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Basics of Flash Photography
Four Fundamentals we must know

  Menu of the other Photo and Flash pages

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Flash pictures are double exposures

Any flash picture is effectively 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, flash and ambient have some different rules about exposure.

The camera system measures ambient and TTL flash separately and independently. The camera meter, and the settings it may choose, indicate only the continuous ambient light it sees (it 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 and ISO settings 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 affects the picture exposure, but it does not affect the exposure of the separate ambient light.

Some Limitations placed on Flash, not related to metered Exposure

The TTL flash metering doesn't ever change automatic camera settings, however the flash presence does have a few effects. If the camera is aware that the flash is present, then:

But these are merely limits, which are not about actually metering anything. TTL flash has its own metering system which is used at time of preflash, which adjusts the amount of flash power that is used to match the camera settings it discovers are in effect then. TTL flash is automatic flash exposure, regardless of any camera settings (including camera M mode, where we can set whatever settings we want).

The TTL flash does not set flash exposure the same way as the camera sets its ambient exposure.

Automation seems a difficult choice for manufacturers. Typically automation tries to help beginners in every possible way to save the picture, but experienced users (who have learned to see the actual scene requirements) typically will have their own better choices.

Flash modes that may be in the flash unit

This is oriented to DSLR cameras. Compacts and cell phones generally have fully automated balanced flash systems, with few options (other than Flash Off).

For DSLR, there can be several options with various modes to use flash, several modes on the flash itself, and also modes on the camera. Some top flash models have most modes, but some low-end may only have one mode, Manual or TTL, or maybe only a couple of modes.

Menus: The flash menu in the camera that selects TTL, Manual, or maybe Commander, only applies to the internal flash when its popup door is open. Then this camera menu has no effect when the popup door is shut. External flashes have their own menu (however Nikon SB-300 and SB-400 are exceptions, they have no menu and must share the camera internal menu). In the automated camera Scene modes, the internal flash may pop up and fire when automation thinks it is needed. But in camera A, S, P and M modes, the internal flash never pops up to fire, but it will always fire if you open it yourself, and a hot shoe flash will always flash with shutter if it is connected and turned on (unless maybe it is incorrectly set to some slave mode).

In other situations in bright sun outdoors, some users instead reduce the bright ambient exposure maybe about two stops, and then do full TTL flash level. This makes the subject stand out against the darker background, and it helps stop subject motion in fast sports (if the flash reaches that far). You would not call it fill flash however.

Cameras use reflective metering (metering is affected by subject colors, so sometimes not precise). Therefore to achieve better results, we may need to use Flash Compensation to tweak what the automation does with flash power level, in either TTL or TTL BL mode. Simply watch the flash exposure result in either mode, and then do what you see you need to do for a perfect flash result. This is key for great flash pictures. Flash is never fully point&shoot, we have to watch and help a little. It is quite easy.

Hint: Scene situations vary, but indoors in normal dim ambient, I like to routinely start balanced flash mode near +⅔ EV or +1 EV Flash Compensation in the general case.

First Basics of Flash:

What we need to know about the basic way the camera operates flash

The next six pictures are about shutter speed choices to control the ambient vs flash, to show what happens. Important tools are: Balanced flash to prevent overexposure of sum of ambient and flash. Camera M mode allows any shutter speed (not exceeding maximum sync speed), specifically faster will keep out off-color indoor ambient to protect flash white balance. And Slow Sync mode allows slower (actually metered) shutter speed, to include dim ambient.

This day was overcast and raining (Not bright). Without flash, the less bright ambient required camera mode A selection of 1/13 second at the selected f/5.6 ISO 400 settings (late afternoon, -8 EV from Sunny 16 bright sun). All pictures were taken within about a minute (in same ambient). #4 is camera M mode, all others are camera A mode. All are bounce flash except #1 is no flash. Flash is non-balanced TTL except only #3 is Balanced flash. All are regular front curtain sync except #5 and #6 are Slow Sync. Nikon D300, 12-24 mm lens at 22 mm. This is just quick point&shoot (but on a tripod).

All six photos are f/5.6 and ISO 400 (not Auto ISO). Again the point is, shutter speed only affects ambient, and NOT the flash, so shutter speed offers some control of ambient (which is an extremely important tool for photographers). Again, all are camera A mode (allowing automatic shutter speed), except #4. The bounce flash does not reach to the fence outside.


