TTL flash question

[Qarik]

okay so I understand the basic concpet of TTL flash. The flash fires and lthe amount of light that returns from the subject is calculated until the camera logic determines correct exposure and the shut ths flash off.

So a basic question.. assume you are shooting manual and you set the shutter speed and aperature to x and y, How is it that you still get overexposed shot?

When your camera meters..if it know the shot will be overexposed then it shouldn't even fire the flash correct? I can see why the over exposure might happen in this case if the the shutter speed was long for example.

But if it meters and determines that flash is needed..you can still get some grossly overexposed shots. Is that because if the flash fires there is some "minimum" output of light that it sends out (which makes sense because I assume you can't flash at say 1/100000 of a sec) and the min amount already is too much?

[ziggy53]

Qarik,

Specifically which flash are you using?

On what camera are you using it?

Do you have any examples, with full EXIF?

[pathfinder]

On Canon and I think Nikon, if you set your shutter speed faster than native flash synch speed, and do not set the flash up for High Speed Synch, the flash resets your shutter speed to the max synch speed ( despite being in Manual Mode ) - about 1/200th or so, and this can result in an overexposure by ambient light if you needed a shutter speed fo 1/500th say for ambient.

This trips lots of folks up at first ( DAMHIK), until they catch on to high speed synch. Manual mode only fully controls shutter speed with High Speed Synch enabled.

[Qarik]

I own d90 and sb800..and I don't have a specific example or exif data. Just a general observation. You can definitely overepose with a TTL flash even in compositions that are not that tricky to meter.

[Qarik]

On Canon and I think Nikon, if you set your shutter speed faster than native flash synch speed, and do not set the flash up for High Speed Synch, the flash resets your shutter speed to the max synch speed ( despite being in Manual Mode ) - about 1/200th or so, and this can result in an overexposure by ambient light if you needed a shutter speed fo 1/500th say for ambient.

This trips lots of folks up at first ( DAMHIK), until they catch on to high speed synch. Manual mode only fully controls shutter speed with High Speed Synch enabled.

ahhh..but the camera can not distinguish between ambient light or flash light..photons are photons. If the shutter speed is reset at 1/200 and the camera knows this and you get an overexposed shot. then either:

1) the metering was off due to some tricky subject matter
2) the flash should have never fired
3) the min flash was "too much" and caused over exposure.

Am I thinking about this correctly?

[ziggy53]

Without seeing an example I can only guess what might be happening and pose some possibilities.

Like Pathfinder said, it's possible that if you start with the flash off and set your manual ambient exposure, turning on the flash might force the shutter speed to a slower speed and subsequent over exposure. The pre-flash might still go off, but the primary flash might not.

Nikon i-TTL partly uses distance information from the lens, assuming that the lens has a distance chip. If the lens is older and does not relay distance information, the system defaults to TTL and that system can be fooled in a number of circumstances (which is why i-TTL was developed.)

Beyond these scenarios I can't think what might be happening, but an example with EXIF could give a hint.

[pathfinder]

ahhh..but the camera can not distinguish between ambient light or flash light..photons are photons. If the shutter speed is reset at 1/200 and the camera knows this and you get an overexposed shot. then either:

1) the metering was off due to some tricky subject matter
2) the flash should have never fired
3) the min flash was "too much" and caused over exposure.

Am I thinking about this correctly?

No, you are incorrect.

Photons are photons, but the shutter speed has been changed and the camera does not readjust the shutter speed because it is in Manual Mode, not Av or P. In effect, the camera in Manual Mode acts as if YOU dropped the shutter speed back to a max synch speed ( 1/200th or 1/250th depending on your camera), not the flash. The flash lighting is not the problem.

The over exposure is from ambient light, not from the flash which will be minimized to the maximum possible.

I said if the ambient exposure at a given aperture, needs a shutter speed of say 1/1000th in Manual mode to properly expose the ambient lit background. Now turn on the flash in iTTL, and the shutter speed is reset to 1/250th to allow the flash to synch ( no High Speed Synch turned on ) and you are 2 stops over exposed before the flash ever goes off.

This is not a ever a problem in dim light, but in bright sunlight it can occur frequently. Turn on High Speed Synch, and the problem is solved, you can now light the background with ambient and the subject with the flash at 1/1000th of a sec and the background is not longer over exposed. YOu can even under expose the background 1 or 2 stops say with a shutter speed of 1/4000th. This is really cool!!

I have had this kind of over exposure when shooting macros because it is so easy to need high shutter speeds in sunlight and the flash reset my shutter speed without my noticing because I thought the shutter speed was fixed/stable/unchanged by the camera in Manual Mode, but found that was not 100% true with flash all the time. There is no warning in the viewfinder that the shutter speed has been altered.

If you meter of a large black tuxedo, you run the risk of over exposing a white wedding dress in the same frame, as the flash will try to create a flash exposure to capture a 'grey' tuxedo if allowed to. This requires Flash Exposure Compensation, but that is a different topic.

[swintonphoto]

Either there is too much dark stuff in the image making it over expose, or your manual settings are already overexposing and the flash is just adding insult to injury.

