Confused about exposure/pre-flash when bouncing

My D90/SB-700 setup gives very good results, as you'd expect.

If I'm pointing the flash head on, the results are near perfect. But the more I bounce the flash, the 'darker' the images become.

Am I correct in thinking that during the whole 'cycle' of taking a photograph, the SB-700 emits a 'pre-flash' which is registered by the D90, which then sets the exposure accordingly prior to opening the shutter, at which point the main flash occurs ?

If so, why does this not work when I'm bouncing the flash ? Clearly less flash light will arrive at the camera when I'm bouncing, but does my D90 not register a weaker 'pre-flash' and compensate accordingly ?

I'm just surprised that I'm having to remember to manually adjust the exposure compensation up a few stops when I'm bouncing the flash. I don't want to sound lazy, it's just that there isn't always time to fiddle with buttons when you're photographing events.

Your flash might not have enough power to provide sufficient
'bounced' light when using smaller apertures.
Open the aperture or use a higher 'ASA'.

jeffcpix said:

Your flash might not have enough power to provide sufficient
'bounced' light when using smaller apertures.

Click to expand...

And even when NOT using smaller apertures, the flash might run out of power! ;-)

Even a good white ceiling reflects only 1/4 of the light that hits it, and, if it is some distance up to the ceiling, the light incident on it will be less than that previously sent straight to the subject.

Furthermore, if the ceiling is a long way up, then it is a long way down again to the subject and the light loses more strength on the return trip.

Moral of this story: Bounce flash uses up a lot of power... quite often more than you have got, especially if the ceiling is NOT a "good white one." Of course, the flash will do its best, and give all it has got....

.... but when it has done so, it hasn't got any more to give, and you have to take other steps... like switching back to pointing the flash directly at the subject, or arranging for an efficient bounce surface that is much closer to flash and to subject.

jeffcpix said:

Open the aperture or use a higher 'ASA'.

Click to expand...

We are calling the numbers 'ISO', now... but don't panic about it. The numbers themselves have not changed.
--
Regards,
Baz

"Ahh... But the thing is, they were not just ORDINARY time travellers!"

Thanks Jeff/Barry,

I'm know how to manually compensate for the fact that less light reaches the camera when bouncing the flash - what I don't understand is why the camera or flash-gun doesn't compensate automatically.

Hence my original question about the pre-flash. Is this not how the camera and iTTL flash work out how to provide the correct exposure ?

When the flash is pointed forwards it goes by the subject distance, as reported by the lens focus distance. That's why forward-facing flash is so accurate. In fact, you can fool the flash by manually adjusting the focus point nearer (too dark) or further (too bright) from the point of focus.

If you are able to get the result you want by adjusting exposure compensation, then you have not run out of flash power. So the problem must lie elsewhere.

Please describe what you mean by "the more I bounce the flash..." I don't understand what that means. Please describe the shooting conditions...subject distance, ceiling height, angle and direction of the flash head, color of ceiling and walls, your aperture setting, shooting mode, and any other flash settings that might be set (example images would be nice too.) Also tell us if you were near any sources of light like a window during the day.

Here's a suggestion. Next time this happens, and after you've adjusted Exposure Compensation, turn the flash off and take the same picture. You might get some motion blur, but that's okay. What you're really checking for is the rendering of the scene luminance. If the ambient exposure appears the same as the flash exposure, then it simply means that your scene requires compensation. If you're shooting test shots in a room with white walls, then yeah, you'll have to apply +2 EC to get the room to look white. That's basic exposure.

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Graystar said:

When the flash is pointed forwards it goes by the subject distance, as reported by the lens focus distance. That's why forward-facing flash is so accurate. In fact, you can fool the flash by manually adjusting the focus point nearer (too dark) or further (too bright) from the point of focus.

Click to expand...