1. No flash, ambient only, 1/13 sec
Overcast or shade can often
be good lighting. This is in an
enclosed patio on a gray rainy day.

2. Non-balanced TTL flash, 1/60 sec
Ambient is -2.33 EV.
Foreground is the bounce flash.
Background change is the 1/60.
The 1/60 is an arbitrary minimum
limit, which is NOT about exposure.

3. Balanced flash TTL BL, 1/60 sec
Ambient is -2.33 EV.
This limit does Not allow slower shutter,
so it holds back the ambient exposure.
And Balanced flash also reduced the
flash exposure.

4. Camera M mode, TTL, 1/200 sec
Reduces ambient -4 EV here.
Faster shutter is useful indoors for
white balance to keep out any orange
incandescent or green fluorescent light,
when desired. Camera M mode is the
tool to set any desired shutter speed.

5. Slow Sync bounce flash, 1/13 sec
Non-balanced bounce flash.
Slow sync uses the ambient metering
but the total with non-balanced flash
is overexposed (bright red and
yellow flowers are clipped)

6. Slow Sync, -1 EV TTL, 1/13 sec,
added -1 EV flash compensation
prevents the clipping of the sum
of ambient and flash (balanced flash
would have been automatic reduction)

With flash, the camera still first meters the ambient light, and those same settings are what it will still use for flash. The TTL flash must adjust its power level for whatever ISO and aperture settings it discovers to be in place then. We might consider choosing the settings differently to help the flash though.

The Exception is that with flash in A or P mode (automation which adjusts shutter speed), the shutter speed is not permitted to be slower than the "Minimum Shutter Speed With Flash", which is menu E2 on many Nikon DSLR models, which allows a Minimum Shutter Speed setting as fast as 1/60 second (default of most cameras is 1/60 second minimum if using flash). Camera models without this menu will simply use a similar minimum with flash, likely 1/60 second. This is an arbitrary limit (probably is about handholding shake, but the thinking is we don't need slower if we are using flash anyway). It is a minimum limit, NOT about metered or a proper exposure. The flash exposure is not even affected by shutter speed, but the ambient light exposure is.

If we had wanted to use the actual slower metered shutter speed for the ambient, we could use Slow Sync mode for the flash, or some cameras can set a slower Minimum. Or more commonly, we could choose Manual camera mode, using any shutter speed settings we wished, which is often a great plan for indoor flash. Or camera S mode will set shutter speed too, but then in a dim situation, camera mode S would likely always use wide open aperture in dim ambient.

Slow Sync ignores the 1/60 second Minimum shutter limit with flash, and uses the slower shutter speed actually metered for the dim ambient. But the sum of any two lights added is brighter than the brightest, so one issue then (Slow Sync or in any properly metered light) is that if we assume properly metered ambient is 100% of a correct exposure, and properly metered TTL flash is 100% of a correct exposure, that sum of both is 200%, which is 1 EV overexposed on the near subject. We see that in #5, where the red and yellow flowers are overexposed (and the sky is overexposed much worse, but it is not considered any issue in this picture). This compensation percentage has a calculator on next page. The definition of Balanced flash mode is that it reduces TTL flash exposure to minimize this overexposure of the sum, but non-balanced TTL does not bother. I still prefer using non-balanced flash, having learned to control the compensation myself, so I'm not always thinking about balanced flash mode, but it has become hard on todays gear to even select non-balanced flash. But indoor flash typically instead forces a 1/60 second Minimum shutter speed which probably underexposes ambient unless ISO is very high, or we choose Slow Sync. My own notions are that Slow Sync mode, camera mode S for flash, or either Auto ISO or High ISO for flash Indoors, are very special cases that I rarely have use for. But they do exist when and if needed.

Balanced flash mode will normally reduce the TTL flash level to minimize overexposure (and here, in #3, due to the f/stop and ISO I selected, the ambient was reduced by the Minimum Shutter Speed With Flash limit). But as always, if ever TTL flash is too bright or too dim, use Flash Compensation to get what you want. Or for Manual flash, if result is not right, correct it's power level (or the f/stop if indoors). Digital cameras make this easy to see and correct instantly, then and there.

And another limit with flash, the shutter speed is not permitted to be faster than the Maximum Shutter Sync Speed, typically about 1/200 or 1/250 second on many cameras, which does require attention in bright sun (camera P mode in sunlight is good about knowing all of that), but the presence of the limit is rarely any concern in dimmer light or indoors (the flash is faster than the shutter speed).

