4. 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, they have different rules.
The camera system measures ambient and TTL flash separately and independently. The camera meter, and 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 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 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:
- The maximum shutter sync speed requirement is enforced, a Maximum shutter speed typically around a 1/200 second ballpark. Focal plane shutter speed cannot and will not go faster if flash is present, which is required by limitations of the focal plane shutter to sync with flash. (See Auto FP section)
- In camera A or P mode, the Minimum Flash Shutter Speed (commonly Nikon menu E2, default 1/60 second) is enforced in low light, so that ANY flash picture indoors probably always uses 1/60 second shutter. The dim ambient likely meters less, but this Minimum won't let the shutter go slower. As you reach up and turn the flash on, see the shutter speed jump up to the Minimum, usually 1/60 second. The purpose of Minimum shutter speed is that if you are using flash, you obviously don't need or want a real slow shutter speed, like 1/4 second.
The exceptions we see in camera A or P modes are if Slow Sync, or Rear Curtain Sync on Nikons - which ignore this Minimum Shutter Speed menu. The purpose of Slow Sync is to "Ignore any Minimum Shutter Speed menu, and use whatever slow shutter the dim ambient metering actually meters", no matter how slow. On Nikons, Rear Sync does too, because it only has meaning for a slow shutter that will cause motion blur, then it makes the blur trail follow the action, instead of lead it (Part 5). In contrast, camera M mode will allow any shutter speed at any time, but will not exceed Maximum shutter sync speed if the camera is aware the flash is present.
- Nikon camera P mode enforces a Maximum aperture value (around f/4) in dim light (charts on next page).
But these are merely limits, which are not about actually metering anything about flash. 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 we want).
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 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 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 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 orangeish 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. 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 choices of course, 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).
Understanding these few fundamentals will greatly simply and explain about everything you see happening with flash.
Camera exposure mode for flash
Some optimum choices (more detail follows) (and there is opinion here, any mode "can" work).
- Fill flash in sunlight - Camera P mode understands how to do this right, with regard to aperture, ambient shutter speed, and maximum sync speed in the sun. Otherwise, you have to figure out that you can only use Sunny 16, and in bright sun, probably f/16 at 1/ISO shutter speed, which cannot exceed maximum sync speed with flash. For automatic flash, TL BL balanced flash mode will know how to compensate the flash exposure lower to be for fill in sunlight. If TTL mode, you have to do this yourself, and -1.7 stops flash compensation is a good start for fill in bright sunlight. Camera P mode with TTL BL flash mode can make fill flash in sunlight be point&shoot automatic.
- Flash indoors, in the usual dim ambient places where flash is needed - Assuming the ambient exposure is hopelessly dim (insignificant, you need flash), and you prefer low ISO, then camera M mode obviously offers the most. TTL flash is still fully automatic flash exposure in camera M mode. If you set camera M mode, and set 1/60 second shutter speed, then this is exactly the same as camera A mode, which would have set 1/60 second too (that is all the camera A mode does with flash in dim light, it sets the minimum shutter speed of 1/60). We have just seen that the flash does not care about shutter speed. You still set the same aperture for both A or M mode, both are same thing then, maybe f/4 for bounce flash. The TTL flash is automatic flash exposure either way. But camera M mode allows setting any faster shutter too, up to maximum sync speed, for control of the ambient indoors. For automatic flash, TTL flash mode is arguably best indoors, or TTL BL if it is the only choice, it works too - sometimes BL just needs a little more compensation, but either may need compensation, so it may be a difference without a distinction (speaking of flash indoors).
- Regardless of your choice, remember it is always important to monitor your TTL results, and use Flash Compensation as necessary to get the result you want.
Fill Flash in Sunlight - Which is quite important, but it will require some compensation. There are a couple of major considerations:
- A proper exposure for the ambient is necessary, but sync shutter speed is limited for the flash. Ambient probably will require aperture to be up near f/16 in bright sun (Sunny 16). Camera P mode is good about understanding all of this, automatic with TTL BL flash, in bright sun. Or, if you want f/2.8, you can sometimes use Auto FP mode for wide apertures at short distance.
- Metering both ambient and TTL flash is two correct exposures, added, totaling one stop overexposure. We can underexpose the flash power (fill flash), or underexpose the ambient exposure a stop or two (highlighted flash).
- If using TTL BL flash mode, you need to realize that TTL BL mode tries to compensate the fill flash automatically (reduces flash to fill level). TTL flash mode does not - you have to do it yourself.
Fill flash in bright sun is an important big deal (at least for portraits of people).
Again, note if you are using fill flash in bright sun, the amount of Flash Compensation needed will depend on which flash mode you use.
- TTL mode: About -1.7 stops manual Flash Compensation is needed (for fill flash in bright sun). -1.7 EV is same as -1 ⅔ EV.