Right, that probably explains it then. I've been shooting in a small room at home, and when I've been pointing the flash at the ceiling, or walls, they are probably the same distance from the camera as the subject I'm photographing directly ahead of me. So this probably means the flash is outputting at the same power regardless of where I point it. I was expecting this to work based on the (probably false) information that the SB-700 emits a pre-flash that enables the camera to control the flash output accordingly (or control exposure by opening the aperture more). So I'd assumed I'd get similarly exposed shots regardless of where I point the flash (up to be point the flash lacks the power to illuminate sufficiently).

Graystar said:

If you are able to get the result you want by adjusting exposure compensation, then you have not run out of flash power. So the problem must lie elsewhere.

Please describe what you mean by "the more I bounce the flash..." I don't understand what that means. Please describe the shooting conditions...subject distance, ceiling height, angle and direction of the flash head, color of ceiling and walls, your aperture setting, shooting mode, and any other flash settings that might be set (example images would be nice too.) Also tell us if you were near any sources of light like a window during the day.

Click to expand...

I wasn't explaining this well. I just meant I was trying everything from pointing the flash forwards at 45° to the ceiling, to pointing it backwards into a corner.

Graystar said:

Here's a suggestion. Next time this happens, and after you've adjusted Exposure Compensation, turn the flash off and take the same picture. You might get some motion blur, but that's okay. What you're really checking for is the rendering of the scene luminance. If the ambient exposure appears the same as the flash exposure, then it simply means that your scene requires compensation. If you're shooting test shots in a room with white walls, then yeah, you'll have to apply +2 EC to get the room to look white. That's basic exposure.

Click to expand...

Yes, it's a room with white walls. I can see how this gives slightly false impressions about whether shot is well exposed, a bit like taking a photo in snow and thinking you need to adjust levels to get the snow white again !

Graystar said:

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Click to expand...

BenGreen said:

Graystar said:

When the flash is pointed forwards it goes by the subject distance, as reported by the lens focus distance. That's why forward-facing flash is so accurate. In fact, you can fool the flash by manually adjusting the focus point nearer (too dark) or further (too bright) from the point of focus.

Click to expand...

Right, that probably explains it then. I've been shooting in a small room at home, and when I've been pointing the flash at the ceiling, or walls, they are probably the same distance from the camera as the subject I'm photographing directly ahead of me. So this probably means the flash is outputting at the same power regardless of where I point it.

Click to expand...

No...the use of distance only occurs when the flash is pointing forward. As soon as you rotate the head to bounce, the flash will only use the pre-flash reading to set exposure, as it can't possibly know the distance that the light has to travel to the subject. In this mode, the metering system is simply trying to make everything gray (which is what meters do for living.)

BenGreen said:

Yes, it's a room with white walls. I can see how this gives slightly false impressions about whether shot is well exposed, a bit like taking a photo in snow and thinking you need to adjust levels to get the snow white again !

Click to expand...

Yeah, I think that's your problem. Nothing wrong here...you simply need to apply appropriate compensation for the tone of the scene. The same thing should happen with an ambient exposure.

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Graystar said:

When the flash is pointed forwards it goes by the subject distance, as reported by the lens focus distance. That's why forward-facing flash is so accurate. In fact, you can fool the flash by manually adjusting the focus point nearer (too dark) or further (too bright) from the point of focus.

Click to expand...

I don't think you wrote that exactly the way you meant it. When pointed straight forward the camera uses the distance information to help determine the correct exposure in TTL mode. The camera still uses preflash information as the main means of determining flash output. The way you wrote this makes it sound like distance is the sole determiner of flash output, which is going to confuse the OP.

--
Mike Dawson

michaeladawson said:

Graystar said:

When the flash is pointed forwards it goes by the subject distance, as reported by the lens focus distance. That's why forward-facing flash is so accurate. In fact, you can fool the flash by manually adjusting the focus point nearer (too dark) or further (too bright) from the point of focus.

Click to expand...