The way Auto ISO is handled with flash can vary with camera model.

Anyway, back to the idea in the six pictures above... So the camera did arbitrarily increase the shutter speed to the 1/60 second Minimum if any flash was detected present, and the ambient was simply underexposed by that shutter speed increase. But still, we are using flash instead.

Look through your old photos if you have Exif data. In A or P or Auto modes, every indoor speedlight flash picture will surely all use this 1/60 second Minimum Shutter Speed With Flash, which is good reason to instead consider camera M mode indoors. If you are setting Camera A mode and f/5.6, and the camera is going to use 1/60 every time (in indoor lighting, maybe faster in stronger light), which is the same as manual mode f/5.6 1/60. Except then you don't have to choose 1/60, it was not about exposure. The 1/60 was Not a metered value. You can arbitrarily select shutter speed to control the ambient as you wish (next section a few lines below). But the TTL flash is still automatic flash even in camera M mode, which still uses the same aperture you'd set in A mode, but it also allows control of ambient with shutter speed.

For formal portraits with flash, it can be important for white balance to use low ISO and to stop down a bit, and to use Maximum Sync shutter speed (camera M mode). Stopping down needs more flash power, but it improves depth of field, and these actions better prevent any stray orange incandescent ambient from affecting your white balance. At ISO 100, typically 1/60 f/4 will let a little of the ambient in, and 1/200 f/8 will generally keep it all out (again, indoors). Faster shutter reduces ambient, but does not affect flash. Low ISO and stopping down more reduces both, but the flash can increase power level to stay the same. Using the same formal settings, you can turn all flashes off and snap off a shot to verify that sees a totally black frame (no ambient leakage).

This case above (1/13 sec changed to 1/60) leaves the ambient underexposed 2 1/3 EV (pretty dark, and indoors is often more difference than that). This is NOT a metered value, it is simply an arbitrary limit (because we're using flash, and wouldn't normally want to handhold 1/13 second). But this 1/60 Minimum with flash is rarely about a proper exposure (not unless it also might happen to properly expose the ambient).

So one big point about flash is, a faster shutter speed is a way to keep the ambient light out of the exposure of the indoor flash picture. A little bit of incandescent ambient (that 1/60 second might provide) can "warm" casual pictures that some consider pleasant, but it doesn't take much to spoil formal portraits.

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. For example, 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 the unwanted light does not affect our picture. One purpose of using ISO 100 and fast Maximum Sync Speed shutter with indoor flash would 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 low ISO and maximum shutter sync speed, for this purpose. And you will want to test your setup and exposure once, say ISO 100 f/8 1/200 second, with all normal continuous room lights on, but all flash disabled, to ensure you do still get a pretty black picture (without effect from the continuous lights). 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 DSLR 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 slow shutter speed 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 handhold 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 orange 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.

Maybe this is overly simplified, but regardless how we might reach the final choices, remember the exposure goal is in fact simple: No matter how mixed and complex the scene, our photo can only set one aperture, one shutter speed, one ISO value, and one flash power level (in each flash unit anyway). Maybe we specify some ISO and aperture, like f/5.6 and ISO 100. If we want to expose the ambient properly, we set shutter speed to do it. If we want to expose the flash properly, we set the flash power to do it. There are always choices, but these two exposures (ambient and flash) is all there is to do. Sometimes these values are a compromise, sometimes a tool, or maybe we ignore a very underexposed ambient, but each parameter has just one value. Look at your results, and when the one value seems wrong for your purpose, simply just fix it. That is your job. Just do what you see you need to do. Flash Compensation is the tool we use to adjust what the automatic TTL flash is doing. I am suggesting we remember to approach it as a simple problem, instead of as incomprehensible magic. Learn a few basics, and it becomes very understandable (honest, it really does).

Flash Compensation only affects relative TTL flash power levels. Exposure Compensation (on Nikons) affects Both ambient exposure and TTL flash exposure. And these methods all add for a final TTL flash value.

Understanding these few fundamentals will greatly simply and explain much that you see happening with flash. See a homework page about trying this concept.

Then, first a quick section on "Modes".   (Balanced TTL vs TTL modes).