[dbd]

ahhh..but the camera can not distinguish between ambient light or flash light..photons are photons. If the shutter speed is reset at 1/200 and the camera knows this and you get an overexposed shot. then either:

1) the metering was off due to some tricky subject matter
2) the flash should have never fired
3) the min flash was "too much" and caused over exposure.

Am I thinking about this correctly?
I think you are making an incorrect assumption in your reasoning. Early film camera implementations of TTL flash control like that in the Olympus OM series starting in the OM-2 performed flash control by turning off the flash after enough light had reached the film. Modern digital electronics perform the function by firing a pre-flash to detetmine how much light to direct the flash to generate later at the time of exposure. See page 3 of:

http://www.manuals.qtm.com/P635A.pdf

This is the manual for a flash that can work with TTL flash control for a number of manufacturers.

The electronics in the camera can tell the difference between flash and ambient light because the time of the test flash is known and the light at the time of the test flash can be measured seperately from the ambient without flash.

Dale B. Dalrymple
http://dbdimages.com

[jeffreaux2]

Aside from the synch speed info you have already gotten, I have seen once where a camera was severely over-exposing some flash shots. This was in conjunction with an off camera shoe type cord. It did not replicate the overexposure when the flash was mounted directly in the shoe. After reattatching the cord everything worked normally. I supposed that the cord had not been attatched correctly or that some contacts had not been made.

FWIW, I use ETTL flash ALL of the time. You would be hard pressed to get a shot to overexpose unless.....

-You are shooting in a situation where you should be using high speed synch ( Like AV mode with an F2.8 aperture set in bright sunny light and your trying to use flash for fill...The proper shutter speed may be something like 1/2000, but if no HSS is selected the default will be 1/250....resulting in a white frame.

-The Flash exposure compensation is cranked way up (this may not cause an overexposure...but I guess its possible)

-The exposure compensation is set in the pluses.

In "M" mode (manual) You really shouldn't be seeing overexposure issues. Not with a Canon flash...on a Canon camera.

[Qarik]

okay I have to clarify. I did some more testing. indeed I was overexposed because I was shooting f/1.4 and the ambient was already overexposed...but cureisouly I was still getting a weak flash going off. I assumed the TTL/metering would keep the flash off...so a more pointed question:

say I have aperture at 1.4 and the shutter speed at 1/200, I meter the shot, and the flash goes off and I get a well exposed shot.

Now say I change my shutter speed to 1/60 for example. Now when I meter the shot with 1/2 depress of the shoot button..does the camera know that I changed my shutter speed and will over expose and shouldn't it not flash? Or does the metering happen independely of what the shutter speed is and so the flash will go off regardless (assuming the subject is dimly lit) but the flash turns off very early due to the TTL. I think this is what is happening.

[jeffreaux2]

okay I have to clarify. I did some more testing. indeed I was overexposed because I was shooting f/1.4 and the ambient was already overexposed...but cureisouly I was still getting a weak flash going off. I assumed the TTL/metering would keep the flash off...so a more pointed question:

say I have aperture at 1.4 and the shutter speed at 1/200, I meter the shot, and the flash goes off and I get a well exposed shot.

Now say I change my shutter speed to 1/60 for example. Now when I meter the shot with 1/2 depress of the shoot button..does the camera know that I changed my shutter speed and will over expose and shouldn't it not flash? Or does the metering happen independely of what the shutter speed is and so the flash will go off regardless (assuming the subject is dimly lit) but the flash turns off very early due to the TTL. I think this is what is happening.

I would think that by lowering the shutter speed THAT much (nearly 2 stops) your background....which is lit primarily by ambient light .....will have a significant increase in exposure. The foreground (lit by flash) should see some increase in the exposure of the shadows, but if using ETTL, the highlights should expose the same. This would be automatically calculated by the camera/flash and controlled by flash duration.

To my knowledge, if the flash is in the hotshoe.....it will fire. If there is enough ambient light that the flash really isn't needed, then the flash will still fire.

[dbd]

okay I have to clarify. I did some more testing. indeed I was overexposed because I was shooting f/1.4 and the ambient was already overexposed...but cureisouly I was still getting a weak flash going off. I assumed the TTL/metering would keep the flash off...

In TTL mode there is still the pre-exposure metering test flash that measures whether, and how much, flash is needed during the exposure.

Dale B. Dalrymple
http://dbdimages.com

[Qarik]

In TTL mode there is still the pre-exposure metering test flash that measures whether, and how much, flash is needed during the exposure.

Dale B. Dalrymple
http://dbdimages.com

The pre-exposure meter flash test..the manual says that this flash is imperceptible though it does happen.

[dbd]

The pre-exposure meter flash test..the manual says that this flash is imperceptible though it does happen.
It is imperceptible when the main flash does fire.

Dale B. Dalrymple
http://dbdimages.com

[ziggy53]

The pre-exposure meter flash test..the manual says that this flash is imperceptible though it does happen.

Understand that the pre-flash does not contribute to the exposure. The pre-flash is only used to determine what the flash contribution "should be".

[time2smile]

Try the pre flash by itself, I think its FN, also are you in TTL BL or TTL. I have noticed that both give significant differences in exposures.