I don't think you wrote that exactly the way you meant it. When pointed straight forward the camera uses the distance information to help determine the correct exposure in TTL mode. The camera still uses preflash information as the main means of determining flash output. The way you wrote this makes it sound like distance is the sole determiner of flash output, which is going to confuse the OP.

--
Mike Dawson

Click to expand...

Thanks for clarifying that Mike, but also many thanks to Graystar. This is all really useful info. Maybe I'm just weird wanting to know this, but this is exactly the sort of information I would hope to find in the user's manual. I just need to know how things work and why they do what they do.

It all seems obvious now, i.e. when I bounce the flash head onto a white wall or ceiling, the camera is using the pre-flash to determine the correct exposure, which in this case leads to a grey wall (which is technically a correct exposure for a white object).

Thanks for your patience and time spent helping. I should have said/admitted at the start that I'm a flash novice, having only used the built-in flash before, and also a non-TTL Auto Thrystor Vivitar.

I understand this photography stuff pretty well.

You wrote > to remember to manually adjust the exposure compensation up a few stops

And in your very first message, the question that puzzles you puzzles me, too, in the context of what you described.

If you're willing to keep going on this...

1/ When you say your photos with the bounced flash are too dark, are you talking about a main subject that fills most of the frame, all pretty much the same distance from the camera, or are you talking about a fairly small subject a fair distance in front of a white wall, so much of your ferame is the wall, not the subject?

Flash metering systems ready vrious amounts of the scene, and I believe the D90 can be changed so it reads more or less of the scene.

SO WHAT? you ask.

If the camera is reading a lot of wall, yes the shot will go dark.

If the camera is just seeing the main subject, there's something wrong if there's an 8 foot ceiling and the subject is 5- 10 feet away and you are at ISO 200 and f8 or wider.

Greater distances, or more white walls int he frame, or higher ceilings all could make the shot dark, too.

Do you allow enough time for the flash to recharge? You could get one good shot, and then the second, takien very soon after, could be dark because the flash is no running out of power.

RE> to remember to manually adjust the exposure compensation up a few stops

And you've never told us (I think) what the camera exposure mode (S, A, P, M) and aperture and metering pattern (matrix, centerweighted, etc.) are.

BAK

BenGreen said:

I should have said/admitted at the start that I'm a flash novice, having only used the built-in flash before, and also a non-TTL Auto Thrystor Vivitar.

Click to expand...

Hey! No shame in that!

It happens that I've always held the simple Auto Thyristor control of the classic Vivitars in high regard... often performing better than non-distance applying versions of TTL, even with bounce. I know of other pros who feel the same. On the other hand, when they do fail it is invariably UNDER-exposure that happens.
--
Regards,
Baz

"Ahh... But the thing is, they were not just ORDINARY time travellers!"

michaeladawson said:

Graystar said:

When the flash is pointed forwards it goes by the subject distance, as reported by the lens focus distance. That's why forward-facing flash is so accurate. In fact, you can fool the flash by manually adjusting the focus point nearer (too dark) or further (too bright) from the point of focus.

Click to expand...

I don't think you wrote that exactly the way you meant it. When pointed straight forward the camera uses the distance information to help determine the correct exposure in TTL mode. The camera still uses preflash information as the main means of determining flash output. The way you wrote this makes it sound like distance is the sole determiner of flash output, which is going to confuse the OP.

Click to expand...

No, the main means to determine exposure in forward facing TTL-BL mode flash is distance. Otherwise, the flash power wouldn't be so affected by the focus distance. In TTL mode, distance isn't used.

If you set the camera to forward facing TTL-BL flash and take a picture of a white subject with a white background, the image comes out white. If you switch to TTL mode, where distance isn't used, the image comes out gray.

In forward facing TTL-BL mode, a preflash is used to determine a power reduction amount for the distance-dictated flash power. This works regardless of the tone of the subject or the effects of reflections from walls. The camera will then reduce the flash power by the amount of light it thinks is already on the subject. This luminance-difference calculation is simply a correction to the real power calculation...which is based solely on distance. that's why manually changing the focus distance affects the exposure so greatly.