Camera modes for flash

Note the camera light meter scale that we see is only about ambient light... the TTL system has its own invisible light meter system that it uses for the flash. TTL flash is automatic, point&shoot flash, metering and responding automatically to whatever aperture and ISO that either you or the ambient metering has set (TTL simply responds to it). However we can control TTL flash using Flash Compensation, to tweak it in.

These camera modes affect how the ambient light is metered. Then the flash modes above affect how the flash power level is adjusted. Camera A mode is very popular for flash, but I would suggest camera M mode indoors, and maybe camera P mode for fill flash in bright sunshine.

D7100 manual page 123, Internal flash in P mode
D7100 manual page 307, External flash in P mode

We specify camera mode appropriately for the property important to our current picture, to ensure it is what it needs to be. Maybe we need wide aperture for more effective flash power capability or a blurred background, or stopped down for more depth of field. Or a fast shutter speed to stop motion in ambient light, or a slow shutter to blur a waterfall. Whatever we think it needs, we have choices. Whereas other than exposure, automation never understands the situation.

Exposure and Flash Compensation controls

Speaking of non-balanced TTL flash: That idea is this: A properly metered ambient exposure is assumed to be correct, 100% solved. A properly metered (non-balanced) TTL exposure is assumed to be correct, 100% solved. But together, two proper exposures is 200%, which is 1 EV overexposed. So to prevent that, we often need to compensate the (non-balanced) TTL exposure. In that 100% ambient situation, -1 EV flash is still 33% flash and the total is -0.58 EV overexposed (calculator next page). But -2 EV TTL is 20% flash and is "only" 1/3 EV overexposed.

That is speaking of a normally exposed ambient, like flash outdoors in sunshine, or Slow Sync, or indoors with outrageously high ISO. Those will need flash compensation (still speaking of non-balanced TTL). But indoors with low ISO is a different story if in camera A mode (if Not using Slow Sync), because the Minimum Shutter Speed With Flash (probably 1/60 second) will underexpose the ambient instead, no longer 100%, no longer 200% overexposed.

I'm an old timer, from before the era of balanced flash, which is a bit newfangled to me. Flash compensation was quite important before, and being old-fogy, I still prefer to do it myself rather than to trust the automation. But today, cameras usually default to balanced flash mode (e.g., Nikon TTL BL or Canon Evaluative) which tries to automate it, and it has gotten better today. It certainly is a big help for those unwilling to think about what they are doing to help out themselves. So it's also very important to realize that cameras today probably default to balanced flash TTL mode, and what that means is that balanced flash mode tries to take care of this situation itself, automation computing what needs to be done. You can still compensate balanced flash to change the result it is getting, but that is a different idea.

Exposure Compensation adjusts ambient exposure of the camera auto modes, like A, S, P, and Auto modes. It does Not change camera Manual mode exposure, however, it does still affect what the light meter reads and shows, perhaps guiding you to adjust manual exposure yourself.

Flash Compensation adjusts metered TTL flash exposure. It does not affect manual flash. Manual is manual. But if the automatic TTL flash exposure is not coming out just right, too dark or too bright, then a bit of plus or minus EV Flash Compensation does the trick.

Auto ISO does affect exposure, including camera Manual mode exposure, and does affect TTL flash metering. Manual flash cannot react to Auto ISO changing, and Auto ISO will stay at Minimum ISO if camera recognizes a manual flash is present.

There may be multiple places flash compensation can be specified (on flash or on camera for example, or in Commander menu if used), and they all add to a total.

On Nikons, Exposure Compensation also adds to Flash Compensation so that TTL flash reacts with the sum total. However, newer models, i.e, D7200 and D810, have a new E4 menu to either does that add (EC + FC), or not. Canon models for example, do not. Pros and cons either way, individual control or collectively. Individually, we can think of Exposure Compensation as affecting the ambient in the background, and Flash Compensation as affecting the flash subject in the foreground. Or collectively, we can change the entire frame with one control.

Nikon likes to discuss flash picture exposure in those terms (background, subject, entire frame), but I think it is more clear in terms of ambient or flash. The flash power cannot reach to the distant background, therefore in general, the background is only ambient light, and the near foreground subject is flash exposure.

Some optimum choices for flash

Regardless of your choice, remember it is always important to monitor your TTL results, and use Flash Compensation as you see necessary to get exactly the result you want.

Continued - Fill Flash in Sunlight

Copyright © 2008-2021 by Wayne Fulton - All rights are reserved.

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