set your cameras exposure mode to Manual M mode for this test. Your TTL flash will still give automatic flash exposure in this camera Manual mode.   Repeat: Your TTL flash will still give automatic flash exposure in this camera Manual mode. Try this once so you will believe it too. No reason to be scared of Manual camera mode with flash indoors, because the TTL flash still gives automatic exposures. You still set aperture yourself, same as in camera A mode. Manual camera mode merely allows you to also specify the shutter speed which controls the ambient in the room. The ambient is near insignificant, and the flash is not a affected by shutter speed anyway. This is kind of a big deal to know, so I will repeat it one more time.

Manual mode: Think of it this way, since flash is not affected by shutter speed, then for flash, M mode is exactly the same as A mode, in that we set aperture for the flash power either way. Camera M mode is good stuff for flash indoors (but matching sunlight usually requires a correct ambient exposure.) Indoors in A mode, you will simply always get 1/60 second shutter speed, which is from your cameras menu for Minimum Flash Shutter Speed. 1/60 second there only means the dim ambient was not stronger than that, so the camera enforced a minimum limit. There are reasons sometimes to use other than 1/60 second, so we might as well set shutter speed for continuous ambient to be the way we want it. M mode allows doing that. This is ALL it does in this case. The TTL flash is still automatic flash exposure, and using the aperture you set anyway, exactly like A mode in this case.

For this test, set the ISO to 200, and set the aperture to f/5.6 or f/8. Open the internal flash door, or use hot shoe flash, in TTL flash mode. Direct flash for this test, but bounce or any other flash setup works the same way.

Take a picture at say 1/30 second (M mode permits that.) Include the table lamp in a picture of the wall.

Take another picture (the same picture), everything the same except at 1/200 or 1/250 second shutter (or 1/500 second on D40/D50/D70 models), or whatever fast shutter speed your camera allows as maximum shutter sync speed. Do stay OUT of FP HSS mode, we are speaking of regular flash here (no faster than maximum shutter sync speed - faster is NOT "flash".)

OK, you should notice that in the slow 1/30 second picture, the table lamp (a continuous light) appears as being lighted, and it is even illuminating a little of the wall and table it sits on. It's power does not reach very far (inverse square law.) We have no reason to believe our f/8 at 1/30 second was a correct exposure for the area of the ambient continuous lamp, but it should be ballpark enough to make the point of this concept. Only the flash is illuminating the farther regions of the wall. The flash exposure was correct, because the camera TTL option metered the flash and adjusted its power level automatically to be correct for the f/8 aperture which we specified. The wall illumination is NOT from the lamp or from any ceiling light. Turn the flash off and repeat the picture (will be pretty dark then) to make this point to yourself.

in the faster picture, 1/200 second or whatever, the continuous light is diminished by these 3 stops of increased shutter speed (1/30, 1/60, 1/125, 1/250 is a 3 stop change.) If your results match mine, the lamp does not even appear to be turned on. The shutter speed was too fast for the degree of continuous ambient we have - the shutter speed we selected simply kept it out. But the flash illumination of the walls is unchanged from the first slow picture.

See? Shutter speed does NOT affect flash exposure, but does affect continuous light. And in such flash pictures, shutter speed is our choice, slow to allow the ambient into the picture, or fast to keep it out. This is such a biggie to know and use. A fundamental of using flash.

In a picture such as this, you may have wanted some of the ambient light to register, warm and cozy so to speak. But if a picture of a person, you don't want the incandescent light making them orange, so you probably want to keep it out. It is our easy choice, whatever you want to do. In a studio flash situation, we never want the ambient, so we use the shutter's maximum sync speed. I might mention this surely requires using the camera's Manual mode.

OK, the room with bright sunshine was mentioned at the start of the homework assignment. That sunlight is continuous too, and works the same as the table lamp, but it is likely much brighter than artificial light, with enough strength to give the flash power a real run for the money. Therefore it is much harder to simply overpower and ignore like we can the table lamp.

But you can change aperture too. If you stop down aperture, the TTL flash will have to use more power to give the same exposure, and it will (up to its power limits.) But stopping down reduces the ambient too (at same shutter speed), and the abmient is unable to compensate and increase (like the flash can.) So stopping down is the second level of tool to reduce the ambient light. Setting shutter to maximum sync speed is obviously the first step.

I will relent and show this, as you need to know, but you really do need to repeat it yourself. Do it! So that you can and will believe it, to enable using it. Any quick flash picture of a lighted table lamp, from not too close. This one is an ordinary incadescent 40 watt lamp, with bright sunshine outside this window. These are ISO 200.

Internal flash - f/5.6 1/30 second

Internal flash - f/5.6 1/200 second

No flash - f/5.6 1/200 second
The camera meter was reading this as 1/3 second.

Note this is a picture OF THE LAMP, which is brighter and harder to handle than the light FROM THE LAMP. Sunshine is mighty bright too. So maybe not perfect, but still an awesome tool, which we need. We have at least a couple of stops of range to use for control. We could go to f/11 here to minimize the ambient even more (at maximum shutter sync speed), but aperture also affects flash power too, so another stop would need twice the flash power. But it is often a very workable choice.

This simple test (to try it and see) is just to prove that flash is NOT affected by shutter speed, but that we can use shutter speed to control the continuous ambient. If there is any question at all, then experiment a bit again, use a little different values but with the same concept in mind. Several values maybe, see how things work. For example, if you used ISO 200 f/5.6 then the fast 1/200 second picture probably can still just barely detect the lamp was on, where f/8 probably shows it dark (however I have not seen YOUR table lamp.) If you used ISO 100 f/8, the 1/30 second picture may not be very impressively illuminated by the lamp. However I am not trying to discuss any specific table lamp or ISO, but only the general concept. Explore the situation though, and you should see the obvious control that you have. It is very useful.

Here is another try (daylight is continuous too.) Approaching sunset, looking east. Bright area in window upper left is a neighbors roof in direct sun, behind a winter bare bush. These are ISO 400.

The flash illumination is the same, unaffected by shutter speed. In flash pictures, shutter speed is merely a choice to keep the continuous ambient light out, or to allow it in, but without affecting the flash exposure. We can use that. This is a N:Vision CFL lamp (camera left, green), and a GE Reveal ("daylight") incandescent lamp (camera right, red.) Neither are ordinary incandescent lamps, but of course, both are continuous (however see more details about fluorescents.)

So shutter speed is a control to change the ratio between continuous ambient and flash. Sometimes we may prefer the "look" of the slower shutter to capture more continous ambient. Me too, sometimes, it can be a warmer tone. But there are cases (especially in studio photography, or high speed flash photography) when all ambient is unwanted, and we need to know that maximum shutter sync speed is the major tool to keep it out. We have the choice, and this one is easy, because it does not affect the flash (and narrow aperture and/or low ISO increase that difference, if we can turn the flash power back up.)

One last point and you've got it. If the ambient is dark (a black picture if flash is not used) then it simply does not matter to anyone what shutter speed is (no ambient to register.) See this link for a sample of high speed photography, where the shutter speed was BULB, manually opened and closed, typically at least 1.5 seconds. The room was dark at f/16, even if there was a table lamp turned on for humans to see. F/16 was to keep out ambient, and for closeup depth of field. Bulb and 1.5 second shutter is quite slow, but the speed of the flash duration is what stopped the action, not the shutter speed (assuming insignificant continuous ambient. The ambient COULD blur what the fast flash stopped, if allowed.) Speedlights are named speedlights because they are incredibly fast at lower power levels. Shutters are simply never that fast, unsuitable for high speed photography.