When bouncing light, the flash has no idea how much flash power is required to properly illuminate the subject. Here, the camera meters the subject and then meters the preflash to see how much the luminance changed. Then it determines how much more power is needed to register a standard exposure. That exposure may or may not be correct...it's simply 18% gray (or there abouts...depending on the metering mode) so the white subject comes out gray. It's a less accurate method.

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if bouncing, you might want to check this video out,
http://neilvn.com/tangents/2011/03/08/video-using-the-black-foamie-thing/
I tried it on my D300 with SB800 in TTL, with a little practice, works great.

Dennis J

Why do you bring TTL-BL mode into the discussion? BL mode is fill mode. I don't recall the OP saying anything about fill flash.

In regular TTL mode flash output is determined mainly by reading exposure information in the camera. Distance readings are not a major determiner of flash output. The exposure starts, the camera measures during the exposure, and sends a signal to the flash to turn off when enough light has reached the sensor.

You can prove this by standing in one place and taking a flash photo. Now put a light robbing diffuser over the flash head without obstructing any of the other sensors on the flash. Take a second photo from the exact same spot of the same subject. You will notice that the flash pumps out more light and the exposure remains the same. If distance was the primary determiner of flash output the flash would put out the same amount of light and the photo would be underexposed.

--
Mike Dawson

michaeladawson said:

Why do you bring TTL-BL mode into the discussion? BL mode is fill mode. I don't recall the OP saying anything about fill flash.

Click to expand...

First, TTL-BL, despite Nikon's confusing description, is not about fill. TTL-BL, while being good anytime, is best for indoor flash use where bright walls and other reflective surfaces may throw off exposure. As Nikon says...

"This mode is especially effective when shooting scenes that include a mirror, white wall, or other highly reflective surfaces"
From the SB-800 example photos booklet.

Second, the SB-700 operates in TTL-BL mode fulltime unless Spot metering is selected. There's no way to select iTTL on the flash. The fact that the SB-700 always operates in TTL-BL mode is further proof that it's not about fill.

michaeladawson said:

In regular TTL mode flash output is determined mainly by reading exposure information in the camera. Distance readings are not a major determiner of flash output.

Click to expand...

In regular iTTL mode distance contributes zero to flash output...it isn't used at all.

michaeladawson said:

The exposure starts, the camera measures during the exposure, and sends a signal to the flash to turn off when enough light has reached the sensor.

Click to expand...

That's not how it works these days. That's easy to prove just by covering the lens after half-pressing the shutter. The flash will not fire at full power (providing, of course, that full power wasn't required in the first place.) That's because it knows the maximum power needed for forward facing flash. For bounce, you can use FV Lock to prove that flash power is determined before hand. Just use FV Lock to fire a preflash and then cover the lens. The same flash power is used as if the lens wasn't covered. So the flash power isn't determined during exposure.

michaeladawson said:

You can prove this by standing in one place and taking a flash photo. Now put a light robbing diffuser over the flash head without obstructing any of the other sensors on the flash. Take a second photo from the exact same spot of the same subject. You will notice that the flash pumps out more light and the exposure remains the same. If distance was the primary determiner of flash output the flash would put out the same amount of light and the photo would be underexposed.

Click to expand...

But if you perform that test by focusing, release the shutter, switching to manual focus, putting the lens cap on the lens, and then fully pressing the shutter, the flash does not fire at full power...not even close. It fires at the full power required for the distance/aperture/ISO combo.

.

I do not own a SB700 and did not know it was always in TTL-BL mode. I thought the article might help the OP in bouncing flash, the principal seems to be that by using (manual setting on SB700) with this black foam in place of using a typical diffuser or none at all, no light is lost by spilling in other directions, instead all light goes directly to the bounce wall or whatever, which seems to increase the amount of power the flash is actually putting out.

Not at all trying to confuse the OP just informing him of something that worked really well for me when bouncing flash in TTL on both SB800 and Nissin Di866.