- TTL BL mode: TTL BL system tries to balance (reduce) fill level automatically, so maybe no extra compensation, at least not so much. Just turn it on (but always be alert to the need to watch results, and compensate a bit if needed).
- The Nikon Internal flash and the SB-400 and SB-700 have no TTL mode, and are always TTL BL mode (except if Spot Metering, which is TTL mode). The internal flash has low power and little distance range - which certainly can help, but the hot shoe flash has much more capability at sunlight exposure levels.
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 (if ISO 200).
This is a narrow window, with 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 P mode flash can adjust automatically when moving outdoors in sun, and then back indoors. But in contrast, indoors in dim light where we simply need flash, P mode simplicity gives up some options. Indoors, 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 (for bounce indoors), and then if in bright sun, the camera will simply fuss and refuse 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 1/ISO shutter speed, which cannot exceed maximum sync speed.
Fill flash in bright sun is a difficult problem, so we may need to seek some shade for our picture. Open shade or overcast is soft light anyway. 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, but we still cannot exceed maximum shutter sync speed. 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.
So then we might try Auto FP High Speed Sync mode (see Part 2B, HSS Auto FP) 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. The rear LCD on the SB-700, SB-800, SB-900 and SB-910 flashes will show that maximum distance range currently in effect.
Balanced flash mode
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 bright sun, if in TTL mode, we normally want to specify about -1.7 EV (stops) flash compensation, so the flash does not overwhelm. Specifically note that TTL BL mode helps us by doing this fill reduction itself, automatically. This is a big difference you should know and expect.
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 twenty 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:
TTL BL - Automatic Balanced Fill-Flash:
The flash output level is automatically
adjusted for a well-balanced exposure of the main subject and background.
TTL - Standard TTL flash:
The main subject is correctly exposed regardless of the background brightness. This is useful when you want to highlight the main subject.
Note: Spot metering always turns TTL BL off, to become TTL mode - which is only way SB-400, SB-700, internal popup flash, or Commander can choose TTL mode. SB-600, SB-800, SB-900, SB-910 have a menu to force TTL (if on hot shoe). Except for Spot Metering mode, the only current flash model that offers TTL override is the SB-910. Otherwise, the Nikon system default is TTL BL mode. However, Spot metering is always TTL mode (there is no concept of background when spot metering, it is only about the spot).
Implications which we should realize, flash in bright sun
Speaking of pictures of people in bright sun, the sun's shadows are dark and harsh, and fill flash makes dramatic improvements to smooth them.
Any Fill flash ought to be weaker than the main light, down a stop or two, to fill dark shadows somewhat, partially but not fully filled. Specifically, towards this end, TTL BL mode reduces flash exposure automatically, especially so in bright sun.
Otherwise, adding TTL mode flash gives TWO correct exposures, one from ambient and one from flash, which is two correct exposures, which add to overexpose the subject 2x, or by one stop. TTL mode does this by default (adds another correct exposure), which is what "without regard for background brightness" means. But TTL BL mode backs off (automatic compensation, for "well balanced exposure"), trying to prevent this by default. Indoors (insignificant ambient), this does not matter, TTL tries to get it right (and TTL BL might come out a little low). But for fill in bright sun, we know TTL ABSOLUTELY MUST use Flash Compensation of about -1.7 EV or so. Or if the flash is to be the main light (to make subject stand out from background, as opposed to fill), many users will manually underexpose the bright ambient a stop or two, and let TTL do its thing. We never want TWO correct exposures worth of light.
Two choices to prevent 2x TTL mode exposure in bright sun
Reduced Ambient - One alternative is to underexpose the bright sun ambient a couple of stops, and then let TTL meter the flash. This causes a darker background, and the flash will make the subject stand out strongly against it (a bit like a spot light on a stage). It is an interesting novelty look for some of your pictures. Stopping fast sports motion in bright ambient is another good reason to do it - due to maximum sync shutter speed, shutter speed is probably limited to ballpark of 1/200 second. But underexposing bright ambient a couple of stops helps prevent the ambient from blurring the fast sports action the speedlight can stop. It does require more flash power (to override the sun), so is easier indoors, like in gymnasiums. This is not fill flash. It highlights the subject.
Reduced Flash - Or, we can expose the sunlight normally, but then reduce the flash power exposure a stop or two, to provide a regularly lighted background and subject, but with subtle fill flash to slightly fill the dark shadows, for a very natural look. TTL BL flash mode attempts to do this automatically by default. TTL mode must be manually compensated to do it. -1.7 EV flash compensation is a good value for this in bright sun.
Or 3, use TTL BL mode instead (but you may still want to control a lesser compensation).
