Why Do Modern Digital Cameras Flash Twice?

[rootone]

I've noticed that when somebody takes a photo of me with a modern digital camera, I experience not one flash but two.
Are there really two flashes for some reason or is it a perception thing?

 

[berkeman]

I've noticed that when somebody takes a photo of me with a modern digital camera, I experience not one flash but two.
Are there really two flashes for some reason or is it a perception thing?

It's to help cut down on red eye...

http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/995/INTERNAL-FLASH/red-eye-reduction.html

 

[rootone]

Ah good, thanks, and I guess it's selectable option for the camera user.
It doesn't bother me but I'm glad to hear I'm not suffering from some kind of visual cortex mess up.

 

[berkeman]

It may also be used to help the camera fine-tune the exposure, but that would be secondary to the red eye reduction function, I believe.

 

[meBigGuy]

There are auto focus assist things that might be happening also (usually a separate light), but it is most likely a pre flash for red-eye reduction as said above.

 

[roberttrotter]

Camera flash used to measure the flash power needed to get an appropriate exposure.

 

[William White]

Also,

When you see many flashes (not just two) it is fast-sync; where the flash is popping continuously during the entire exposure, from the front shutter curtain opening to the rear curtain closing.

It allows the use of flash with shutter speeds faster than the cameras sync speed (for 35mm frame, often 1/60th or 1/125th sec , sometimes 1/250th sec for newer cameras).

 

[sophiecentaur]

Camera flash used to measure the flash power needed to get an appropriate exposure.

That can be worked out during the period of just one flash and you can then turn it off when you have gathered enough energy for an exposure. Otoh, focussing takes time.

 

[meBigGuy]

If the OP posts a camera model that exhibits this behavior we can probably pin it down (if he cares). But at least he knows it can be considered normal behavior.

 

[rootone]

No that's OK, I just wanted to understand the purpose of the double flash, and I've seen this with more than one camera model.
There have been enough satisfactory explanations now, thanks guys.

 

[olivermsun]

That can be worked out during the period of just one flash and you can then turn it off when you have gathered enough energy for an exposure. Otoh, focussing takes time.

That used to be the case with film (meter off the reflection from the film), but digital sensors typically don't have the same reflectivity -- now a pre flash is typically used and metered off the shutter curtains before they open. This ought to be extremely quick, though. Very noticeable pre flashes are probably intended as red-eye reduction (albeit not very effective) as several posters have already said.

 

[William White]

That used to be the case with film (meter off the reflection from the film),

brilliant

that was first introduced on the OM2 was it not? (Imho the OM line were the best 35mm cameras ever made and the fast zuikos the best manual lenses . period. (how did they makes such fast high resolution lenses so small?)

The OM4 had the best metering of any camera I have every used. Enormous, bright, viewfinder and multispot metering - you spotted where you wanted the camera to meter, be it a 18%, a highlight or shadow (the highlight and shadow buttons were right next to the shutter button so you didnt need to take your eye off the viewfinder) and the spots were all arranged on the exposure bar at the bottom of the viewfinder. There has not been a camera before or since with such brilliant, quick, intuitive, accurate exposure. I can't even understand why olympus themselves did not keep this functionality for the digital OMS. (it was also the first 35mm horizonal shutter camera to allow fast sync flash at any shutter speed up to 1/4000th sec)

 

[davenn]

That used to be the case with film (meter off the reflection from the film),

HUH ?
I think you need to reconsider that comment

how can you meter off the reflection from the film ? the film is hidden ( in the dark) behind the shutter and the metering is done before the shutter opensDave

 

[olivermsun]

HUH ?
I think you need to reconsider that comment

how can you meter off the reflection from the film ? the film is hidden ( in the dark) behind the shutter and the metering is done before the shutter opensDave

The (final) flash metering happens during the exposure, as sophiecentaur mentions above. Thus the shutter is open and the flash is reflecting off the film.

 

[William White]

Dave
The OM had a rather spiffy metering method (which became widely copied and never matched, even now)

The first curtain was patterned and looked liked an array of black and white dots that was determined to have the same reflectance as the "average scene" would have off the film. The light reflected down to a little light sensor that adjusted the exposure. (the camera had a split mirror to facilitate this)

http://www.mir.com.my/rb/photography/hardwares/classics/olympusom1n2/images/om2shutter.jpg

As the shutter was fired, and the first curtain opened to show the film, the light reflected OFF the film was analysed and the shutter speed was adjusted on the fly to give accurate exposure.

And it was the best exposure method ever used in a 35mm camera.

You could use some very fussy reversal films and get spot on exposures every time.

So for example, one takes a meter reading and the shutter speed is determined to be 1/500th sec. Once the shutter is fired, the light is analysed during the exposure, first from the pattern of the first curtain, and then off the film - and because the light is measured off the film, this allows better exposure. The shutter is controlled, and may change the effective shutter speed say, to 1/470th sec, or 1/530th sec. (it required a shutter mechanism tolerance far in advance of competing cameras)

It was actually very useful for slower exposures when 1/3 stop was in the realm of a seconds or large fractions of a second 1/4; 1/2 etc.So yes, a meter reading was taken before the shutter is pressed. But the metering which determined the final exposure was taken during exposure.

 

[olivermsun]

The real time OTF metering of the OM was impressive then, and it still is!

 

[William White]

https://en.wikipedia.org/wiki/Through-the-lens_metering#Off_the_film_meteringhttp://www.mir.com.my/rb/photography/hardwares/classics/olympusom1n2/shared/flash/

For any knowledgeable photographer, it is hard not to associate the amazing TTL "OTF" Auto capability with Olympus, in particular the OM-2 which was their first successful attempt in realizing this revolutionary flash exposure control method. The flash is totally controlled by the camera so there is no need for any exposure calculations or settings even for bounce flash, diffused flash lighting, off-the-camera flash or simultaneous use of multiple flash units. Any aperture of the taking lens can be selected freely, the flash can be used over a distance range from about twice as close to twice as far as with normal auto flash and the angle of flash measured is always precisely the same as the angle of the taking lens.

 

[sophiecentaur]

HUH ?
I think you need to reconsider that comment

how can you meter off the reflection from the film ? the film is hidden ( in the dark) behind the shutter and the metering is done before the shutter opensDave

iirc, the Olympus OM10 did its through the lens metering using the film surface.
Edit: The thing in favour of this method is that the metering, by the very nature of the system, decides that it's had enough light when it actually has had enough - whatever the lighting of the scene. In dim light it waits - as it would have to, anyway.
I scoffed when I first heard of the system. lol

 

[William White]

yes, it was great for long exposures; if one was taking a night scene, with a filter, slow film and small aperture ; you just set your aperture, hit the shutter and waited...and several minutes later the shutter closed and you had a spot on exposure. (I think the limit for the OM4 was an hour? I can't remember exactly). The only thing you had to worry about was reciprocity failure which affected some films; but that wasnt a problem, you just dialled in + 'x' stops for the film you were using.

It made long exposures so easy.

New digital cameras are starting to have a similarly useful feature where the image builds up on the screen, and when you think it is correct you stop the exposure.