Dennis J

Graystar said:

If you set the camera to forward facing TTL-BL flash and take a picture of a white subject with a white background, the image comes out white. If you switch to TTL mode, where distance isn't used, the image comes out gray.

Click to expand...

The simplest tests show this is not true. It would be true if it were about black instead of white. The D lens distance info is used by TTL BL mode in direct flash, but apparently only as an overexposure safeguard (guide number override). Therefore, it can prevent black frames from being overexposed to gray, but it does not affect white frames, which still come out gray. Simple observation confirms this (black vs white paper backgrounds, etc).

There are many fanciful notions about how this TTL stuff works, but most miss the mark of matching results.

My fanciful notion: TTL BL is NOT actually about subject fill, and is NOT about backlight, or background, not about "balancing" flash with ambient, or any of that, not as such. Stupid camera computer cannot even recognize what is the subject, and has no clue how it ought to look.

The physics is much simpler. It is about any two lights adding, to be brighter than the brightest. TTL flash mode will properly expose the subject distance regardless of ambient, and the camera meter will properly expose the continuous ambient. That is two proper exposures on subject, 2x exposure, and one stop overexposure. Guaranteed. So we just learn that TTL fill in bright sun needs about -1.7 EV flash compensation (for this reason). TTL BL mode tries to do that for us, and it backs off on the flash to minimize this overexposure, for this reason. TTL mode does not.

Direct flash falls off with the inverse square law, i.e., the background is necessarily dark, and preflash meters it that way. Bounce flash fills the room, generally most of the room is more equal distance from the ceiling, the far walls are brighter, and preflash meters it that way. All things are made to be a middle gray result, so darker direct flash is prone to overexpose, and brighter bounce is prone to underexpose. Seems very simple, but is not anything that the camera can figure out.

Film TTL flash always seemed more accurate than digital (my notion). This is because film TTL metered the final result (from all lights) in real time at the film surface, and simply terminated the flash when actual exposure was sufficient. iTTL cannot do that, it has to meter a weak preflash, and then set the flash level to be maybe 6 stops more light than preflash. This process is very much less precise, esp with speedlight truncated duration trying to mimic a power level. Digital is a giant step forward, but not sure that preflash is.

BenGreen said:

My D90/SB-700 setup gives very good results, as you'd expect.

If I'm pointing the flash head on, the results are near perfect. But the more I bounce the flash, the 'darker' the images become.

Am I correct in thinking that during the whole 'cycle' of taking a photograph, the SB-700 emits a 'pre-flash' which is registered by the D90, which then sets the exposure accordingly prior to opening the shutter, at which point the main flash occurs ?

If so, why does this not work when I'm bouncing the flash ? Clearly less flash light will arrive at the camera when I'm bouncing, but does my D90 not register a weaker 'pre-flash' and compensate accordingly ?

I'm just surprised that I'm having to remember to manually adjust the exposure compensation up a few stops when I'm bouncing the flash. I don't want to sound lazy, it's just that there isn't always time to fiddle with buttons when you're photographing events.

Click to expand...

On the SB-900 there is a icon that will display if the flash is not powerful enough to apply enough light to properly expose the image. It is a triangle with a - sign next to it. You might want to check your flash LCD to see if you have a similar icon showing when you get the underexposed image (I'm not familiar with the SB700). I too agree that it sounds more like a exposure problem than a flash power problem, but that icon is a helpful indicator.

My Sunpak 555's are a dammed sight more powerfull and accurate for bounce flash with the sensor pointing at the subject than my Canon 580exII shooting at 400 iso. Vivitar 283 is also more accurate shooting at 800 iso. F stop is f8 for both.

Film style TTL metering of flash worked beautifully -- assuming
you had a flash that was powerful enough.

The current method for digital (preflash) sucks by comparison.
But one 'pop' and the results speak for themselves -- instant feedback
via LCD screen... and so much cheaper than Polaroids.