Compensating Fill Flash
Here are the quickest simple samples of flash compensation for fill flash in bright sun. All are the same ambient exposure, f/10 1/100 second, ISO 200. Nikon D300, camera A mode, center weighted metering (seeing substantial shadow areas). Hot shoe SB-800, for fill flash, as noted.
A clear day, 1:30 PM shade on the brick wall, camera facing East.
That is a White Balance card stuck in the bush, in the shade. Most of the picture is in shade, but the bush in sunlight is already overexposed a bit (Sunny 16 would predict by two stops, and the bush does have blinkies in it). I am only trying to show that the flash can easily fill the dark shadows. Note there are two lights added, the sun and the hot shoe flash. The sun makes shadows, which the reduced flash fills somewhat, to look more natural. Also judge the highlights on the bush, and the dark shadow at the far right of the card. WCV below is my "white card value", the actual RGB value on the card (0..255 range).
No flash, WCV 169 |
TTL BL 0 EV(BL does automatic Flash Comp.) WCV 229 |
TTL 0 EV(too much in the sun) WCV 255 |
TTL -2/3 EV Flash Compensation, WCV 242 |
TTL -1 1/3 EV Flash Compensation, WCV 237 |
TTL -1 2/3 EV,Flash Compensation WCV 232 |
TTL -2 EV Flash Compensation, WCV 226 |
Same TTL BL 0 EV, just repeated again down here |
Note that TTL mode 0 EV fill (no compensation) did illuminate the sun's shadows well, but it did not leave them as being shadows. It added flash shadows. The effect of the ambient is ignored by TTL mode, adding to make the highlights be overexposed (the white card is clipped at 255, as are the bushes). TTL mode works better indoors (where there is insignificant ambient), or of course, this is also what flash compensation is for. We just know to dial in about -1.7 EV flash compensation for fill in bright sun.
Compensation is our choice, but third stops are a subtle change to differentiate. And it is difficult to predict precisely what TTL BL automation (automatic flash compensation) will do, it varies with situation and exposure (but is often fairly close to acceptable). In bright sun, the automatic TTL BL compensation is usually ballpark of around -1.7 EV (1 2/3 stop, or a little more or less - closer to -2 EV here). Indoors in dim but meterable ambient (less contrast), maybe -1 EV (no bets though). We don't ever see that TTL BL automatic number, only the result. Technically, we do have more control in TTL mode, when we do see the number, and it is our choice. Ideally, when indoors with flash as sole light (insignificant ambient), the system ought to know to use 0 EV (not reduced), however TTL BL seems likely to slightly underexpose the flash then, where TTL does not. Indoors as sole source is not a fill flash situation, with no shadows to fill.
For TTL mode,with manual flash compensation, -1 2/3 EV (-1.7 stops) is a very common fill flash preference in bright sun. It is a preference, the way you want your picture to look, and normally, anything is better than nothing. But TTL -1.7 EV (or TTL BL) really works wonders for pictures of people in bright sun, to help lighten harsh dark shadows, looking natural, without looking obvious. But yes, you really should want to adopt fill flash. Compare same picture with and without fill flash, at a couple of levels, so you can truly appreciate the difference. Even the camera popup flash, which does not have great range, but if not too far, and that's all you have with you, it can really help for fill in bright sun. Try it.
The term Balanced Fill Flash does not mean flash level equal to ambient. It means sufficient flash level to help, but not enough to mess it up, or be identifiable as such. You can always add Flash Compensation also to TTL BL to adjust its results, but automation is a moving target, it does things on its own. If you are going to specify your own flash compensation, then why not use TTL mode instead? (then you have a zero base line, more predictable with only one cook salting the broth, no one to fight you). The difference is, TTL BL mode does this fill reduction automatically, and doesn't need much extra compensation, but TTL mode definitely will need maybe the full -1.7 stop flash compensation (in bright sun). Just don't misunderstand and try both -1.7 EV and TTL BL (which would be less effective).
Nikon's system is a TTL BL system, and it's internal popup flash, and the SB-400, and the SB-700, and Commander remote flash, are always TTL BL mode (when automatic flash, unless Spot Metering, which becomes TTL). You may need to realize that. The other flash models have a mode switch to force TTL mode. If you don't have menu choices showing both TTL and TTL BL, then your system default is TTL BL (unless Spot metering). Manual flash mode is of course whatever level you set.
What does TTL BL mode do, and Why does it do it?
The entire TTL BL concept (Automatic Balanced Fill-Flash) is about multiple light sources adding to be brighter than the brightest. Two equal light sources (which could be ambient and flash exposures), each properly exposed, add to 2x exposure, which is one stop overexposure. So, TTL fill flash in bright sun will seriously overexpose the near subject if without any compensation attention. We can manually compensate the TTL flash, or TTL BL mode tries to automatically compensate the flash to prevent this, but it has some limitations.
iTTL meters the preflashes individually, and meters ambient individually, so it never sees the total sum (and this added sum is variable, depends on positions and angles and shadows, overlapped coverage, ratio, etc). The camera metering cannot see this actual sum to know any details. The highlights ought to add, if aimed at same thing, but the shadows don't (and metering is a larger averaged area). TTL BL mode plays safe (underexposes), because it doesn't know, cannot see result.
The commander runs risk of multiple flashes and ambient too, lots of lights adding, so its pretty cautious, because digital does need to avoid overexposure. There's worse things, we can always crank it back up in post... best in Raw, but that's what higher ISO does. Or we can see result on the rear LCD, and compensate it.
It is all pretty much about two lights adding to be brighter than the brightest. In the studio with manual lights, we meter to set main to f/8 and set fill to f/5.6. We meter them together and set f/10 aperture to keep from overexposing the sum. ITTL would instead set f/8 and turn the fill flash down until it could work adequately. It cannot meter the sum. The sum of two equal lights combining perfectly could be as much as one stop greater than one of them (normally somewhat less though, ratios, shadows, etc).
We can set ratio in Commander (multiple lights), which affects sum addition, but which added result is unknowable to the system, so it plays very safe. I always start with +1EV FC (umbrellas indoors). Instead of Main +0 and Fill -1EV and +1 EV FC - instead I usually use Main +1 EV and Fill 0, which is same thing.
TTL BL or balanced fill flash or whatever you call it, has the one job to crank the flash down to minimize overexposure of the sum. Humans can think of it balancing flash with background if that's easier to grasp, but that is really not what it does. That may be a result, but the intent is more basic than that. The background simply does whatever ambient meters, and whether the flash is present or not has no effect on it. It's not about backlighting or any of that stuff. We humans can only see what we can see, which confuses us, we can imagine things. :) The camera has no clue what or where the subject is, but the flash does try not to blow the center out. It's nothing new, and not rocket science. There is only one ambient exposure and one flash power value, so it's not that hard (except iTTL system cannot see the sum. Human photographers can, on the LCD). The Commander might use two flashes and no ambient, which is still two lights summed. System sets them equal (compensated ratio), and they add, but sum cannot be metered. We tweak exposure with flash compensation.
There is one more difference for TTL BL mode, only if using direct flash on the hot shoe (flash head not tilted). Nikon TTL BL Direct Flash sometimes uses the D lens distance information (the lens reports the focused distance). Nikon doesn't explain this well, but simple experiments clearly show that the D lens can sometimes be a safety check to prevent overexposure for overexposed meter readings (TTL BL and direct flash). See a TTL BL and D lens Test showing that.
TTL mode does not worry about any sum, the flash just does its own thing without concern, which is perfect for one light indoors, but which will be unacceptable as fill in bright sun, until we manually compensate it lower. No big deal, we have just always known to do this. Concept of fill is a lower light level anyway, hopefully not even noticeable to us. But if we do this to highlight or make the subject "stand out", we have to start with ambient a stop or two down, because two proper exposures add to overexpose a stop. It is all about that sum of two lights adding.
It seems pretty clear what it does. It seems pretty clear what it has to do. It is just basics.
But the above is the meaning of these words from the flash manuals:
TTL BL - Automatic Balanced Fill-Flash:
The flash output level is automatically adjusted for a well-balanced exposure of the main subject and background.
TTL - Standard TTL flash:
The main subject is correctly exposed regardless of the background brightness. This is useful when you want to highlight the main subject.
This actually reads pretty coherently if you interpret background as ambient, and subject as flash (what else could they mean?), and if you understand that any two lights add to be brighter than the brightest. Some of the camera manuals (section about internal flash) add the word ambient for TTL BL: "for natural balance between main subject and ambient background". Basically, TTL BL turns the flash down. TTL does not.
TTL BL mode typically is for fill flash outdoors in daylight, when there is enough ambient light to give a good regular exposure. The fill flash is added and matched (reduced) to balance (not overexpose) that background exposure. The camera meters the background for proper exposure, and the flash is balanced (reduced) to that. The flash power is typically reduced a good stop, maybe more, as needed in sunlight. TTL BL often seems slightly underexposed indoors, but all auto flash needs to be watched for the need of compensation.
Some people resort to Spot metering (forces TTL mode) to bring the usual underexposure of Commander TTL BL back up. Understand that Spot metering is not a general purpose metering method. It just makes that one spot come out middle gray (should that spot even be middle gray?), instead of making the overall image average be middle gray. Seems better in general to address the situation directly, and just use maybe + EV flash compensation, if needed. It often needs to be done anyway, and will have greater effect. Flash compensation is something we MUST learn.
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.
Continued - Flash Indoors, and Auto ISO
Copyright © 2008-2013 by Wayne Fulton - All rights are reserved.
